Aperture and seeing?
stjar...@juno.com
I understand the theoretical limits ( Dawes Limit) on the resolving power
of telescopes of various apertures, however when the statement is made,
"larger scopes are hurt more by poor atmospheric seeing", I am somewhat
confused. If a large scope of say 18 inch aperture and a small scope of 5
inch were placed side by side on a night with seeing conditions allowing
1 arcsecond, would the smaller instrument show greater detail? Does the
quote above merely state that a larger aperture is affected MORE because
it can resolve down to a lower limit and since the atmosphere will only
allow you to resolve to a certain point it is therefore hurt more than a
smaller scope which can only reach a less impressive resolution. Anyone
that can clear this up for me? Clear skies...
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Mike
I have never compared an 18" and a 5" reflector but I can attest to an
atmospheric phenomenon that afflicts the larger aperture scopes in varying
degrees. Realizing that light gathering power is a function of the mirror's
surface area one can easily see that an 18"scope gathers nearly 13 times more
light than a 5 inch. It must also look through a column of air 13 times
larger as well. These columns of air contain various"cells" which may have
varying degrees of density which in turn will affect atmospheric diffraction
etc.,etc. causing the image to break down in a light bucket. This is
especially noticeable at moderate to high power. ( This is how it has been
explained to me) I remember a time this past summer when it was really
apparent . The 10" and larger scopes began to exhibit "poor seeing" while the
6 to 8" ones were still crisp and contrasty.I understand it can be a bear of
a problem with the monstrous observatory scopes.
Clear Skies and Good Seeing,
Mike
Mike Fleenor
Knoxville,TN
please remove nospam to reply
Visit Mike's Home Planet: http://user.icx.net/~mfleenor
Bill Greer
I once spent most of night (with poor seeing) studying Mars with a 5"
scope. After spending a great deal of time achieving 'best' focus, I
noticed that it was possible to pick out an occasional detail in the
'bubbly' image. My intrique with this ability to detect detail under
poor seeing kept me at the eyepiece until dawn.
I started a Mars sketch, but the planet rotated faster than I could
pick out the fleeting details. One moment a detail would be 'there',
not to be seen again until literally minutes later.
What would a larger scope have seen on that night? My best guess is
that the area of Mars in sharp focus (for the briefest of instants)
would be smaller with the larger scope. In effect, instead of 1/8th
of the planet's surface popping into a rare crisp view, a scope of
double the aperture would show 1/64th of the planet's surface popping
into a rare crisp view.
The initial impression would be that the image in the larger scope
would seem to be 'messier' than the image in the smaller scope. But
with great care, a meticulous observer would be able to pick out finer
details with the larger scope, despite the fact that at any one time,
more of the planet would be 'fuzzy' with the larger scope. Working
against the larger scope would be the greatly increased time needed to
extract a complete image of the planet.
Higher magnification is needed with the larger scope, even under poor
seeing, in order to exceed the resolution of the smaller scope.
Under equal magnifications, as long as the magnification is not 'too
much' for the smaller scope; the smaller scope will show an image
that is preferrable to that seen with the larger scope. This is
because the smaller scope will show crisp detail over a larger angular
area than will the larger scope at any one instant.
Much will depend upon the skills of the observer. To (perhaps) most,
the image in the larger scope will be too messy to attempt to unravel
despite the greater 'hidden' detail contained in that mess. It's so
much easier to just use a smaller aperture and see more of the planet
in focus at any one instant.
Example of observer differences: One evening I was sketching subtle
shadings on Mercury as seen with an 8" scope. A much less experienced
observer looked at Mercury through the same scope. The seeing was
poor. The less experienced observer was unable to tell what phase the
planet was displaying -- let alone the 'hopeless' task of picking out
subtle surface markings. To myself, the phase was very obvious
(though far from steady) and I was busy searching out finer details.
That was an educational experience for both of us. I was quite
surprised at how important experience was for picking out planetary
detail.
Bottom line: Aperture always wins (assuming equal optical quality) --
unless one is interested only in brief glimpses through the eyepiece.
In the latter case, the smaller aperture will appear to show a better
image under poor seeing. No one ever said visual astronomy was easy.
Jeff Beish
No, the smaller scope would not show greater detail. It would
just not show as much apparent turbulence. We refer to this myth, and
that is what it is, as the "Questar effect." It encompasses a lot of
theoretical mumbo jumbo, or jargon depending on your point iof view.
In the case of the theoretical jargon people with less experience in
optics are often confused by this and talk shold remain simple.
Jeff
Jeff Beish
On Wed, 24 Dec 1997 05:29:38 GMT, verdn...@mcn.net (Bill Greer)
wrote:
>I once spent most of night (with poor seeing) studying Mars with a 5"
>scope. After spending a great deal of time achieving 'best' focus, I
>noticed that it was possible to pick out an occasional detail in the
>'bubbly' image. My intrique with this ability to detect detail under
>poor seeing kept me at the eyepiece until dawn.
>
Planetary observers usually work the focuser to death anyway
regardless of the seeing. While drawing at he telescope I have to
readjust my eyes to look at the drawing paper then re-focus again back
at the telescope, nothing surprising about that. The effects aperture
has on seeing, or at least how were perceive it, is only apparent
anyway. When I observe a planet a larger aperture operating fully is
always desirable. There are always moments during periods of less than
desirable seeing when it steadies out and the aperture is there for
you right then. If you switch to a smaller telescope because you
think you see more details hen when the seeing gets good you are
stuck.
Jeff
gser...@vgdirect.ca
jbe...@gte.net (Jeff Beish) wrote:
>On Wed, 24 Dec 1997 05:29:38 GMT, verdn...@mcn.net (Bill Greer)
>wrote:
>
>>I once spent most of night (with poor seeing) studying Mars with a 5"
>>scope. After spending a great deal of time achieving 'best' focus, I
>>noticed that it was possible to pick out an occasional detail in the
>>'bubbly' image. My intrique with this ability to detect detail under
>>poor seeing kept me at the eyepiece until dawn.
>>
> Planetary observers usually work the focuser to death anyway
>regardless of the seeing.
Glad to hear I'm not the only one who observes with one hand on the
focuser all the time.
Regards,
Gary in Vancouver
(Member: Village Green Preservation Society)
(NOTE: Remove "VG" from e-mail address to reply.)
Jeff Beish
That may be why I opted to do away with the motorized focuser
and just started back with the hand adjustments. It seemed to be
slower. I eventually made a massive German mount so that wouldn’t
rattle the scope so much. Yeah, the constant re-focusing is due to
several things -- the iris will open and close relative to how bright
the image is, especially while using filters in from of the eyepiece.
When you look away to make the drawings the eye then takes up a new
focus, and the iris will probably close up a little due to your light
on the drawing board. But, the changing atmosphere is the primary
reason you have to change focus so much.
Seeing comes is several forms that not many people talk about.
In Florida, the seeing was poor after a “cold” front came through. It
wasn't the typical poor seeing stuff where the image appeared like is
was swimming in water, but the fuzzy ball type. This defies
explanation because it indicates high frequency turbulence and usually
this it at high altitudes and will not go away soon enough. The usual
poor seeing make an image appear like it grows in size, fuzz’es up, or
thermals will run through it like waves of water. This seeing is
do-able because it will usually get better and worse as the night goes
by. If it gets better as a mater of habit, then you might need to move
your telescope or find and fix the tube currents. The fuzzy ball
seeing is something that is long lasting and you might as well pack it
in.
Jeff
AndersonRM
In article <34a09c34.6774943@news>, verdn...@mcn.net (Bill Greer) writes:
>Higher magnification is needed with the larger scope, even under poor
>seeing, in order to exceed the resolution of the smaller scope.
Only up to the point where the larger scopes diffraction limit is
perceived by the observer. At 200x, the diffraction limit of an
8 inch telescope becomes just visible to an observer. You could
put the magnification of a 4-inch scope up to 200x but the 8-inch
scope will show you more detail even though it's magnification is
the same as the 4-inch scope.
>Under equal magnifications, as long as the magnification is not 'too
much' for the smaller scope; the smaller scope will show an image
that is preferrable to that seen with the larger scope.
Preferable is a loaded term. You might see a more aesthetic image in the
smaller scope because you will see the image as subjectively clearer, depending
on the seeing conditions and the sizes of the scopes used. However, the
smaller scope will not show more detail.
-Rich
size of the two scopes.
AndersonRM
>Planetary observers usually work the focuser to death anyway
>regardless of the seeing.
One thing all planetary observers should do is use filters when needed
and wear a black eyepatch over the non-observing eye. Filters are
needed to reduce brightness levels on bright planets that washes out
contrast and to improve contrast by using non-complementary colours
(filter and planet detail). The black eyepatch is to prevent the non-observing
eye from seeing anything because that visual information is superimposed
by the brain onto the image from the other eye. This tends to drown out
planetary detail. Lastly, planetary observers should sit down because it
lessens microtremors of the body that "jitter away" planetary detail.
-Rich
Paul Schlyter
In article <882843671....@dejanews.com>, <stjar...@juno.com> wrote:
> I understand the theoretical limits ( Dawes Limit) on the resolving power
> of telescopes of various apertures, however when the statement is made,
> "larger scopes are hurt more by poor atmospheric seeing", I am somewhat
> confused. If a large scope of say 18 inch aperture and a small scope of 5
> inch were placed side by side on a night with seeing conditions allowing
> 1 arcsecond,
> would the smaller instrument show greater detail? Does the quote above
> merely state that a larger aperture is affected MORE because it can
> resolve down to a lower limit and since the atmosphere will only allow
> you to resolve to a certain point it is therefore hurt more than a
> smaller scope which can only reach a less impressive resolution. Anyone
> that can clear this up for me? Clear skies...
The typical turbulence cells in the atmosphere are some 4-6" (10-15
cm) large.
If the telescope has an aperture not exceeding this, the image will
be moving back and forth, but remain fairly sharp.
If the telescope's aperture is several times larger than this, it
will see through several adjacent turbulent cells at once. Thus
there will be several superimposed images, each moving back and forth
independently of the others, which results in a smeared image.
Therefore the smaller scope may, under poor seeing, produce a sharper
image than the larger scope.
--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pau...@saaf.se paul.s...@ausys.se pa...@inorbit.com
WWW: http://spitfire.ausys.se/psr -- updated daily!
Jeff Beish
On 25 Dec 1997 12:03:16 +0100, pau...@electra.saaf.se (Paul Schlyter)
wrote:
>That's good seeing, not poor seeing!
True......indeed.
>
>The idea behind this is:
>
>The typical turbulence cells in the atmosphere are some 4-6" (10-15
>cm) large.
>
May I play devil’s advocate here for a moment. You say, “the
typical turbulence cells in the atmosphere are some 4-6" (10-15 cm)
large. “ This may be true or not, the point I wish to raise is this:
at what distance are these so-called 4-6 inch cells from your
telescope? If the cells are very near to the entrance of your
telescope then they will be superimposed on the optical path and will
be seen as 4-6 inch cells, however, if they are 10,000 feet up then
will they not represent a smaller angle relative to the aperture of
your telescope?
I know the above is a loaded question, but it does present a
curious point to those who do not understand the relationship of
atmospheric turbulence to aperture. Oh yeah, does the distance these
cells are from the optical path effect their focus in the telescope
eyepiece?
Jeff
Now, where is that old bearded guy with the bag of new
eyepieces!!! It's Christmas!
Jeff Beish
There have been amateur astronomers who rejected the book I am
about to recommend, but after reading J.B. Sidgwick’s _ Amateur
Astronomer’s Handbook _, it has one of the best sections (section 26,
pages 445 - 470) on astronomical “seeing” that has ever been printed.
It covers so much that has been asked here that the only way to answer
questions is to recommend that people read this book first to set the
pace. I have seen it in several book stores, even near Washington,
DC, where science is abstract thought better left to people with
brains :)
Jeff
Sue and Alan French
Paul,
I've run this idea about cells by three meteorologists by now, and they
all say they have never heard of such a thing. Astronomers talk about them
a lot. What published evidence is there that they even exist?
Clear skies, Alan
Paul Schlyter <pau...@electra.saaf.se> wrote in article
<67tehk$1...@electra.saaf.se>...
Kenneth Drake
I agree with Paul completely. This is precisely what I see, the image in
the four inch just wanders around but appears sharp while in the 24 inch
it gets shreaded. In some cases, when we get "fast seeing", like looking
through the exhaust of a jet engine, no image in any scope is sharp.
This seems to happen just after the passage of a cold front.
Kenneth Drake
Paul Schlyter wrote:
>
> The typical turbulence cells in the atmosphere are some 4-6" (10-15
> cm) large.
>
> If the telescope has an aperture not exceeding this, the image will
> be moving back and forth, but remain fairly sharp.
>
> If the telescope's aperture is several times larger than this, it
> will see through several adjacent turbulent cells at once. Thus
> there will be several superimposed images, each moving back and forth
> independently of the others, which results in a smeared image.
>
> Therefore the smaller scope may, under poor seeing, produce a sharper
> image than the larger scope.
>
> --
Fred Fuentes
I'm not positive, but I think the idea may come from the Russian
mathematician Kolmogorov. I believe he did some landmark work demonstrating
that turbulent flows can be described mathematically *AS IF* they consisted
of small cells.
Fred
Sue and Alan French wrote in message
<01bd1142$9f9fb6c0$1c032399@sue-alanfrench>...
>Paul,
> I've run this idea about cells by three meteorologists by now, and they
>all say they have never heard of such a thing.
snip
> Clear skies, Alan
Jeff Medkeff
I am pretty sure that jbe...@gte.net (Jeff Beish) said the
following, though I may be wrong:
> There have been amateur astronomers who rejected the book I am
>about to recommend,
No way! Who were these, er, so-called amateur "astronomers"?
>but after reading J.B. Sidgwick’s _ Amateur
>Astronomer’s Handbook _, it has one of the best sections (section 26,
>pages 445 - 470) on astronomical “seeing” that has ever been printed.
I second the recommendation. This book is easily found,
having gone through a reprinting within the last 20 years
(at least) by Dover in the US. Note that in different
editions the page numbers are different.
>It covers so much that has been asked here that the only way to answer
>questions is to recommend that people read this book first to set the
>pace.
Note also that Sidgwick does include citations for the
technical literature in the bibliography at the back of the
book (at least in my copy). I have followed through by
reading some of that literature, which was all very
interesting. From one source I learned that the "seeing
cells" which are so often discussed were brought up by a
professional astronomer as a manner of describing the
effects and characteristics, though not the mechanisms, of
seeing. Naturally, the amateur community took this
descriptive analogy way too far, perhaps in no small part to
the Questar ads of the 1970s (or earlier?).
Jeff Medkeff | Check out the s.a.a. photos page at
Rockland Observatory | http://shutter.vet.ohio-state.edu/saa.htm
Sierra Vista, AZ |
Jack Schmidling
Jeff Beish wrote:
" I know the above is a loaded question, but it does present a
curious point to those who do not understand the relationship of
atmospheric turbulence to aperture. Oh yeah, does the distance these
cells are from the optical path effect their focus in the telescope
eyepiece?
One of the other frequently reported factoids is that one can actually
measure the altitude of these cells simply by measuring the eyepiece
travel to "get them in focus".
One typically has to move the focuser a good fraction of an inch to
see turbulence well. This would mean they are within a few tens of
feet from the scope or infinitely beyond infinity.
js
--
Visit our WEB pages:
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ASTROPHOTO OF THE WEEK..... New Every Monday
Chris Marriott
In article <67u510$e...@argentina.earthlink.net>, Fred Fuentes <fredfuent
e...@nospam.earthlink.net> writes
>I'm not positive, but I think the idea may come from the Russian
>mathematician Kolmogorov. I believe he did some landmark work demonstrating
>that turbulent flows can be described mathematically *AS IF* they consisted
>of small cells.
>
Are you sure that he wasn't just talking about the standard mathematical
modelling technique of "finite element analysis"? In FEA, you model a
physical process such as air flow by dividing it up into a "mesh" of
cells, and then describing each cell by one or more partial differential
equations which are all solved simultaneously. This is the standard
technique used to generate computer models of air flow around aircraft,
cars, etc.
The fact that you model the real word *as* a mesh of small cells in FEA
doesn't mean that that's the way the atmosphere really *is*, however!
Chris
----------------------------------------------------------------
Chris Marriott, Microsoft Certified Solution Developer.
SkyMap Software, U.K. e-mail: ch...@skymap.com
Visit our web site at http://www.skymap.com
Jeff Beish
Jack, right on. You can also see tube currents that way too. A
common heat current from components is heat rising from the secondary.
But, that isn't the jist of the question......
Jeff
On Thu, 25 Dec 1997 10:04:06 -0800, Jack Schmidling <a...@mc.net>
wrote:
Jeff Beish
Fred,
Much of these terms, i.e., seeing cells, turbulence cells, and
I use "thermal cells," etc., are used because of a lack of proper
words. Meteorologists have their terms and we seem to come up with
our own and would probably be better if we learned more about their
definitions.
The cause of bad seeing is usually down in altitude around
where your telescope is, but the upper atmosphere can contain winds
with currents, thermal currents maybe, and when they cross each other
the refraction of the different levels, air densities and all that
will cause seeing to be poor. The density of air increases and
decreases as the temperature decreases or increases and so goes
refraction of air too, so you get a lot of little bubbles of hot and
cold air, like floating lenses, in the optical path.
I put a brief paper on astronomical “seeing” in the
observatory newsletter and the superintendent told the editor I had
some "wrong weather science" in it. For the life of me I can’t find
out what that is, but I’ll take is word for it -- he is a
meteorologist. Maybe I used thermal cells in it! :)
Jeff
On Thu, 25 Dec 1997 09:28:27 -0800, "Fred Fuentes"
<fredf...@nospam.earthlink.net> wrote:
>I'm not positive, but I think the idea may come from the Russian
>mathematician Kolmogorov. I believe he did some landmark work demonstrating
>that turbulent flows can be described mathematically *AS IF* they consisted
>of small cells.
>
Jeff Beish
I think some people forget that we astronomers use quote
around some of the jargon we come up with when it pertains to the
atmosphere and that stuff. The people I referred to were over on
GEnie several years ago. One guy said my choice in astronomical
literature was hopefully out of date. He really objected to
Sidgwick’s books for some reason. of course, he never replied to my
questions. Several of his gang sided with him, not unlike people
arguing about religion -- the two sides divided up and flames each
other, it ended up nasty. I concluded they had no clue at what they
were saying...... Genie was a strange place <G>.
Merry Christmas, hope all your astronomical socks were full of goddies
:)
Jeff
Fred Fuentes
Chris Marriott wrote in message <4Zk3qIAy...@chrism.demon.co.uk>...
>Are you sure that he wasn't just talking about the standard mathematical
>modelling technique of "finite element analysis"?
snip
>The fact that you model the real word *as* a mesh of small cells in FEA
>doesn't mean that that's the way the atmosphere really *is*, however!
>
>Chris
>
>----------------------------------------------------------------
>Chris Marriott, Microsoft Certified Solution Developer.
>SkyMap Software, U.K. e-mail: ch...@skymap.com
>Visit our web site at http://www.skymap.com
Yes, my limited impression is that he was talking about what you call FEA.
I'm not an expert, so I'll defer to you, but I think he did pioneering work
in this area, maybe fifty (?) years ago, and I couldn't help wondering if
maybe some people around that time, getting fragmentary information, began
to think that this modeling represented the way the atmosphere really is.
Fred
Paul Sventek
Hi Jeff,
Jeff Beish wrote:
>Genie was a strange place <G>.
Yeah, but it was fun, wasn't it? Nice to run into you here.
Todd Gross
>I agree with Paul completely. This is precisely what I see, the image in
>the four inch just wanders around but appears sharp while in the 24 inch
>it gets shreaded. In some cases, when we get "fast seeing", like looking
>through the exhaust of a jet engine, no image in any scope is sharp.
>This seems to happen just after the passage of a cold front.
Whatever the reason.. indeed, without a doubt, I need to stop down my 16" in
times of certain poor-mediocre seeing conditions for consistently better
planetary views. (off-axis mask) There is no question that the off axis
aperture mask has aided in my quest for detailed planetary images.
In bad seeing both apertures are hit hard
In mediocre-good seeing, the larger aperture can glean more detail...with
patience.
Thanks! - Todd
_________________________________
BOSTON TV METEOROLOGIST TODD GROSS
Weather/Astronomy Home Page: http://www.weatherman.com
Administrator, Meade Advanced Product User Group: ma...@shore.net
Administrator, New England Weather Observer Mail List: wxob...@shore.net
IRC Channel Operator: #Weather (Undernet)
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_________________________________
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AndersonRM
In article <01bd1142$9f9fb6c0$1c032399@sue-alanfrench>, "Sue and Alan French"
<Sue_and_A...@msn.com> writes:
>I've run this idea about cells by three meteorologists by now, and they
all say they have never heard of such a thing. Astronomers talk about them
a lot. What published evidence is there that they even exist?
These cells do exist. They produce "waves" of irregular refractive
index that are about 1"-4" inches in size. You can see them visually
if you have a scope of about 10-14" in size. Put a bright star in the
telescope, remove the ocular, watch the "waves" cross the field.
If you observe a star diffraction pattern in a 4-inch scope during periods
of bad seeing, the diffraction pattern will seem to be intact but in motion.
If you do the same with a 10-inch, the diffraction pattern will be distorted
(perhaps to the point of mush) but the pattern will be stationary. This is
because the 4-inch is seeing through one "cell" while the 10 is seeing more
than
one.
-Rich
AndersonRM
>Several of his gang sided with him, not unlike people
arguing about religion
>-- the two sides divided up and flames each
other, it ended up nasty. I
>concluded they had no clue at what they
were saying...... Genie was a strange
>place <G>.
I tend to give credence to observers who practiced visual work
like Sidgwick over modern astronomers who seldom look through
any telescopes except with ccd's.
-Rich
Jeff Beish
Your name rings a bell. Getting old and forgetful. Does
GEnie still exist?
Jeff
On Thu, 25 Dec 1997 21:47:43 -0600, Paul Sventek <sve...@ibm.net>
wrote:
>Hi Jeff,
>
>Jeff Beish wrote:
>>Genie was a strange place <G>.
>
Jeff Beish
A lot of "fragmentary information" is somehow etched in stone
when it comes to astronomy, especially when talking about our
atmosphere. It is quite complex but some elements like how it effects
telescopic images came be simply explained. The one type of "seeing"
that puzzles me is when an image, planetary usually, gets fuzzy and
details are completely wiped out.
The usual atmospheric effects on images is to see them as
though they are swimming in water, so to speak. Sometimes the image
will dart around the field or oscillate a bit as it would when a cloud
drifted by and blocked off the image. Actually, you can see a bright
images in the telescope field even though it may be blocked off by
clouds if you looked at it naked eye. So, the atmosphere is composed
of water vapors, at times water droplets and ice crystals, so this too
may wipe out an image. Maybe the ice crystals are the reason the
images get real fuzzy and the image grows many time larger that it
should be......
Jeff
K4MSG
For a thorough and exhaustive description of atmospheric effects on
seeing, including citations wherein professional scientists have actually
measured the size and disposition of atmospheric irregularities (they tend to
exist in a relatively thin band situated between 1.5 and 5 km above the Earth's
surface), as well as a methodology by which amateurs may actually "view" the
irregularities, plus discussions of seeing versus aperture and methods for
qualitatively evaluating seeing conditions, read the "Amateur Astronomer's
Handbook" by J.B. Sidgwick, sections 26.7-26.10, pp. 449-460 (Enslow
Publishers, Hillside, NJ, 1980). It's all there.
Clear skies,
Paul
Paul H. Bock, Jr. Hamilton, VA U.S.A.
K4MSG since 1957 Member - ARRL, SOWP, VWOA
Member - A.L.P.O., RASC (Kingston Centre), William Herschel Society
Jeff Beish
Rich,
I will ask again. How do you know these "cells" are the size
you indicate? The existance of "cells" may be, but what size are they
and how far away from your telescope they are is another question.
Sometimes I get the impression that some “amateur” astronomers
have little regard for “professional” astronomers. I too have said
things, that if taken out of context, could be misconstrued as
negative feeling toward the “professional” astronomer types. This is
probably my fault in that for the lack of time or space statements are
made that after re-reading them seem disconnected -- even to myself.
In this case it is just not laziness on my part but a lack of skill in
one line, sound bite type statements. I usually write something for
publication that has little resemblance to the original string or
words when it goes to press. Several people get into the act and
ideas fly, editors may change things, but the final product may be
good enough to represent an idea.
“Professional” astronomer may be used too often when
describing those few people who work at universities and receive
research grants for projects that are associated with astronomical
observations, etc. There are a hand full of people who work directly
for an astronomical observatory or indirectly through other
institutions of science. Since my contact with astronomer types other
than Solar System researchers, is limited to a few radio astronomers,
I can’t say much about that part of the “profession,” except that they
appear to wear pin-striped suits and drink martinis, and talk above my
head <G>. “Planetary astronomers” are more down to Earth, so to
speak.......
So, it may be that some amateurs think that because some
people use technology to enjoy the celestial sphere they are somehow
not amateurs as some would like them defined. Then, what doesn all
this have to with astronomical "seeing"?? :)
Jeff
Jeff Beish
I too side or lean toward the visual observer, but what does
that have to do with the subject at hand?
Those people I know who use CCD cameras usually observe
visually as well. At least a couple I know who use cameras have been
known to do visual observing for years before taking up the
technological side of astronomy. You may have been talking about the
"stereo-typed professional" astronomer. My association with the “deep
sky” type professional is limited, but I can probably count the number
of professional planetary astronomers on my hands, maybe a few toes
too, that I know personally or know of. It is a small club. There is
just not many of them left. So, it has been up to advanced amateurs to
do the bulk of the imaging of planets, comets, etc., and to some
extent other electronic measuring data of the Solar System, and they
provide this information to the few professionals that are data
starved by the funding cuts of late.
Anyway, you can measure "seeing" using CCDs and even film.
Film that has star streaks can be used to measure the amount and
frequency of the wiggles, as well as the thickness of the streaks, to
quantify astronomical "seeing" for that period of time. This too is
an interesting side of astronomy, to quantify astronomical “seeing,”
even though some of the attempts to do this seems a little off track.
Jeff
K4MSG
Jeff said:
>I will ask again. How do you know these "cells" are the size
you indicate? The existance of "cells" may be, but what size are they and how
far away from your telescope they are is another question.
Here is a quotation from Sidgwick (see my previous post), p.451:
"That movement of the image can be converted into confusion of the image
by changing from an aperture of 10 cm to one of 150 cm suggests that the
distance between adjacent convexities and concavities in the theoretically
plane surfaces of equal refraction lies between these limits; observations of
shadow eclipse bands suggest that it lies nearer the former, and Anderson
accepts about 15 cm as the order of size. Ellison and Seddon found
indications that the amplitude of scintillation reaches a maximum at about D =
75 mm, from which they deduced this to be the order of size of the refractional
irregularities causing the scintillation. As regards the height of the
distorting layer above the Earth's surface, it was found at Edinburg that
Jupiter and Saturn (angular diameters 49" and 20") showed no trace of stellar
scintillation; the scintillation of Mars (4.4") was of about half the amplitude
of that of a comparison star; and that of Jupiter III (1.5") was purely
stellar. They concluded that the angular size of the refractional
irregularities was of the order of 3", (which is) the angular subtention of 75
mm at a distance of about 5 km. Since the observations were made at an
altitude of 30 degrees this indicates a vertical height of about 2 1/2 km."
"The region of these refractional irregularities thus lies at a
considerable height above the Earth's surface - probably between 1 1/2 and 5
km. There is also evidence that they are confined to a comparatively thin
stratum."
There is more which precedes and follows which amplifies on the above, but
that's enough bandwidth for this go..... ;-)
Sue and Alan French
These cells do exist. They produce "waves" of irregular refractive
index that are about 1"-4" inches in size. You can see them visually
if you have a scope of about 10-14" in size. Put a bright star in the
telescope, remove the ocular, watch the "waves" cross the field.
Rich,
Bullshit. I have a 14.5" scope. You are describing radiational cooling
effects.
Clear skies, Sue
Sue and Alan French
Rich,
To expand somewhat on what my wife said...
Simply restating your belief that these cells do exist does not make it
so. Our atmosphere is an every-changing mix of air of varying temperature,
density, and moisture content. To say that it contains "cells" of some
particular size seems like nonsense. If you were to say that the wavefront
is distorted as it passes through the atmosphere, and that on average a
sample of the wavefront reaching your telescope 4 inches in diameter is
less distorted than another sample 20 inches in diameter, I might agree.
But to attribute this to "cells" that actually exist stretches credulity.
Clear skies, Alan
AndersonRM <ander...@aol.com> wrote in article
<19971226130...@ladder01.news.aol.com>...
> In article <01bd1142$9f9fb6c0$1c032399@sue-alanfrench>, "Sue and Alan
French"
> <Sue_and_A...@msn.com> writes:
>
> >I've run this idea about cells by three meteorologists by now, and they
> all say they have never heard of such a thing. Astronomers talk about
them
> a lot. What published evidence is there that they even exist?
>
Lance Olkovick
On Thu, 25 Dec 1997 12:55:44 GMT, jbe...@gte.net (Jeff Beish) wrote:
>On 25 Dec 1997 12:03:16 +0100, pau...@electra.saaf.se (Paul Schlyter)
>wrote:
>>That's good seeing, not poor seeing!
> True......indeed.
>>The idea behind this is:
>>The typical turbulence cells in the atmosphere are some 4-6" (10-15
>>cm) large.
> May I play devil’s advocate here for a moment. You say, “the
>typical turbulence cells in the atmosphere are some 4-6" (10-15 cm)
>large. “ This may be true or not, the point I wish to raise is this:
>at what distance are these so-called 4-6 inch cells from your
>telescope? If the cells are very near to the entrance of your
>telescope then they will be superimposed on the optical path and will
>be seen as 4-6 inch cells, however, if they are 10,000 feet up then
>will they not represent a smaller angle relative to the aperture of
>your telescope?
I think some of our notions about "turbulence cells" or "seeing cells"
are highly mistaken. Here's my take on things:
Turbulence cells are *not* blobs of air of different refractive index
than the surrounding air that act as lenses that distort the incoming
light (although such blobs may exist and may contribute to the
degradation of seeing conditions); a cell is simply an arbitrary
section of the atmosphere extending from observing level to just
beyond the turbulent depth of the atmosphere. If we set up a 4"
telescope, the section of atmosphere through which the light of the
object under study must pass (extending from the objective to beyond
the turbulent layer) defines a 4" cell. The incoming wavefront passes
through the atmosphere and is affected by various turbulent features
of the atmosphere (the jet stream, shear, blobs of differing
refractive index, whatever). If the phase shift of the incoming
wavefront is less than that which adversely affects a 4" objective, we
say that the turbulence cells are at least 4" big at that time. If we
look through a 10" objective, the atmosphere may not be steady enough
to allow a 10" to be used to best effect (the phase shift in the
larger cell is great enough to compromise the 10"er's performance), in
which case we might say that the larger instrument is actually being
forced to look through a number of the smaller "perfect" turbulence
cells.
By this reckoning, cells are more like devices that describe seeing
conditions rather than explain them.
Does this sound feasible, or am I out to lunch on this one? The notion
that cells are like the blobs in a lava lamp that must be over a
certain size or must line up somehow to provide good seeing seems
ludicrous to me.
--
Lance Be seeing you. -- Number Six
David Tyler
"Sue and Alan French" <Sue_and_A...@msn.com> writes:
>Paul,
> I've run this idea about cells by three meteorologists by now, and they
>all say they have never heard of such a thing. Astronomers talk about them
>a lot. What published evidence is there that they even exist?
> Clear skies, Alan
see
[1] D.L. Fried, ``Optical resolution through a randomly inhomogeneous
medium for very long and very short exposures," J.Opt.Am.Soc. 56
(1966)
[2] F. Roddier, ``The effects of atmospheric turbulence in optical
astronomy, in _Progress in Optics_, E. Wolf, ed., vol. XIX, North-
Holland, New York (1981)
and perhaps the most comprehensive and readable of these
[3] M.C. Roggemann and Byron Welsh, ``Imaging Through Turbulence,"
CRC Press, New York (1996).
the last reference is a research monograph and comes loaded with more
references than you could possibly look up, as well as a very readable
introduction.
oops, here's another:
[4] J.W. Goodman, ``Imaging in the presence of randomly inhomogeneous
media," in _Statistical Optics_, Wiley, New York (1985).
dave
______________________________________________________________________
-David W. Tyler "It seems you feel our work is not
-USAF Phillips Laboratory of benefit to the public."
-Albuquerque, New Mexico
-ty...@plk.af.mil --Rachel
David Tyler
"Fred Fuentes" <fredf...@nospam.earthlink.net> writes:
>Chris Marriott wrote in message <4Zk3qIAy...@chrism.demon.co.uk>...
>>Are you sure that he wasn't just talking about the standard mathematical
>>modelling technique of "finite element analysis"?
>snip
>>The fact that you model the real word *as* a mesh of small cells in FEA
>>doesn't mean that that's the way the atmosphere really *is*, however!
>Yes, my limited impression is that he was talking about what you call FEA.
>I'm not an expert, so I'll defer to you, but I think he did pioneering work
>in this area, maybe fifty (?) years ago, and I couldn't help wondering if
>maybe some people around that time, getting fragmentary information, began
>to think that this modeling represented the way the atmosphere really is.
no, that's not it. since astronomical objects are so distant relative to
the earth's atmosphere, the atmosphere can be modelled as a thin phase
screen at the telescope pupil. by ``phase screen," i mean a layer with
a spatially-varying refractive index. a plane wave incident on the screen
will not be planar after passing through the screen, since the refractive
index variations will cause one part of the wave to be delayed relative to
another. the index variation obviously doesn't occur in discrete steps;
the index varies continuously over the telescope pupil. if you have a
very small pupil, you won't see any variation _at all_ (no turbulence
effects will be observed in your imaging). if you observe a star with
increasingly larger pupils, you'll notice a point where the star image
no longer becomes sharper as the pupil size increases; your pupil is now
roughly the size of a ``turbulence cell." in other words, you are start-
ing to measure an index variation across your pupil.
finite element analysis is a numerical scheme to treat continuously-vary-
ing fields, and can be applied to the study of turbulence. however, the
atmosphere in many cases behaves as though it really is made up of ``see-
ing cells."
K4MSG
Another reference which relates seeing to weather (and describes the
controversial "cells") can be found in "Observing and Photographing the Solar
System," by Dobbins, Parker & Capen (Willmann-Bell, 1988), section 2.13, "The
Atmosphere and Observing," pp. 17-19.
The following quotation is instructive:
".....'seeing' is caused by moving cells of air at altitudes ranging from
several hundred feet to ten miles that have different temperatures and hence
different indices of refraction. **At most locations these cells range in size
from four to eight inches in diameter, although research by atmospheric
physicists and meteorologists has revealed that they can vary tremendously in
size ** (emphasis added). Each cell acts as a lens, changing the focal
position of the image by bending rays of light differently."
(p.17)
FWIW,
Kenneth Drake
Paul Sventek wrote:
>
> Hi Jeff,
>
> Jeff Beish wrote:
> >Genie was a strange place <G>.
>
> Yeah, but it was fun, wasn't it? Nice to run into you here.
Nice to see you here, Paul. You may not remember, but, it was you who
introduced me to the "what's this" of local seeing effects. In Jan. of
'84 I had borrowed a Tinsley 8" classical Cassegrain from our local
club. I took in out of a 70 degree house into the 28 degree air to
observe on the front porch. After trying to focus the beast for 10
minutes, I called you to help out. You were kind enough to explain that
until the mirror cooled down to near ambient, that it wouldn't focus. It
took over two hours. Thanks Paul.
--
Clear skies and skinny moons------Drake
Paul Schlyter
In article <680blp$og5$3...@gte1.gte.net>, Jeff Beish <jbe...@gte.net> wrote:
> The usual atmospheric effects on images is to see them as though they
> are swimming in water, so to speak. Sometimes the image will dart
> around the field or oscillate a bit as it would when a cloud drifted
> by and blocked off the image. Actually, you can see a bright images
> in the telescope field even though it may be blocked off by clouds if
> you looked at it naked eye. So, the atmosphere is composed of water
> vapors, at times water droplets and ice crystals, so this too may
> wipe out an image. Maybe the ice crystals are the reason the images
> get real fuzzy and the image grows many time larger that it should
> be......
Sometimes the Sun can be seen comfortably by the naked eye, if viewed
through clouds of a suitable optical thickness.
If the Sun appears sharp to the naked eye, these clouds are composed
of water droplets. If you aim a telescope at the Sun at such a time,
the air is often unsteady, but sunspots can easily be seen, if they
are present.
But if the Sun appears smeared out (as seen with the naked eye),
these clouds are composed of ice crystals. If you view the Sun
through a telescope at such an occasion, the image will be so
smeared out that even large sunspots will be invisible.
--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pau...@saaf.se paul.s...@ausys.se pa...@inorbit.com
WWW: http://spitfire.ausys.se/psr -- updated daily!
Jeff Beish
We are talking about refraction integral that is dependent on
the refractive index, 1 + x, where x is the refractivity.
Refractivity is a function of wavelength and of the sate of the
atmosphere. Temperature and pressure effect the refractivity of the
atmosphere. Confusing at best. Although we usually equate the effect
of air on refraction with zenith distance a particular object is
observed at, the refractive properties of air can vary considerably
from small pockets of air to others, therefore giving the apparent
effect as though one is looking through a bunch of floating lenses.
The atmosphere usually has streams of air currents that differ in
temperature and pressure. As air decreases in temperature the density
increases, so the refractivity of that region changes. Heck, it even
gets more mumbo jumbo, jargonized as we go!
So, effectively, if the "cells" exist, refraction is a factor.
But, the question of the existance of "cells" hasn't been answered.
You are not out to lunch and are close to the answer.
Jeff
On Fri, 26 Dec 1997 23:52:52 GMT, lolk...@sfu.ca (Lance Olkovick)
wrote:
Jeff Beish
A telescopic image “swims” around in the field may be as a
result of low altitude waves of warm or cold air moving along by wind.
The experts write that you shouldn’t observe objects near the horizon
and the higher the object the better it will appear in a telescope.
Before reading their stuff I tried observing Mars around 20 degrees
off the horizon and after it rose though 30 deg, 40 deg, then higher
I found that 30 degrees altitude was a bit better than it was at 50
degrees! At times these thermal waves of air would slowly flow
through the image field, but Mars just became distorted and remained
fairly clear and details were as sharp as ever.
So, that was curious, Mars was pretty good at 30 degrees
altitude, but seemed to vary in quality as it rose. Near 70 degrees
it then became better again indicating that something was effecting
the air in my backyard is some strange way. That was before hurricane
Andrew blew all the trees away. Afterward this phenomena was not so
apparent, the “seeing” didn’t seem to change much as the object rise
in altitude. The only conclusion is that the trees were causing
turbulence at some level about my backyard and the objects were just
not seen through stable air until it rose above the wake of turbulent
air left by the trees. Trees are living matter and will out-gas
vapors, at times vapors warmer than the ambient air surrounding them.
Pine trees are worse than others, according to some literature I have
read. I can say from experience as a former glider pilot that pine
forests were great places to find “thermals” during the day light
hours.
Jeff
Jeff Beish
On 27 Dec 1997 14:54:23 +0100, pau...@electra.saaf.se (Paul Schlyter)
wrote:
>Ice crystals will indeed make your image REALLY fuzzy !!!!
>
>Sometimes the Sun can be seen comfortably by the naked eye, if viewed
>through clouds of a suitable optical thickness.
>
>If the Sun appears sharp to the naked eye, these clouds are composed
>of water droplets. If you aim a telescope at the Sun at such a time,
>the air is often unsteady, but sunspots can easily be seen, if they
>are present.
>
>But if the Sun appears smeared out (as seen with the naked eye),
>these clouds are composed of ice crystals. If you view the Sun
>through a telescope at such an occasion, the image will be so
>smeared out that even large sunspots will be invisible.
>
This, of course, is similar to using colored or neutral
density filters for observing planets. That is, if the filters that
are not marred or cracked <G>. Water droplets, i.e., hazes, clouds,
are more evenly reflects light, usually an even distribution from
droplet to droplet, whereas ice crystals mix up the reflections
because they are probably rotating, acting like spinning mirrors.
Jeff
K4MSG
Jeff said:
> telescopic image “swims” around in the field may be as a
>result of low altitude waves of warm or cold air moving along by wind. The
experts write that you shouldn’t observe objects near the horizon and the
higher the object the better it will appear in a telescope.
<Description of very interesting and worthwhile experiment snipped>
Jeff, I believe that whenever an observer is located at ground level, in
the vicinity of trees *or* near uneven terrain, and/or in the proximity of
structures (especially complex structures like houses), that "all bets are off"
in the matter of studying atmospheric effects on seeing. There is simply too
much air motion along the surface of the ground due to foliage, buildings,
unequal cooling of ground objects, etc., even when there is no apparent wind.
One effect is slowly shifting thermal gradients; ever walk down an incline
after sunset and note that it is several degrees colder at the bottom than the
top? Happens on the back acre or so of my property all the time, but
fortunately it's several hundred feet from where I observe.
A couple of solutions are (1) to wait until after midnight, when these
types of effect tend to reduce, or (2) to build an enclosed observatory which
places the telescope at least 15 feet above ground level and preferably higher.
Only then, I believe, can one collect qualitative and quantitative data on
"seeing" and be reasonably assured that the effects are atmospheric (meaning
well above the observer) and not "ground effect" (I apologize for the use of an
aeronautical term, but the words seem to fit).
Of course, observing from the peak (or near peak) of a high hill or
plateau may also mitigate such effects *provided* that there are no
updraft/downdraft/crosswind effects. "Observing and Photographing the Solar
System" recommends the lee side of a plateau which gently slopes upward from
windward to leeward, and notes that Lowell Observatory in Flagstaff, and
particularly "Mars Hill" (site of the 24-inch Clark refractor) is at the very
edge of such a mesa on the northeast (leeward) corner.
I'd like to see someone critically research the phenomenon of "seeing" in
the classical way, i.e., form a hypothesis, research all previous work, create
and conduct experiments and collect data, and then publish a *readable*
up-to-date treatise on the subject. Unfortunately, in amateur astronomy as in
many other fields these days, some people tend to jump directly to step three,
so the best that can be said about reports of such efforts is that they are
opinions based on personal observation, and may in fact have some degree of
validity even when different individuals observe different effects - but they
are *not* science, and basing any set of "general rules" on them is perilous at
best .
Such a project *might* qualify for a Master's thesis, or possibly a Ph.D
dissertation. It would also make a stunning achievement for an amateur in the
field - assuming, of course, that new discoveries were made which would stand
up to critical scientific review by professionals.
Cheers,
SSqui
> Jeff, I believe that whenever an observer is located at ground level, in
>the vicinity of trees *or* near uneven terrain, and/or in the proximity of
>structures (especially complex structures like houses), that "all bets are
>off"
>in the matter of studying atmospheric effects on seeing. There is simply too
>much air motion along the surface of the ground due to foliage, buildings,
>unequal cooling of ground objects, etc., even when there is no apparent wind.
>
Never thought of 'seeing' as being solely atmospheric. From the 35th floor of
my office building I note that when things are breezy at ground level, lights
have a noticeable shimmer and twinkle on the ground off in the distance. This
leads me to call everything, whether ground effect or the stuff way up there as
related to seeing. As I read all of these posts, there may well be
justification for some form of 'granulation' (in the 1-4" or 4-6" cell size) in
the atmosphere from 1.5-5km. Here in Denver that would be called ground
effect:-} Wish we weren't so narrow minded about what may be reality. Looking
forward to getting into some of the references cited.
-
Stewart Squires
"Islands lie beyond the sun that I shall raise ere day is done" Bilbo's Last
Song, J.R.R. Tolkien
K4MSG
My apologies, Stewart, I implied something unintended and see now that my
wording was faulty. What I meant to convey was that studies (and conclusions)
about higher-altitude "seeing" effects might be distorted by lower altitude
"seeing" effects in the vicinity of the observer *unless* care is taken to
avoid them. Certainly some of the reported observations and phenomena may be
quite valid because ground-level effects played no part, but it may have been
more by chance than by design that such was the case. So, the first step is to
*consciously* try and eliminate (or minimize, or at least understand) what the
ground-level effects are before digging into the upper level effects.
Obviously, for some observers (and I'm one of them) this isn't possible
without "hitting the road" to find a suitable site. :-(
Certainly, Denver or vicinity might be a good location to study upper
altitude "seeing" precisely because it is situated above a significant part of
the atmosphere. The same precautions apply with regard to local, low-level
(0-15 feet) air disturbances caused by trees, buildings, etc., but with those
neutralized or avoided by choice of observing location some interesting studies
of the upper atmosphere might be possible.
Regarding "narrow-minded": There is an unfortunate tendency in this day
and age to want things quantified into simple, neat little fits-it-all
explanations, or to accept the expressed opinions of someone "knowledgeable" as
absolute truth. The first tendency is simply silly, because that isn't the way
the Universe, or life, or Nature, or much of anything really is; and the second
tendency is potentially downright dangerous. And, by the way, I apply that
same statement to *published* works which are not backed up by research,
citations on the work of other persons considered reputable, etc. Any fool can
write a book and a lot of them do, but fortunately for us not usually in the
scientific genre (religion, "conspiracy theories," and political ideology seem
to have more than their share, however).
Enjoy the referenced works; bear in mind that there are others; if
something doesn't make sense, mentally challenge it and seek proof through the
referenced citations in each work; and finally, ask the opinions of others who
should know (or who claim to know) and compare their responses with documented
studies.
A lot of work, to be sure, but it is the only *real* way to approach something
akin to "absolute" truth or understanding.
Clear skies (man, I sure wish we'd get some),
Paul (in snowy northern Virginia)
Paul Schlyter
In article <19971228003...@ladder02.news.aol.com>,
SSqui <ss...@aol.com> wrote:
> Never thought of 'seeing' as being solely atmospheric. From the 35th
> floor of my office building I note that when things are breezy at
> ground level, lights have a noticeable shimmer and twinkle on the
> ground off in the distance. This leads me to call everything,
> whether ground effect or the stuff way up there as related to seeing.
neither the stars nor those ground-based lights in the distance
will twinkle.
SSqui
> My apologies, Stewart
Thanks for the clarification. My narrow minded comment was about those that
can't believe that a cellular structure as described 'might' exist. The fact
that meteorologists, and even Todd Gross (as close as they can come to an
astronomical meterologist), hadn't heard anything about them, doesn't mean that
they don't. Without having read the referenced articles I can't say that I
agree that they are there or not. My other point was that seeing is not going
to be affected solely by these possibly mid altitude cells. There is ground
effect and as Todd (or sonmeone else) pointed out, possible Jet Stream effects,
and a host of other disturbances possible. I think the whole kettle of soup is
out to get me most of the time.:-} Yet if all else is quiet and these cells (
for whatever reason) exist and are active, then the whole stopping down
argument and the 'Questar myth' might in fact be valid. As I said before,
looking forward to reading these papers and forming a conclusion.
And you raise an excellent point in that the credibility of anything published
here is open to question; it is, after all, the Internet. One would hope that
technical reference works would go through a little more scrutiny and journal
articles at least a peer review process. In any event, the reader is always
left to form his own conclusions. And as many have found, (as each and every
one of us is always trying to beat this particular problem) even a rough
application of the scientific method can lead to something that works if we are
persistent enough to continue the pursuit.
Lastly, while high altitude is nice (we have the highest continental US?
observatory at Mount Evans (+14,000'), oxygen deprivation affects thinking and
vision at that altitude and even my dark site at 11,000' suffers from
turbulence associated with the Continental Divide. You just can't win:-))
This has got to be one of the most important threads that has been posted. I
have excellent quality telescopes, but am hammered by seeing most of the time
here in Denver, along the Front Range. Anything that helps me better
understand the nature of the problem is greatly appreciated.
Martin Tom Brown
In article <4Zk3qIAy...@chrism.demon.co.uk>
ch...@chrism.demon.co.uk "Chris Marriott" writes:
> In article <67u510$e...@argentina.earthlink.net>, Fred Fuentes <fredfuent
> e...@nospam.earthlink.net> writes
> >I'm not positive, but I think the idea may come from the Russian
> >mathematician Kolmogorov. I believe he did some landmark work demonstrating
> >that turbulent flows can be described mathematically *AS IF* they consisted
> >of small cells.
>
> Are you sure that he wasn't just talking about the standard mathematical
> modelling technique of "finite element analysis"? In FEA, you model a
> physical process such as air flow by dividing it up into a "mesh"
No - I'm fairly sure it was done analytically.
Kolmogorov was a brilliant theoretician who developed analytic
theory to describe turbulent flow. Essentially his model has
large scale turbulance with energy transfer to shorter and shorter
length scales until it is finally dissipated to heat by viscousity.
He also contributed some excellent statistical analysis methods...
"Seeing cells" are a corruption of a rule of thumb derived by Fried
in the 60's which gave a sort of scale length of tolerable phase error.
The real world is not so clear cut.
The seminal work on this is still probably Tatarski 1961, 1971
"Propogation through a turbulent neutral atmosphere" I think.
Regards,
--
Martin Brown <mar...@nezumi.demon.co.uk> __ CIS: 71651,470
Scientific Software Consultancy /^,,)__/
Jeff Beish
You may have to remove the moisture from your eyes as well.
Our eye plays a role in this here "seeing" stuff :)
Jeff
On 28 Dec 1997 14:17:15 +0100, pau...@electra.saaf.se (Paul Schlyter)
wrote:
>In article <19971228003...@ladder02.news.aol.com>,
Jeff Beish
The “seeing cells” we talk about may or may not exist.
Sometimes we just accept things because it is in books, or that
everyone says it. I just never gave it much thought, just accepted
that the atmosphere is composed of small air cells to astronomers and
streams or currents of air to meteorologists. When you think about
it, how could air be broken up into cells anyway? If the atmosphere
was static or more like jelly then cells might be more acceptable.
But, it is not jelly. It is a combination of gasses, particles and
vapors. It acts like a fluid, and sometimes we use fluid mechanics to
quantify it.
The more I think about it the less likely I accept seeing
cells exist. When we see air bubbles rising in a glass of carbonated
water we see different size bobbles streaming to the top from
somewhere neat the bottom, but we don't se then form up like cells, as
one might see a honey comb or skin cells on our arm. If one shakes
the water it may foam a bit, so this may result in a large number of
interconnection bubbles that appear like cells. Granted, carbonated
water does not bubble up due to temperature changes, but I would
expect an air cell to form some kind of barrier at the edge of its
cell like air does in water. Some adhesive force in the liquid makes
air bubbles form spherical pockets within the liquid, so they appear
like cells.
The density of air changes with temperature. If it increases
the air decreases density and tends to be displaced upward by the
heavier, cooler air. This is not usually homogenous, or exactly the
same throughout a large body of air but small streams or currents of
air moving in all directions due to temperature differences. I would
think that cells is not the best choice of terms for these air
currents, so what would replace it if my contention is right?
Does any of this make sense? :)
Jeff
On 28 Dec 1997 14:01:46 GMT, ss...@aol.com (SSqui) wrote:
Brent Hutto
SSqui wrote:
> Thanks for the clarification. My narrow minded comment was about
> those that can't believe that a cellular structure as described
> 'might' exist. The fact that meteorologists, and even Todd Gross
> (as close as they can come to an astronomical meterologist), hadn't
> heard anything about them, doesn't mean that they don't. Without
> having read the referenced articles I can't say that I agree that
> they are there or not.
Well, in that sense you could just as well say that *anything* a
person might happen to imagine "might" exist. However, it is not
narrow minded at all to say (as I believe) that the existence of
"cells" in the atmosphere which
a) are said to be bounded in some way, one cell from another,
b) have a measureable or observable characteristic size and
c) act as distinct units in their effect on light passing through them
conflicts with my understanding of the most common behaviors of fluid
like air and therefore I would need to see pretty strong experimental
evidence to consider the concept anything more than fanciful.
Just because some concept (like "cells" in the atmosphere) can be
described in a way that:
a) seems consistent with certain transient atmospheric conditions and
b) seems to make intuitive sense to some people (although not to me)
does not make it any more valid a theory than saying that the flights
of angels between heaven and earth are responsible for bad "seeing"
conditions on some nights.
I have no understanding at all about the mechanisms that cause the
view through different-size apertures to vary on a given night.
But don't call me "narrow minded" just because folk wisdom or urban
legend attributes the phenomina to "cells" or "angels" and I don't
but it.
K4MSG
Brent said:
>But don't call me "narrow minded" just because folk wisdom or urban legend
attributes the phenomena to "cells" and I don't
buy it.
Brent, please don't be offended; while an unfortunate choice of words may
have been used I feel sure it was without malicious intent. This dialogue has
been, for the most part, quite informative to me personally (and to others as
well, I'm sure) and I'd like to see it continue.
However, I must take issue with your statement above about the purported
existence of "cells" being attributable to "folk wisdom" or "urban legend." As
I have tried to point out, there are numerous published references which
discuss astronomical "seeing" in these terms and which include an abundance of
citations on previously published work *which includes* scientific studies.
And, these published references were written by people considered to be highly
skilled and knowledgeable in the science of astronomy - at least, at the time
that they wrote.
Now, having said all that I am also bound to say that just because these
references exist does not *absolutely* make the phenomenon a "fact," but until
someone conducts an objective scientific inquiry and is able to
(a) Prove that the cells do not exist,
AND
(b) Demonstrate exactly what the mechanism really is,
AND
(c) Explain why the "cell theory" ever came to be gleaned from the collected
data and reported by experienced scientists in the first place,
then my contention is that statements made which refute the "cell theory",
while interesting and thought-provoking and perhaps even the impetus for more
real research, are in reality nothing more than isolated opinions with no
supporting scientific evidence. In point of fact, until the "cell theory" is
disproved these opinions are much closer to "urban legend" than the cell theory
itself.
Now, please don't take offense at what's written above; I'm merely trying
to inject scientific objectivity into the process. To be truthful, I've never
understood the "cell theory" myself and I agree that *intuitively* it seems
pretty far-fetched. But I've been a professional practicioner in science and
engineering far too long (38 years, to be precise, starting with work which
supported the manned space program in 1960) to simply dismiss a wealth of
previously published scientific data because it "doesn't make sense."
The logical first step here is to develop a comprehensive background of
where the theory came from and what "data" is available to support it, and then
apply more advanced knowledge about the nature of the atmosphere and fluid
mechanics in an attempt to explain the "data" in another way. The result may
only be a "rival theory," but it will be a start and perhaps new data
( taken objectively, of course) will point the way toward further
enlightenment.
Hmmmm, I like that last phrase; as a Freemason I'm supposed to be
constantly "seeking light" (i.e., enlightenment) anyway..... :-)
Paul
SSqui
> However, it is not
>narrow minded at all to say (as I believe) that the existence of
>"cells" in the atmosphere which
>
>a) are said to be bounded in some way, one cell from another,
>b) have a measureable or observable characteristic size and
>c) act as distinct units in their effect on light passing through them
>
>conflicts with my understanding of the most common behaviors of fluid
>like air and therefore I would need to see pretty strong experimental
>evidence to consider the concept anything more than fanciful.
>But don't call me "narrow minded" just because folk wisdom or urban
>legend attributes the phenomina to "cells" or "angels" and I don't
>but it.
Okay, you're not narrow minded. Are you from Missouri?:-}
>I have no understanding at all about the mechanisms that cause the
>view through different-size apertures to vary on a given night.
I would like to gain a better understanding of this phenomena. It seems to me
that if it doesn't fit the fluid nature of air flow or turbulence, then it's
got to be due to some other phenomena. What else is going on up there? Enough
on this subject from me until I've read a little of what's been offered. Sorry
if I offended.
Paul F. Gustafson
Paul,
No offense, but I disagree with your contention that the onus is on those
who feel the "cell theory" is nonsensical to "(a) Prove that the cells do
not exist, AND (b) Demonstrate exactly what the mechanism really is, AND (c)
Explain why the "cell theory" ever came to be gleaned from the collected
data and reported by experienced scientists in the first place."
I've presented the "cell theory" to a wide variety of experienced scientists
who are experts in the fields of fluid mechanics, meteorology, and
atmospheric turbulence as it applies to military optics. Not only have these
people never heard of atmospheric cells, they all found the idea ludicrous.
It is a "theory" that flies in the face of all that is known about the
behavior of the medium (air), and cannot be explained or supported by modern
fluid mechanics. In fact, the "theory" only exists in the field of astronomy
(but is by no means recognized by all astronomers) and is supported only by
word of mouth and by certain articles written by astronomers. It is, in
fact, the statements of those who don't believe the theory that are
supported by scientific evidence and current knowledge, while it appears
that those who support the "cell theory" really have no solid scientific
basis (or RECENT published references) for it.
Whatever causes the improved seeing of smaller scopes (the evidence for this
is also primarily anecdotal, and not all astronomers feel this actually
occurs), invoking atmospheric cells to explain what people feel they are
seeing is closer to unsubstantiated urban legend or parable than scientific
theory. If one thinks about it, the idea that you are looking through many
different "layers" of atmosphere, differing in temperature, density, and
movement both within each layer and movement of each layer relative to the
others, and the resulting turbulence at every such interface, for the cell
theory to make any sense at all, each cell would have to be a cylinder of
still air 4 to 6 inches in diameter and many miles long extending from your
telescope through all this moving "fluid" all the way into the upper
atmosphere. And if all these "tubes" extend from the upper atmosphere down
to my telescope, what about my friend's scope several miles away? Do they
all also extend down to him?
Many old theories slowly fade away as knowledge is gained, without the need
to prove them false. Since there is no support for this theory in any other
field of science, including meteorology and fluid mechanics, the burden of
proof really is on those who feel that these tubes or cells exist. First,
they would need to prove in an objective scientific manner (no anecdotes)
that smaller scopes are less affected by seeing conditions than larger
scopes OF EQUAL QUALITY. Second, they would have to prove that these "cells"
actually exist. Third, they would have to explain the mechanism by which
these cells exist and by which they differentially affect scopes of
different aperture, consistent with current knowledge of optics and fluid
mechanics. Their data would need to stand up to repeated scientific
scrutiny. Not one of these has been done.
It is an interesting discussion. One man's fact is is another man's fallacy.
BTW, my dad is an extremely active Freemason. He interviews students for
scholarships--they gave out almost $200,000 last year.
Regards and clear skies,
Paul
K4MSG wrote in message <19971229023...@ladder02.news.aol.com>...
K4MSG
Having quoted from Sidgwick ("Amateur Astronomer's Handbook" and
"Observational Astronomy for Amateurs") and Dobbins-Parker-Capen ("Observing
and Photographing the Solar System") more or less in support of the "cell
theory," it is only fair that I mention other references which do *not* discuss
"seeing" in this manner:
"A Complete Manual of Amateur Astronomy", P. Clay Sherrod - Very little is
mentioned about seeing per se, except to point out that given the variations in
seeing in any given location it is a simple fact that a smaller telescope will
be usable for detailed planetary studies more nights than a much larger
aperture.
"The Amateur Astronomer"s Handbook", James Muirden -
Rather extensive discusion of seeing and the various effects of the atmosphere
are given without *once* mentioning "cells." Very worthwhile reading.
"Observing Visual Double Stars", Paul Couteau - This one is a biggee, IMHO.
Couteau is a *professional* astronomer, and here is how he describes the
atmospheric effects of seeing on telescopic images (pp. 81-81):
"Light rays are subject to six main perturbations by the atmosphere, which
refracts, absorbs, diffuses, disperses, breaks up, and smears out the light.
Each of these contributes to the deterioration of the images and often makes
them unrecognizable and useless. In general, refraction, absorption
(extinction), and diffusion are of less importance for high-resolution
observations. Dispersion is nothing but differential refraction: it makes the
stars look like little spectra......By contrast, image motion and blurring are
responsible, in most cases, for image degradation. Images move because plane
wavefronts of random inclinations, caused by slow variations in the refractive
index of the atmosphere, pass in front of the objective. Blurring is produced
by a deformed wavefront which can no longer be considered as plane. Image
motion is not seen through large apertures, because the wave, deformed by rapid
variations in the refractive index of air, is never plane over the whole
objective. With a small aperture, well-formed images can be seen on some
nights; but they move about a mean position, whereas in a big instrument they
appear stable but blurred."
"It is the combination of image motion and blurring that is called seeing.
Mathematically, it is defined as the mean angular deviation between the light
ray and its theoretical direction. The deviation varies in a random manner
inside a small cone of very small semivertical angle (at most a few
arc-seconds). Danjon defined a scale of seeing giving the order of magnitude
of this angle as a function of the appearance of the image. When the seeing
reaches the angular value of the resolving power, the diffraction image is much
altered; its form tends to disappear, and the rings vanish. When the seeing is
considerably less than the resolving power, for example about half, the Airy
disk remains easily visible; the instrument can perform to its full capacity.
For example, an instrument of 50 cm aperture, with a resolving power of 0.24
arc-seconds, cannot tolerate seeing above 0.12 arc-seconds without the images
it forms being noticeably affected. Since the seeing is rarely less than 0.1
arc-seconds, it is obvious that large apertures are seldom able to work at full
capacity."
There is much more, but you get the drift. The book is copyright 1982 by
The MIT Press, Cambridge, MA. Since Couteau's double-star work was done using
large refractors, this a a refractor-lover's book; in fact, if you own a
first-rate refractor, apo or achromat, 3" or larger, this book has a wealth of
observational technique and insight which will prove invaluable. It's my
favorite!
Cheers,
Jeff Beish
I'm afraid this stuff, as in "seeing cells" gets into books
that are only carry overs from other books, all are probably wrong and
should be corrected. Someone should define exactly what a "seeing
cell" is. Better still, what is astronomical "seeing" anyway? I use
it, write about, and all that, but that doesn't make it right. It's
like turning to look the other way when I do include such terms in
articles, I will stop it <G>. There just has to a better way to
define how the atmosphere effects a telescopic image.
Jeff
On 27 Dec 1997 03:29:20 GMT, k4...@aol.com (K4MSG) wrote:
> Another reference which relates seeing to weather (and describes the
>controversial "cells") can be found in "Observing and Photographing the Solar
>System," by Dobbins, Parker & Capen (Willmann-Bell, 1988), section 2.13, "The
>Atmosphere and Observing," pp. 17-19.
>
> The following quotation is instructive:
>
> ".....'seeing' is caused by moving cells of air at altitudes ranging from
>several hundred feet to ten miles that have different temperatures and hence
>different indices of refraction. **At most locations these cells range in size
>from four to eight inches in diameter, although research by atmospheric
>physicists and meteorologists has revealed that they can vary tremendously in
>size ** (emphasis added). Each cell acts as a lens, changing the focal
>position of the image by bending rays of light differently."
>(p.17)
>
> FWIW,
>
Jeff Beish
Interesting Paul. So, I am probably guilty of using that term,
"seeing cell" or like terms to describe the way air effects
astronomical "seeing." Even the term "seeing" is usually written with
quotes because there is no clear definition of what that even means.
Even though I use it as most other astronomers do, it is just
ambiguous. In fact, we use a lot of terms that are ambiguous and
appear in a lot of astronomical literature as facts. Some of our
terms should show up in the myth department.
Jeff
Jeff Medkeff
I am pretty sure that "Paul F. Gustafson" <dr...@erols.com>
said the following, though I may be wrong:
>No offense,
No offense is intended in the following, either, even though
it is bound to raise someone's ire:
>I've presented the "cell theory" to a wide variety of experienced scientists
>who are experts in the fields of fluid mechanics, meteorology, and
>atmospheric turbulence as it applies to military optics. Not only have these
>people never heard of atmospheric cells, they all found the idea ludicrous.
>It is a "theory" that flies in the face of all that is known about the
>behavior of the medium (air), and cannot be explained or supported by modern
>fluid mechanics.
Personally, I don't think the participants in this thread
are paying enough attention to the thread itself, nor are
they looking at the published literature in any useful way,
and IMO they are displaying all the worst characteristics of
amateur astronomers by saying a whole bunch of things that
are not substantiated - and I think all these things are
happening on both sides of this debate. I believe a reality
check is in order, and really doubt I am the one to give it,
but here is the old college try:
A review of the literature (some of which is cited below)
will show that in astronomical seeing two principal effects
on a telescopic image may be discerned, that of deformation
of the image (with or without motion), and that of motion of
the image (without a significant degree of deformation).
[There are also other effects, such as changes in the EFL of
the telescope, etc, but these are comparatively minor in and
of themselves.]
It is shown in the literature that in small apertures motion
occurs, while in large apertures deformation occurs, all
other things being equal. At extremely small apertures (such
as the aperture of the eye), it is shown that scintillation,
or "twinkling" (rather over simply put), is an effect of
seeing conditions.
If the literature is accepted, this is completely enough to
show that identical seeing conditions effect varying
apertures in different manners, and certainly ought to close
debate on that particular question. Whether it will or not,
of course, very much remains to be seen, as I detect a
tendency in this thread for folks to demand that all the
observations be done over again to fulfil some arbitrary
criteria (of recentness, for example, as though the
atmosphere has changed appreciably in 50 years; or of
comprehensiveness, as though one single big experiment will
settle the question forever).
Visually, it is well known (and shown in the citations) that
image distortion is much more detrimental to a "clear view"
than image motion. Since image distortion is shown in the
literature to occur more easily in larger apertures (see
above), it is clear that there are grounds for amateurs who
note that their smaller scopes are "less effected" by seeing
than their larger ones (e.g., Todd Gross springs to mind).
It is also shown that the so-called "cell theory" is not a
theory at all, but originated _merely_ and _solely_ as an
analogue used to explain the net detrimental effects of
seeing on a telescopic image, and _nothing_ else. The
"theory" behind astronomical seeing does indeed have its
roots in meteorology and fluid physics, as the literature
shows. But the literature ALSO shows that the concept of
seeing cells is a useful manner of thinking about the theory
when also thinking about the difference between
(seeing-caused) deformation and motion in a telescopic
image.
It is _also_ shown in, or at least it can be inferred from,
the literature that the concept of seeing cells has been
taken far to literally by the amateur community. My personal
pet conspiracy theory is that Questar ads had much to do
with this.
>First,
>they would need to prove in an objective scientific manner (no anecdotes)
>that smaller scopes are less affected by seeing conditions than larger
>scopes OF EQUAL QUALITY.
This can easily be done, as long as you agree that image
motion is preferable to image distortion visually.
Physiologic studies have shown that most people find this to
be so, but by no means all of them have (e.g., Sue French
might be one of these, IIRC). Photographically, a completely
different set of rules exists, which can be discovered by a
review of the fundamental literature of astrometry.
>Second, they would have to prove that these "cells"
>actually exist.
Of course, they do not actually exist. However, the concept
of "cells" is probably a valid way of explaining the effects
(not the origins) of seeing as it relates to astronomical
observation. Do note that various sources (Anderson,
Ellison, Seddon) give sizes of various things in inches;
note also that the things they are giving sizes for are
_not_ cells.
>Third, they would have to explain the mechanism by which
>these cells exist and by which they differentially affect scopes of
>different aperture, consistent with current knowledge of optics and fluid
>mechanics.
The literature cited contain explanations of how seeing
differentially affects telescopes of different sizes (but of
course the literature does not show cells to exist in fact
nor does the literature provide such a mechanism). Part of
the differential effects, as far as visual stargazers are
concerned, are physiological, and you can not limit the
discussion to merely optical and fluid mechanical subjects.
If we wish to speak also of photographic effects, then we
need to include the photographic sciences in the mix.
>Their data would need to stand up to repeated scientific
>scrutiny.
And the literature has done so, AFAIK, as I have trouble
finding recent revisions of this _fundamental_, if somewhat
dated, literature. This despite the advent of huge
astronomical telescopes, adaptive optics, and so on. I
believe that in some cases the fundamental literature may
have been improved upon (especially in mathematical
quantification of the phenomena), but I do not find that any
of the observations have been thrown out nor do I discover
that any of the conclusions in the citations below have been
shown to be completely wrong.
>It is an interesting discussion.
I kind of thought it was a disgrace. There are a heck of a
lot of opinions flying around and no appearance of actual
knowledge of the literature of the field, and it even seems
to be getting to the point where people are getting offended
with each other. Some citations, with indications of the
ones I have read:
J A Anderson, Astronomical Seeing, J. Opt. Soc. Amer., 25,
No. 5, 152 *
D Brunt, Physical and Dynamical Meteorology, Cambridge
University Press, 1939
H E Butler, Observations of Stellar Scintillation, Proc. R.
Irish Acad., 54A, 321, 1952 *
H E Butler, Scintillation and Atmospheric Seeing, Irish
Astr. J., 1, No. 8, 225 *
A E Douglass, Atmosphere, Telescope, and Observer, Pop.
Astr., 5, No. 2, 46 *
A E Douglass, Scales of Seeing, Pop. Astr., 6, No. 4, 193
A E Douglass, The Study of Atmospheric Currents by the Aid
of Large Telescopes and the effect of such currents on the
Quality of the Seeing, Meterological Jrnl., USA, March 1895
* (really dated, really interesting, and easily reproducible
experiments with current amateur apertures!)
M A Ellison & H Seddon, Some Experiments into the
Scintillation of Stars and Planets, MN, 112, No. 1, 73
A T Young, Seeing and Scintillation, Sky&Tel, Sep 71,
139-150 *
In addition to these older and more fundamental references,
a number of papers have been written since expanding on the
material in the citations given above. Those interested
primarily in recent citations (for whatever reason) can find
a number of those papers here (you all know how to do a
search, I assume):
Jeff Medkeff | Check out the s.a.a. photos page at
Rockland Observatory | http://shutter.vet.ohio-state.edu/saa.htm
Sierra Vista, AZ |
Jeff Medkeff
I am pretty sure that "Paul F. Gustafson" <dr...@erols.com>
said the following, though I may be wrong:
>No offense,
No offense is intended in the following, either, even though
it is bound to raise someone's ire:
>I've presented the "cell theory" to a wide variety of experienced scientists
>who are experts in the fields of fluid mechanics, meteorology, and
>atmospheric turbulence as it applies to military optics. Not only have these
>people never heard of atmospheric cells, they all found the idea ludicrous.
>It is a "theory" that flies in the face of all that is known about the
>behavior of the medium (air), and cannot be explained or supported by modern
>fluid mechanics.
Personally, I don't think the participants in this thread
>First,
>they would need to prove in an objective scientific manner (no anecdotes)
>that smaller scopes are less affected by seeing conditions than larger
>scopes OF EQUAL QUALITY.
This can easily be done, as long as you agree that image
motion is preferable to image distortion visually.
Physiologic studies have shown that most people find this to
be so, but by no means all of them have (e.g., Sue French
might be one of these, IIRC). Photographically, a completely
different set of rules exists, which can be discovered by a
review of the fundamental literature of astrometry.
>Second, they would have to prove that these "cells"
>actually exist.
Of course, they do not actually exist. However, the concept
of "cells" is probably a valid way of explaining the effects
(not the origins) of seeing as it relates to astronomical
observation. Do note that various sources (Anderson,
Ellison, Seddon) give sizes of various things in inches;
note also that the things they are giving sizes for are
_not_ cells.
>Third, they would have to explain the mechanism by which
>these cells exist and by which they differentially affect scopes of
>different aperture, consistent with current knowledge of optics and fluid
>mechanics.
The literature cited contain explanations of how seeing
differentially affects telescopes of different sizes (but of
course the literature does not show cells to exist in fact
nor does the literature provide such a mechanism). Part of
the differential effects, as far as visual stargazers are
concerned, are physiological, and you can not limit the
discussion to merely optical and fluid mechanical subjects.
If we wish to speak also of photographic effects, then we
need to include the photographic sciences in the mix.
>Their data would need to stand up to repeated scientific
>scrutiny.
And the literature has done so, AFAIK, as I have trouble
finding recent revisions of this _fundamental_, if somewhat
dated, literature. This despite the advent of huge
astronomical telescopes, adaptive optics, and so on. I
believe that in some cases the fundamental literature may
have been improved upon (especially in mathematical
quantification of the phenomena), but I do not find that any
of the observations have been thrown out nor do I discover
that any of the conclusions in the citations below have been
shown to be completely wrong.
>It is an interesting discussion.
I kind of thought it was a disgrace. There are a heck of a
Brent Hutto
K4MSG wrote (quoting Couteau):
> "It is the combination of image motion and blurring that is
> called seeing. Mathematically, it is defined as the mean angular
> deviation between the light ray and its theoretical direction.
> The deviation varies in a random manner inside a small cone of very
> small semivertical angle (at most a few arc-seconds). Danjon
> defined a scale of seeing giving the order of magnitude of this
> angle as a function of the appearance of the image. When the seeing
> reaches the angular value of the resolving power, the diffraction
> image is much altered; its form tends to disappear, and the rings
> vanish. When the seeing is considerably less than the resolving
> power, for example about half, the Airy disk remains easily visible;
> the instrument can perform to its full capacity. For example, an
> instrument of 50 cm aperture, with a resolving power of 0.24
> arc-seconds, cannot tolerate seeing above 0.12 arc-seconds without
> the images it forms being noticeably affected. Since the seeing is
> rarely less than 0.1 arc-seconds, it is obvious that large apertures
> are seldom able to work at full capacity."
FWIW, this theory seems to me to be using concepts that are in keeping
with how I understand fluids like air to work. The advantage is that
it doesn't require the air to be segregated into heterogenous "cells"
but relies on continuously-varying bulk characteristics of the air yet
still explains the observed phenomina.
I suspect that his (Danjon's) phrase "a scale of seeing giving the
order of magnitude of this angle as a function of the appearance of
the image" is getting at the same thing as conventional wisdom would
call "cells". I am familiar with meterological articles that analyze
atmospheric motion in terms of various "scales" or "characteristic
length scales" for purposes of describing the way that patterns of
motion vary with altitude above the ground (in convective activity
mostly).
P.S., I was certainly not offended by the reference to "narrow minded"
although I see how my earlier wording may have given that impression.
I am harder than that to offend - generally it requires active effort
to do so ;-)
Jeff Beish
Sometimes you have to define the elements of a debate, such as
aperture and seeing, related elements such as "cells" and what effect,
if any, they have on seeing then if it has any relationship to
aperture and seeing. I think you are a little harsh on the
participants of this thread when It seems to be a very mild mannered
discussion, not even up to the debate level IMO yet. It seems all in
order as to the subject of the thread, seeing, aperture, a little
meteorlogy a little astronomy, no one has mentoned the "M" word, of
"C" word, and so on.
Maybe, in the tradition of this newsgroup, we should all begin
calling each other, and their mothers, names and yell, scream, and
kick at everything each participant says?
Jeff
K4MSG
Paul Gustafson said:
>No offense, but I disagree with your contention that the onus is on those who
feel the "cell theory" is nonsensical to "(a) Prove that the cells do not
exist, AND (b) Demonstrate exactly what the mechanism really is, AND (c)
Explain why the "cell theory" ever came to be gleaned from the collected data
and reported by experienced scientists in the first place."
First, let m say in my defense than I based those statements on *only*
what published information I had reviewed. I have not discussed this with
anyone before, so I have no basis for believing that reputable scientists
reject the theory. My baseline was simply what I'd read up to the time I wrote
the quoted statement.
Having checked additional references, especially Couteau (in whom I have
the most faith), I must say that I now agree with you. In fact, Couteau's book
has a diagram which, in a very simplified form, shows that the actual effect on
a large aperture is one of diverging light-ray paths, since the large aperture
is "seeing" a larger portion of the incoming (plane) wavefront which is being
distorted by atmospheric turbulence. The latter is shown as a slowly varying
"wave" which cause the incoming light wavefront to still appear plane to a
small aperture (because the small aperture is "looking" at only a relatively
small incremental portion of the "wave" and thus the lightwave can appear
roughly plane) while the larger aperture sees a non-planelight wavefront
because it is "looking" across more of the "wave." No "cells" are mentioned.
Couteau lists eight references including C. Roddier's thesis from the
University of Nice, 1976: "Etude de effets de la turbulence atmospherique sur
la formation des images astronomiques", literally "Study of the effects of
atmospheric turbulence on the formation of astronomical images."
If anyone knows how I might get a copy of that thesis, let me know; my
French is pretty rusty, but I'll "relearn" it just to read this study. Having
written a thesis myself, I know that Roddier's work no doubt contains a wealth
of valuable scientific citations as part of the "literature search" portion of
the thesis (typically chapter 2 in the U.S.) to support his study, and it is
that list that I want most of all.
Cheers,
Jeff Beish
From the chapter “The Theory of Visual Lunar and Planetary
Observations,” by: Charles H. Griffen and Clark R. Chapman in the book
_Observing the Moon, Planets, and Comets_, Clark Chapman and Dale
Cruikshank: “‘Seeing’ refers to the relative turbulence of the
atmosphere through which the planet’s light passes. If the air is
turbulent, the image is blurred and the eye cannot resolve features as
small as should be theoretically possible. Turbulence may be caused
by winds and convection high in the atmosphere or by unequal
temperatures in the immediate vicinity of the observer or within the
telescope tube.” It goes on to describe the effects on telescopic
images, etc., and how to estimate “seeing” but does not mention
anything about cells. It also goes on later about aperture and
seeing, but it has nothing more than what I had read here.
Another great reference, _Exploration of the Universe_, By:
George Abell, Chapter 12.2 (j), “Atmospheric limitations,” pp. 253.
“The earth’s atmospheric, so vital to life, is the biggest headache to
the observational astronomer.” Slipped that in to show he did have a
sense of humor. He writes some other stuff then, “Finally, the air is
unsteady, star images are blurred. In astronomical jargon, the
measure of the atmospheric stability is the ‘seeing’.” He continues
about stars twinkling, composition of air, etc., but never mentions
the “cell” thing. So, I took up reading _Elements of Meteorology_,
Miller and Thompson, and saw nothing that suggested “cells” or
anything like what we astronomers refer to as “seeing cells,” or the
other things we call them. Hum, this is not easy to pin down. There
are astronomy related books with this stuff in them, but not much out
side that circle of literature.
So, we can call our so-called seeing cells, it is astronomical
jargon meaning nothing to anyone else but us. Such a deal :) Notice
that most if not all references made to astronomical “seeing” is
enclosed by quotes. This should be the case always because “seeing”
is not defined in any dictionary that I know of..
BTW, the book referred to in the first paragraph was the ALPO
handbook for years until Dobbins, Parker, and Capen, with my help,
rewrote much of it and added a lot more information to produce
“Introduction to Observing and Photographing the Solar System.” Some
of the stuff that could be considered “for astronomers eyes only,” may
have been handed down from one to the other book.
Jeff
On Tue, 23 Dec 1997 17:11:36 -0600, stjar...@juno.com wrote:
>I understand the theoretical limits ( Dawes Limit) on the resolving power
>of telescopes of various apertures, however when the statement is made,
>"larger scopes are hurt more by poor atmospheric seeing", I am somewhat
>confused. If a large scope of say 18 inch aperture and a small scope of 5
>inch were placed side by side on a night with seeing conditions allowing
>1 arcsecond, would the smaller instrument show greater detail? Does the
>quote above merely state that a larger aperture is affected MORE because
>it can resolve down to a lower limit and since the atmosphere will only
>allow you to resolve to a certain point it is therefore hurt more than a
>smaller scope which can only reach a less impressive resolution. Anyone
>that can clear this up for me? Clear skies...
>
>-------------------==== Posted via Deja News ====-----------------------
> http://www.dejanews.com/ Search, Read, Post to Usenet
K4MSG
After studying Couteau some more in between working on a new equatorial
mount (I'm taking this week off from work) here's the way I now understand the
"seeing" issue:
If we accept Couteau's premise that "seeing" can be mathematically defined
as the mean angular deviation (in arc-seconds) between the light ray and its
theoretical direction (taking the latter to mean "if the wavefront remained
perfectly plane from source to observer"), and also accept his premise that
"acceptable" images are formed if the "seeing" is equal to one-half the
resolution of the optics in use, and assign this condition as "average" seeing,
then we may say the following:
If a hypothetical observer is using an aperture x, and the seeing is rated
as "average" through that aperture at the time in question, then the
cross-section of the atmosphere through which the observer is looking
(including the total depth, i.e., he/she is really looking through a "volume")
is representative of the incremental cross-section of the atmosphere which will
produce an angular deviation in arc-seconds equal to one-half the resolution of
a telescope of aperture x. This *could* be thought of as a sort of "cell" of
cross-section x, but one not distinct or separately bounded; it exists as part
of a more or less homogeneous whole.
Since this amount of cross-section (I will no longer refer to it as a
"cell") causes an angular deviation equal to 1/2 the resolution of aperture x,
then for any telescope larger than x the angular deviation will be a greater
percentage of the resolution, getting worse as the aperture increases. Thus,
the larger aperture suffers from "poor seeing," but it is only "poor" in a
qualitative sense *for that large aperture* if we accept the mathmatical
definition of "seeing", since a mathematical definition can be, in itself,
neither poor nor good.
This kind of approach explains why average seeing in a small scope will be
poor seeing in a large one, and excellent seeing in a small one might be
average in a large one, and so on. As the size of the cross-section of the
atmosphere necessary to cause a particular angular deviation increases (i.e.,
the atmospheric turbulence reduces), we say that the seeing "improves."
Does this make any sense?
jwinston
As more of a lurker than a participant, and as more a theoretician than
someone freezing his butt off at an eyepiece, I'm hesitant to jump in,
here, but having found this thread very interesting, I can restrain
myself no longer.
Little mention has been made of the arduous math analysis mined by the
early Russian writers up to Tatarski, and the later American authors
including (from memory) Hufnagel at Perkin Elmer in the early 60's, I
believe. Maybe late 50's. Atmospheric turbulence was originally analyzed
in terms of ensemble averages of index of refraction expressed as
correlation length. Without the detailed math, it is impossible to
express just what the hell this is supposed to mean to the man on the
street, but interestingly enough, the early literature dexcribed the
ensemble average correlation length as being in the 10-20 cm range. It
is a small step to start using correlation length for an averaging
process as if it were the dimension of something called a cell.
Extremely few of us would care to wade through the math of atmospheric
turbulence and its optical effects. It is a thicket of log-normal
statistics and bizarre coefficients. There are more effects than have
been discussed here so far. Some would make a whole other thread. Also,
there has been no mention of the temporal rate of change of the
atmosphere. What would constitute a long exposure with a CCD, for
example. I believe that the actual atmosphere in the line of sight of
our telescopes can change many times a second. Consider the wind speeds
at the tropopause. Compare this to our aperture diameters. How long
does it take for a whole new atmosphere to obstruct our LOS?
My point is only that we should hesitate more to speculate on causes,
and continue to discuss actual observational experiences. I think that
as amateurs, we could make real contributions with reports of very
careful and documented reports of seeing vs equipment and other relevant
data. Let's not get excited about what cells are, since they are an
abstract concept substituting for a lot of complex math. I think that
Paul had it right when he pointed out that that the test of any theory
is whether it usefully describes what guys can go out and measure or
see.
Its been a long time since I've opened Tatarski, so I hope that I
haven't mangled it too badly.
Winston
Jeff Medkeff
I am pretty sure that jbe...@gte.net (Jeff Beish) said the
following, though I may be wrong:
> Sometimes you have to define the elements of a debate, such as
>aperture and seeing, related elements such as "cells" and what effect,
>if any, they have on seeing then if it has any relationship to
>aperture and seeing. I think you are a little harsh on the
>participants of this thread when It seems to be a very mild mannered
>discussion,
Ok, I defer to your opinion on that. Sometimes I get
over-excited when I see people spinning their wheels.
However, folks should take a look at some of the citations I
posted and understand the distinctions I drew between "cell"
as an analogous explanation and the true _mechanism_ of
astronomical seeing.
Martin Tom Brown
> From the chapter “The Theory of Visual Lunar and Planetary
> Observations,” by: Charles H. Griffen and Clark R. Chapman in the book
> _Observing the Moon, Planets, and Comets_, Clark Chapman and Dale
> Cruikshank: “‘Seeing’ refers to the relative turbulence of the
> atmosphere through which the planet’s light passes. If the air is
> turbulent, the image is blurred and the eye cannot resolve features as
> small as should be theoretically possible. Turbulence may be caused
> by winds and convection high in the atmosphere or by unequal
> temperatures in the immediate vicinity of the observer or within the
> telescope tube.” It goes on to describe the effects on telescopic
> images, etc., and how to estimate “seeing” but does not mention
> anything about cells. It also goes on later about aperture and
> seeing, but it has nothing more than what I had read here.
Unfortunately, a lot of the books have well intentioned but
inaccurate or at least only partly true descriptions in them.
Turbulence affects the size of blobs of air with similar temperature,
and more turblence breaks things to ever smaller length scales.
Strong wind shear is generally bad news.
Since the work of Tatarski in 1961 it has been known in professional
astronomy that in the optical range phase errors are mainly due to
temperature variation along the ray path through the atmosphere.
(Water vapour and ionosphere mainly affects radio astronomers).
See for a reasonable exposition Thomson, Moran & Swenson's book
"Interferometry & Synthesis in Radio Astronomy" - despite the
title the chapter on Neutral Atmospheres is quite general.
> So, we can call our so-called seeing cells, it is astronomical
> jargon meaning nothing to anyone else but us. Such a deal :)
Yes it is a rough and ready rule of thumb based on the maximum rms
phase error you are prepared to accept over your working aperture.
I believe "seeing cells" is a corruption of the approximate scale
length of turbulence or Fried length, derived by Fried in 1965.
Turbulence and temperature variation is fractal in nature -
rather like invisible clouds. It has variations on many scales.
"Seeing-cells" are as you say a misnomer, but are still
a reasonable if rough guide as to maximum useful aperture.
K4MSG
Jeff Beish (aka "Mr. Mars") said:
>it seems to be a very mild mannered discussion, not even up to the debate
level IMO yet. It seems all in order as to the subject of the thread, seeing,
aperture, a little meteorlogy a little astronomy, no one has mentoned the "M"
word, or "C" word, and so on.
I tend to agree, Jeff. However, to give the other Jeff his due he is
correct in that the discussion has been too "unByronesque" to be called
anything like science. My latest posting about Couteau's work (the one which
says I'm reading Couteau in between working on my new mount) does apply some
simple math into the discussion, so perhaps that's more on the right track.
I now believe that the best way to view (no pun intended) the subject of
seeing is to accept Couteau's mathematical definition, and to view the entire
atmosphere between the observer and the edge of space as a fluid in motion,
which motions may be of different intensities and directions at different
altitudes. What we are interested in is the *net effect* of all this motion
(or turbulence) on what we are trying to observe, and that effect has been
defined by Couteau in terms of angular displacement of the light rays. The
greater the turbulence, the greater the net displacement over any given cross
section through which we may be looking at a given moment (and dependent solely
on the aperture of our instrument) and the poorer the seeing, becoming "bad"
when the net displacement over a cross sectional area equal to our telescope
aperture exceeds one-half the resolving power of the optics.
We can mitigate this effect to some extent by (a) elevating the observing
position above ground level to eliminate local building, tree, etc. effects;
and by (b) relocating to higher elevations to put less atmosphere between us
and the object viewed, taking care that such a move does not introduce another
"local" factor such as mountain crosswinds and updrafts/downdrafts, etc.
And if we can do neither, then we just have to live with the problem and
maybe own two telescopes (a big 'un and a small 'un) so as to maximize our
data-collecting efficiency.
Cheers,
Paul Bock
P.S. As the good Lord Byron said (in part), "When you can express it in numbers
you know something about it.....when you cannot, your knowledge is of a most
meagre and unsatisfactory kind, and is not science....."
Jeff Beish
Your proposition sounds sound. As long as you enclose the
words cell, seeing, other related terms with quotes. But, what you
say is true.
Jeff
Jeff Beish
Much of it is not very clear and seems to be off track, but it
seems to be following some progression towards an showing that there
are misconceptions and misunderstandings in the affect of aperture has
on seeing, or visa versa. You are right, some wheels are spinning but
it seems to me some discussion will clear it all up (no pun intended
<G>).
Jeff
On Mon, 29 Dec 1997 21:39:27 GMT, med...@NOBULKc2i2.comMERCIAL (Jeff
Medkeff) wrote:
>Ok, I defer to your opinion on that. Sometimes I get
>over-excited when I see people spinning their wheels.
>However, folks should take a look at some of the citations I
>posted and understand the distinctions I drew between "cell"
>as an analogous explanation and the true _mechanism_ of
>astronomical seeing.
>
>
Atiya Y. Hakeem
ty...@ug2.plk.af.mil (David Tyler) writes:
>[1] D.L. Fried, ``Optical resolution through a randomly inhomogeneous
> medium for very long and very short exposures," J.Opt.Am.Soc. 56
> (1966)
>[2] F. Roddier, ``The effects of atmospheric turbulence in optical
> astronomy, in _Progress in Optics_, E. Wolf, ed., vol. XIX, North-
> Holland, New York (1981)
>and perhaps the most comprehensive and readable of these
>[3] M.C. Roggemann and Byron Welsh, ``Imaging Through Turbulence,"
> CRC Press, New York (1996).
>______________________________________________________________________
>-David W. Tyler "It seems you feel our work is not
>-USAF Phillips Laboratory of benefit to the public."
I'm glad to see someone posting real references!
If you want proof of some kind that smaller telescopes are more likely to
have better seeing, at this the moment the UH 2.2m on Mauna Kea
where I am observing has 0.4" seeing at I, whereas Keck is around 1".
The 2.2m routinely has much better seeing than the bigger scopes like
Keck (often diffraction limited in the near-IR, i.e. 0.3").
This is mostly because the smaller mirror has fewer seeing cells across it -
a mediocre night on Keck will often be superb here, because we have fewer
random phase elements to deal with.
If people are interested, they should read about adaptive optics. Stephan
Beckers (sp?) had a very nice review article in Annual Reviews in Astronomy
and Astrophysics a few years ago.
- Jason Surace
ja...@galileo.ifa.hawaii.edu
>
Jeff Beish
I know you just slipped the tongue with "smaller telescopes
are more likely to have better seeing," cause scopes don't have
seeing, or at least without our eye <G>. The different "seeing" on
that hill you speak of is different from hill to hill. Before they
torn down the UH24 next to the UH88 the "seeing" there was better for
the most part than over at the Keck anyway.
Jeff
Jeff
On 30 Dec 1997 07:39:34 GMT, at...@alumnae.caltech.edu (Atiya Y.
Jeff Beish
One method to determine "seeing" was used by ALPO observers in
years past that has quantitative results. Some of us would endure it
one a year but not each observation period as recommended. We were to
stop our telescopes down to one inch (1") then observe a few selected
double stars and note the separation, then the same pair at full
aperture and note the separation. I forget the equation, but the
separations were used to derive angular "seeing." It took longer than
I liked so it was only done to "calibrate" my mind as to what the
"seeing" estimate was. At least three of us who lived within a few
miles of each other would compare seeing over the phone and were very
close to each other every time. We used the 0 - 10 scale, probably
close to the Perkering scale. All three of us would be within a half
point of each other, that is pretty good estimating.
Jeff
On Mon, 29 Dec 1997 15:41:41 -0800, jwinston
<jwin...@worldnet.att.net> wrote:
<snipped for your convenience>
David Randell
Hi,
I was looking through Texeraux's ('How to make a
Telescope', Willman-Bell, 2nd ed, p309-310) and Dragesco's
('High Resolution Astrophotography', CUP, p3-4) and
noted the following quantitative scale to estimate seeing based
on the work of Danjon and Couder (1935). It also gets a
mention in Sidgwick (AAH, 2nd ed, p454-455). What I like
about this scale is that it provides an absolute notion of seeing
expressed in arcseconds and is not tied to any specific
aperture, unlike some of the other scales in common use e.g.
Antoniadi. Here I have assumed the Rayleigh limit (140/Dmm)
as the baseline measure. In use one simply compares the
degree of turbulence in the Airy pattern with the description,
and then reads off the value.
a (in arcsecs) = 140/D(mm)
Scale t-value Description
I t > 1.5a Image tending towards a planetary
appearance
II t = a Strong turbulence; rings weak or
absent
III t = 0.5a Medium turbulence, diffraction rings
broken, central spot having
undulating edges
IV t = 0.25a Complete rings crossed by moving
ripples
V t =< 0.25a Perfect images without visible
distortion and little agitated
Jeff, do you know if the scale you mentioned above has been matched
against this one?
Regards,
Dave Randell
Jeff Beish
Nice post Randell. Thanks for digging that up. I use a
subjective scale for estimating "seeing" but it is from thousands of
hours at the telescope. No matter what scope I am using, 6, 12.5, or
16 inch, the estimate is the same so it takes into accont the
aperture. Different people have reported observations to me using a
variety of scales and the "arcsec" scale is understandable.
When I observe Mars it is burned into my mind the relative
size of things and when the planet fuzzes up or swells, moves about
the image it is easy to estimate how many seconds of arc the error is.
I also use a filar micrometer a lot and that too gives you the
"calibtated eye" for estimating the size of things in the field.
Jeff
K4MSG
Jeff said:
>Nice post Randell. Thanks for digging that up.
>Jeff
When Dave Randall said:
>>noted the following quantitative scale to estimate seeing based
>>on the work of Danjon and Couder (1935). It also gets a
>>mention in Sidgwick (AAH, 2nd ed, p454-455). What I like
>>about this scale is that it provides an absolute notion of seeing
>>expressed in arcseconds and is not tied to any specific
>>aperture, unlike some of the other scales in common use e.g.
>>Antoniadi. Here I have assumed the Rayleigh limit (140/Dmm)
>>as the baseline measure. In use one simply compares the
>>degree of turbulence in the Airy pattern with the description,
>>and then reads off the value.
Muirden ("The Amateur Astronomer's Handbook", p. 76) uses the same type of
scale based on the appearance of the Airy disk and diffraction rings, but it
has ten steps or levels rather than five.
The five-step approach based on Danjon seems easier to use.
FWIW, the resolution for a given aperture listed in "The Observer's
Handbook" of the Royal Astronomical Society of Canada lists 120/Dmm rather than
140/Dmm as Dave used above.
Both are less optimistic than the Dawes limit, but 140/Dmm seems overly
stringent and I cannot find a derivation for it - Dave, can you help?
Dr. Donald G. Bruns
I have started a series of simulations of what a star looks like as seen
through different apertures. I hope to post some more in January.
You can also read about adaptive optics and how it relates to seeing and
cell size. Please check out www.stellarproducts.com for my simulations.
> noted the following quantitative scale to estimate seeing based
> on the work of Danjon and Couder (1935). It also gets a
> mention in Sidgwick (AAH, 2nd ed, p454-455). What I like
> about this scale is that it provides an absolute notion of seeing
> expressed in arcseconds and is not tied to any specific
> aperture, unlike some of the other scales in common use e.g.
> Antoniadi. Here I have assumed the Rayleigh limit (140/Dmm)
> as the baseline measure. In use one simply compares the
> degree of turbulence in the Airy pattern with the description,
> and then reads off the value.
>
K4MSG
David Randall said:
> Here I have assumed the Rayleigh limit (140/Dmm)
>as the baseline measure. In use one simply compares the
>degree of turbulence in the Airy pattern with the description,
>and then reads off the value.
>
>a (in arcsecs) = 140/D(mm)
David, I answered my own question. I went back to Couteau (p. 31) and
found the relationship for the angular radius of the first dark ring outside
the Airy disk as a + 14/D with D in cm where it is assumed that the visual
wavelengths are centered at 0.55 um. This is your 140/Dmm. I confused this
with resolving power, also defined by Couteau (p.33) as follows: "The
resolving power is, by definition, 85 percent of the angular size of the first
dark ring; that is, in arc-seconds p = 0.85a = 12/D." (again, D in cm) This
is the 120/Dmm noted in the RASC Handbook and a couple of other places.
Sorry for the confusion.....
David Randell
K4MSG wrote:
>
> David Randall said:
>
> > Here I have assumed the Rayleigh limit (140/Dmm)
> >as the baseline measure. In use one simply compares the
> >degree of turbulence in the Airy pattern with the description,
> >and then reads off the value.
> >
> >a (in arcsecs) = 140/D(mm)
>
> David, I answered my own question. I went back to Couteau (p. 31) and
> found the relationship for the angular radius of the first dark ring outside
> the Airy disk as a + 14/D with D in cm where it is assumed that the visual
> wavelengths are centered at 0.55 um. This is your 140/Dmm. I confused this
> with resolving power, also defined by Couteau (p.33) as follows: "The
> resolving power is, by definition, 85 percent of the angular size of the first
> dark ring; that is, in arc-seconds p = 0.85a = 12/D." (again, D in cm) This
> is the 120/Dmm noted in the RASC Handbook and a couple of other places.
>
> Sorry for the confusion.....
>
> Paul
Hi Paul,
No problem ... The 140/Dmm value I cited comes from the Dragesco ref I
cited; it is also used by Rutten and van Venrooji as an approx
value to the Rayleigh criterion. In terms of the derivation, I
guess it comes from the value derived from the the standard
equation: i.e. 1.22*Lambda*206265/D(mm) where Lambda is
set at 555nm (green light), i.e. the radius of the Airy disc. Similarly
Rutten and van Venrooij use 117/D(mm) for Dawes limit - others use
116/D(mm) ...
Anyway, whatever the value, I find in practice that none is
absolute in practice: for example resolving xi Uma (abutting Airy discs)
with my 70mm Pronto at an estimated separation of ~1.5
arcseconds - below that defined by Dawes limit for this
aperture. No doubt this all has a bearing on the accuracy of
the estimation of seeing using the Airy pattern in the manner
suggested. Similarly, the quality of the optics and whether or
not the optical train has a central obstruction will no doubt
impose additional variables into the equation. For example, the larger
the central obstruction the brighter the Airy rings which would
have a definite bearing on the degree of turbulence one
ascribes to the Airy pattern. To what extent all this affects the
scale I mentioned, I don't know to be perfectly honest; but I like
the idea of using this absolute notion of seeing in principle if
nothing else. If anyone has other information on this, I'd
welcome any additional pointers, thanks.
Regards,
Dave Randell
PaulHBock
Dave said:
> In terms of the derivation, I
>guess it comes from the value derived from the the standard
>equation: i.e. 1.22*Lambda*206265/D(mm) where Lambda is
>set at 555nm (green light), i.e. the radius of the Airy disc.
Yes, that's what I found in Couteau; shoulda looked first. ;-)
>Anyway, whatever the value, I find in practice that none is
>absolute in practice: for example resolving xi Uma (abutting Airy discs) with
my 70mm Pronto at an estimated separation of ~1.5
>arcseconds - below that defined by Dawes limit for this
>aperture.
Right on! Bell ("The Telescope", 1981, pp 263-264) noted that Burnham
typically resolved to 0.53 of Dawes' Limit using a 6-inch refractor, and was
also well inside Dawes' Limit with a 9.4 inch instrument. But when he used
larger instruments (12, 18.5 and 36 inches) he typically fell short of Dawes'
Limit by anywhere from 15% to 60%. Bell comments,
"All observations being by the same notably skilled observer and
representing discoveries of doubles, so that no aid could have been gained by
familiarity, the issue becomes exceedingly plain that size with all its
advantages in resolving power brings serious countervailing limitations due to
atmosphere."
But Bell then goes on to point out that in the matter of light grasp,
there is no doubt where the advantage lies.
I like your use of 140/Dmm as the "standard" for use with the I-V scale
you described, and I think it's a good way to measure seeing because it has
some quantitative basis yet isn't hard to understand and use.
FWIW, Couteau says that the120/Dmm definition for resolving power
corresponds "to two Airy disks, not separated but discernible as a flattened
figure-eight."
It may be worth mentioning that the ability to discern extended objects
(i.e., planetary detail) is well above the Dawes' Limit; for example, a 3-inch
refractor will show Cassini's Division in Saturn, yet this feature subtends an
angle of about 0.5 arcseconds, roughly one-third of the "resolving power" of
that aperture using Dawes' criterion (Muirden, p. 176). To be sure, it is a
very high-contrast feature.
BTW, I changed my e-mail address for this newsgroup (and other
astronomy-related things).
Paul
Paul F. Gustafson
Thanks for the info. I like my SCT, but I LOVE my refractors. :-)
Regards,
Paul
K4MSG wrote in message <19971229054...@ladder01.news.aol.com>...
snip
>"Observing Visual Double Stars", Paul Couteau - This one is a biggee, IMHO.
>Couteau is a *professional* astronomer, and here is how he describes the
>atmospheric effects of seeing on telescopic images (pp. 81-81):
>
snip>
> There is much more, but you get the drift. The book is copyright 1982
by
>The MIT Press, Cambridge, MA. Since Couteau's double-star work was done
using
>large refractors, this a a refractor-lover's book; in fact, if you own a
>first-rate refractor, apo or achromat, 3" or larger, this book has a wealth
of
>observational technique and insight which will prove invaluable. It's my
>favorite!
>
> Cheers,
Paul F. Gustafson
Spending all day explaining sometimes complex mechanisms in my field to
people with very little knowledge in the subject, I use a variety of overly
simplistic or not-quite-accurate but descriptive examples to help them
understand what is occurring. It usually serves the purpose, but on one
occasion in which my client sat calmly through a simplistic explanation,
then revealed to me that his knowledge of the involved mechanisms is
probably better than mine, I felt like an idiot. I probably should retire
some of those terms and explanations, too. :-)
Regards,
Paul
Jeff Beish wrote in message <687vtl$8hm$1...@gte2.gte.net>...
Paul F. Gustafson
This makes sense. Thanks. A friend of mine uses telescopes (among other
things) to study the effect of atmospheric turbulence on optical systems for
a Defense Dept contractor. I'll ask him if he can track it down for you.
Regards,
Paul
K4MSG wrote in message <19971229155...@ladder02.news.aol.com>...
Dave Tandy
Obviously, this does not by any means prove the existence of similar cells in the vastly turbulent and complex atmosphere. I just thought I'd mention that cells very similar (at least in the basic concept to what we are discussing) have been created in scientific experiments. I have scanned the image and can e-mail it to anyone who wants to take a look. - Dave Tandy
Paul Schlyter wrote:
In article <882843671....@dejanews.com>, <stjar...@juno.com> wrote:
> I understand the theoretical limits ( Dawes Limit) on the resolving power
> of telescopes of various apertures, however when the statement is made,
> "larger scopes are hurt more by poor atmospheric seeing", I am somewhat
> confused. If a large scope of say 18 inch aperture and a small scope of 5
> inch were placed side by side on a night with seeing conditions allowing
> 1 arcsecond,
That's good seeing, not poor seeing!
> would the smaller instrument show greater detail? Does the quote above
> merely state that a larger aperture is affected MORE because it can
> resolve down to a lower limit and since the atmosphere will only allow
> you to resolve to a certain point it is therefore hurt more than a
> smaller scope which can only reach a less impressive resolution. Anyone
> that can clear this up for me? Clear skies...
The idea behind this is:
The typical turbulence cells in the atmosphere are some 4-6" (10-15
cm) large.If the telescope has an aperture not exceeding this, the image will
be moving back and forth, but remain fairly sharp.If the telescope's aperture is several times larger than this, it
will see through several adjacent turbulent cells at once. Thus
there will be several superimposed images, each moving back and forth
independently of the others, which results in a smeared image.Therefore the smaller scope may, under poor seeing, produce a sharper
image than the larger scope.--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pau...@saaf.se paul.s...@ausys.se pa...@inorbit.com
WWW: http://spitfire.ausys.se/psr -- updated daily!
PaulHBock
For Jeff Beish:
I found the A.L.P.O. procedure you mentioned; it's on pp. 11-12 of Julius
Benton's "The Saturn Handbook." As I understand the procedure, the first step
is to determine the observer's "personal constant" by using several double
stars on a "night of exceptional seeing", and with the aperture stopped down to
1". This "personal constant, r, is the separation in arcseconds of the closest
pair which can barely be separated.
Step 2 requires that, on a night of actual observing, the observer find
the closest double star which can be resolved using the full aperture and then
multiply that separation by the aperture in inches, yielding a value r'. This
is used along with r (as found above) to calculate the telescope efficiency, e,
as
e = r/r'
and the effective aperture, D', can be determined from
D' = (rD)/r'
where D is the telescope aperture in inches.
Now, this is no doubt very useful, but it occurs to me that in step 1 the
observer would perhaps be better served by using the methodology described by
Couteau in Chapter 4 of "Observing Visual Double Stars" where he explains in
detail how to use artificial lighting and small ball bearings to create
artificial double stars located some distance away from the observer. In his
own words (p. 89):
"You will have a stable stellar image, unaffected by seeing, that can
easily be examined comfortably, without twisting your neck. Reflections from
two lamps, side by side, will give a beautiful double star. The separation can
be varied at will, up to the limit of resolution, and even differences in
brightness can be created by moving one lamp with respect to the other."
By using the formula Couteau provides, all variables (ball bearing radius,
distance between the lamps, distance from lamps to ball bearing, and distance
from telescope objective to ball bearing) are used to define the separation of
the artificial pair in arcseconds. In his example, he uses a 4mm ball bearing,
lamps separated by 10cm and located 1m from the bearing, and an observer 100m
away, to yield a 0.2 arcsecond separation.
Obviously, a suitable "test stand" could be built to allow the "personal
constant" to be determined without regard to whether or not it was a "night of
exceptional seeing." Such a test stand could also be used to compare
telescopes of the same aperture to determine which had the better absolute
resolution. Might be a real "attention-getter" at a star party, what?
Jeff Beish
Paul,
The information you describe comes from the old Chapman and
Cruishank book ALPO used as a handbook for years. Can't remember when
they published it, but people still use the method in books. I used
the method for a while but found my estimationswere just as good.
Doing all that to find the "seeing" is a little overboard. Of course,
Julius dosen't seem to be able to let go with the old, out dated
methods.
Jeff
AndersonRM
In article <01bd1252$4ef773a0$69032399@sue-alanfrench>, "Sue and Alan French"
<Sue_and_A...@msn.com> writes:
>Rich,
Bullshit. I have a 14.5" scope. You are describing radiational
>cooling
effects.
Clear skies, Sue
Why should I believe you over Sidgwick?
-Rich
Jeff Beish
Paul,
I have Couteau’s _Observing Visual Double Stars_ and will look
through it again. It is about the only book on the subject. Maybe
someday the re-formed Double Star newsletter editor will write a book
on the subject. Our double star expert, Charles Worley, just retired
from the USNO and he only left a couple people to do the work.
The method Benton describes seems to be different than the
original version in the old ALPO handbook. But, either one is tedious
and seems to me a waste of time to repeat it more than once in a life
time. To each to his or her (P.C.) own. I prefer to use a micrometer
occasionally to derive my "personal constant" for "seeing." Like I
said, between the three of us Mars observers who lived south Florida
we could always count on being within a half a point difference in our
seeing scale from each other. Using a micrometer it is easy to
measure the expansion of an images, frequency was noted, and a value
of image error could be obtained in seconds of arc.
While at the USNO station in Miami I used selected plates from
the volumes collection of PZT film plates that were taken from 1948
throughout 1989 to determine the typical seeing there. During any
seasonal period each of the years covered by observations I would
select a plate with star trails on them and measure the oscillations
and star trail widths to determine the seeing in seconds of arc. The
parameters of the PZTs were well documented and the time was no
problem because time was what they were used for. I must have
measured 900 or a thousand plates over the years and I determined the
astronomical seeing over the station to be typically less than one
arcsec. It would be nice to report such findings, but hurricane
Andrew came and blew my logs and note across the Everglades. Maybe
some lonely Indian out on the reservation is using them for toilet
paper now <G>.
While observing a planet one can estimate seeing by knowing he
apparent diameter of the planet and how much it moves about or expands
due to turbulence. There is always a hunk of dust on the field lens
of our favorite eyepiece or a reticule micrometer eyepiece and that
can be used to gauge the size of the images. Even using the Moon the
apparent size of the dust particle can be measured, noted, and used
later as a course gauge. Observers should learn these tricks and what
the image scales of each of their eyepieces are if the intend to do
serious planetary observations.
PaulHBock
Jeff said:
>Observers should learn these tricks and what
>the image scales of each of their eyepieces are if they intend to >do serious
planetary observations.
Well, there is the crux of the matter. Some of these tricks are not
well-documented, or they're documented in several places. I wish there was a
single reference work which covered all these "good" things and also
established quantitative & qualitative standards which everyone could agree on,
but I guess that's asking too much.....
Sorry about your encounter with Andrew.....did you save your 'scopes?
Didn't Andrew also "do in" (Dr.) Jim Phillips?
Jeff Beish
On 31 Dec 1997 14:01:33 GMT, paul...@aol.com (PaulHBock) wrote:
>
> Well, there is the crux of the matter. Some of these tricks are not
>well-documented, or they're documented in several places. I wish there was a
>single reference work which covered all these "good" things and also
>established quantitative & qualitative standards which everyone could agree on,
>but I guess that's asking too much.....
>
paper for the ALPO Mars Section with these tricks, techniques, or what
ever is that they seem to be on the fringe of good observing
techniques. Several of us planetary discuss this stuff, but it is like
second nature to do them and we forget that it is not common
knowledge. A case in point, a dust particle on the field lens would
probably be in focus with the image if one see the dust and without
thinking focuses it with the image. This is the way you would
position a reticule micrometer, with both the reticule and image in
focus or the micrometer wires in a bifilar type. But, when someone
hears us suggest using a speck of dirt or dust to estimate the
apparent size of an image they may laugh. So, we just don;t think to
include stuff like that in our papers.
> Sorry about your encounter with Andrew.....did you save your 'scopes?
>Didn't Andrew also "do in" (Dr.) Jim Phillips?
>
> Paul
Hey, I remember Jim Phillips. He worked at the same hospital
as Don Parker and was member of our astronomical society. As I
remember he went of to get his Ph.D in astrophysics or the like and is
not doing work in that field. Hope he finds riches in astronomy, no
one else has <G>. Andrew did us all in, if Jim was still there in
Miami at that time. Haven't seen him in years. I ask Don but he had
bnot seen him either. Where do you know Jim from?
Jeff
jwinston
Dave Tandy wrote:
>
> I'll add my twist on this discussion. I too have read about "cells"
> is some astronomy texts (don't remember which ones). I did find a
> reference to a phenomenon called Bernard Cells in the book Frontiers
> of Complexity by Coveney and Highfield. I quote: "If a thin layer of
> silicon oil is heated carefully from below, the initial featureless
> uniformity of the liquid suddenly gives way to an array of hexagonal
> convection cells, forming a honeycomb pattern. This pattern of
> so-called Rayleigh-Bernard cells, caused by convection, can be easily
> seen if some aluminum dust is sprinkled into the oil." (pg. 155)
> Obviously, this does not by any means prove the existence of
> similar cells in the vastly turbulent and complex atmosphere. I just
> thought I'd mention that cells very similar (at least in the basic
> concept to what we are discussing) have been created in scientific
> experiments. I have scanned the image and can e-mail it to anyone who
> wants to take a look. - Dave Tandy
>
>
1. Yes, but please post it for all of us who have been following this
thread.
2. Does it strike anyone else that the "C" word is getting closer? The
discussion of convection cell effects reminds me of a similar experiment
involving chaotic behavior.
3. But really before taking this experiment seriously as an analog to
atmospheric effects, there must be more of a showing that the physics is
comparable. It is very interesting, but we are far from agreeing that
the compariosn is helpful.
Winston
PaulHBock
Winston said:
>But really before taking this experiment seriously as an analog to atmospheric
effects, there must be more of a showing that the physics is comparable. It is
very interesting, but we are far from agreeing that the comparison is helpful.
And I agree. I was initially willing to accept the "cell" theory at face
value because I found references to it in several otherwise reputable published
texts (but without a real explanation), and because I know so little about
upper atmosphere mechanisms.
Subsequent research having identified an equal or greater number of texts which
don't mention it at all, and based on some very lucid and rational explanations
both in those texts and by other posters, I now side with the "show me and then
I'll believe it" group.
Here is yet another explanation from _The Telescope_, originally
published in 1922 by Louis Bell, PhD, pp 255-256:
"The relation of air waves and such like irregularities to telescopic
vision was rather thoroughly investigated by Douglass more than twenty years
ago (Pop. Ast. 6, 193) with very interesting results. In substance, from
careful observation with telescopes from 4 inches to 24 inches aperture, he
found that the real trouble came from what one may call ripples, disturbances
from say 4 inches wave length down to 3/4 inch or less. Long waves are rare
and relatively unimportant since their general effect is to cause shifting of
the image as a whole rather than the destruction of detail which accompanies
the shorter waves."
" This rippling of the air is probably associated with the contact
displacements of air currents such as on a big cale become visible in cloud
forms. Clearly ripples, marked as they are by difference of refraction,
located in front of the telescope objective, produce different focal lengths
for different parts of the objective and render a clean and stable image quite
out of the question."
"In rough terms Douglass found that waves of greater length than half the
aperture did not materially dteriorate the image, although they did shift it as
a whole, while waves of length less than one-third the aperture did serious
mischief to the definition, the greater as the ripples were shorter, and the
image itself more minute in dimension or detail."
"Hence there are times when decreasing the aperture of an objective by a
stop improves the seeing considerably by increasing the relative length of the
air waves. Such is in fact found to be the case in practical observing,
especially when the seeing with a large aperture is decidedly poor. In other
words one may often gain more by increased steadiness than he loses by lessened
'resolving power,' the result depending somewhat on the class of observation
which chances to be under way."
FWIW.....based on what is said above about "disturbances of 4 inches wave
length", etc., I can see how someone might migrate to viewing these phenomena
as a sort of "cell," but that's a pretty loose analogy and one which has good
chance of being misunderstood and misapplied, IMHO.
Craig Kulesa
I've been following this thread with much interest, and couldn't resist
jumping in. Hope I don't make a mess out of things. :-)
Jeff M.'s summary really hits the nail on the head, IMHO. The notion
of "cells" is phenomenological -- it explains the result you see at
the telescope but is not a satisfactory physical explanation for the
atmosphere as a whole. In other words, Paul G's objections about the
atmosphere as "cellular" are well grounded.
However, looking at the atmosphere from an astronomer's point of view,
the atmosphere is simply a time-varying optical element (well, there's an
awful lot of cool physics going on too, but let's skip that for now).
Without it, plane-parallel light waves coming from an astronomical source
would still be plane-parallel when they reach your telescope's primary
mirror.
Of course, this never happens. :-) The actual wavefront has deviations
from the perfect plane wave. *The size scale of these deviations is where
the notion of "cells" arises*. A "cell" sets the physical scale for
optical wavefront distortions due to the atmosphere. Nothing more.
However, atmospheric turbulence doesn't have a *single* size scale.
Large scale changes in the wavefront (relative to the size of the
aperture) result in a "tilt" of the wavefront and a shifting of the
image. The secondary mirror can be tilted to correct for this motion, and
in fact some telescopes are being outfitted with "tip-tilt" secondaries to
account for this. A nearby "tip-tilt guide star" is monitored to provide
simultaneous real-time feedback to the secondary mirror as to which way it
is supposed to move.
That only takes out the large-scale motions though. Small, sharp
discontinuities in the wavefront (high spatial frequency) can't be removed
solely with tip-tilt. Fully deformable secondaries are being constructed
to be able to adapt to these sharp undulations in the wavefront. One such
system is being built here at the U of A. It consists of over 300
voice-coil actuators that will adjust the secondary mirror (a 2 mm thick
facesheet) 1000 times per second. It will be placed on the Multiple Mirror
Telescope after it is upgraded to a single 6.5-meter 'scope (which is
supposed to happen this spring). Pretty exciting stuff.
The physical reason for the incoming optical waveform to behave the way it
does is buried in the complicated structure of our atmosphere. Admittedly,
the study of turbulence in hydrodynamical fluids is a really tough nut to
crack. Numerical simulations seem to be our (collective) best hope right
now... but since turbulence cascades from size scales as large as the
fluid itself to the microscopic realm, numerical models must have
insanely fine resolution. Via this brute force method, even the most
massively-parallel supercomputers fall far short. This stuff has wide
astronomical importance as well -- studies of the interstellar medium,
star formation, galaxy structure... even the formation of structure in the
Universe when it was young -- these can all be modeled very effectively as
hydrodynamical fluids.
I'm sure everyone's tired of this thread, but if anyone's interested in
physically what contributes to atmospheric seeing, there's a lot of cool
stuff to talk about there too. :-)
Happy New Year!
Craig Kulesa
Steward Observatory, University of Arizona
cku...@as.arizona.edu, http://loke.as.arizona.edu/~ckulesa/
=========================================================================
Jeff Medkeff
I am pretty sure that paul...@aol.com (PaulHBock) said the
following, though I may be wrong:
> Here is yet another explanation from _The Telescope_, originally
>published in 1922 by Louis Bell, PhD, pp 255-256:
>
> "The relation of air waves and such like irregularities to telescopic
>vision was rather thoroughly investigated by Douglass more than twenty years
>ago (Pop. Ast. 6, 193) with very interesting results."
Excellent posting, Paul. Thanks for the long excerpt, it was
quite interesting. Douglass was one of the sources I cited
in my longish, somewhat irate posting to this thread, and
IIRC I mentioned that present amateur equipment was well
suited to reproducing these results. It is nice to see a
summary of his experiments.
> "In rough terms Douglass found that waves of greater length than half the
>aperture did not materially dteriorate the image, although they did shift it as
>a whole, while waves of length less than one-third the aperture did serious
>mischief to the definition, <snip>"
I want to leap in here and point out that "definition" as
used here is a technical term, the meaning of which is IMO
largely (but not wholly) lost on today's amateurs, at least
in the US. In a somewhat oversimplified explanation,
definition refers to the discernability of small extended
details, such as on the planets. OTOH, Bell and Douglass
both would have used "resolution" almost exclusively in
reference to the ability of an instrument to split double
stars.
The distinction is probably important to this discussion. My
experience suggests that one can often increase definition
by stopping down during times of poor seeing, but I have not
really noticed that effective resolution is _ever_ helped by
stopping down. Perhaps one of the more experienced double
star observers here can comment on that.
> FWIW.....based on what is said above about "disturbances of 4 inches wave
>length", etc., I can see how someone might migrate to viewing these phenomena
>as a sort of "cell," but that's a pretty loose analogy and one which has good
>chance of being misunderstood and misapplied, IMHO.
Very well put.
Jeff Medkeff
I am pretty sure that cku...@loke.as.arizona.edu (Craig
Kulesa) said the following, though I may be wrong:
>I've been following this thread with much interest, and couldn't resist
>jumping in. Hope I don't make a mess out of things. :-)
>
>Jeff M.'s summary really hits the nail on the head, IMHO. The notion
>of "cells" is phenomenological -- it explains the result you see at
>the telescope but is not a satisfactory physical explanation for the
>atmosphere as a whole.
Arrrrgh! THAT was the word I was groping for when I made
that post - 'phenomenological'. Where were you last week
when I was trying so hard to get my brain to let go of that
word? :-)
>I'm sure everyone's tired of this thread,
Actually, IMO it has taken a marked turn in interesting
directions.
>but if anyone's interested in
>physically what contributes to atmospheric seeing, there's a lot of cool
>stuff to talk about there too. :-)
>
>Happy New Year!
And to you!
>Craig Kulesa
>Steward Observatory, University of Arizona
>cku...@as.arizona.edu, http://loke.as.arizona.edu/~ckulesa/
>=========================================================================
PaulHBock
Jeff Medkeff said:
>I want to leap in here and point out that "definition" as
>used here is a technical term
>definition refers to the discernability of small extended
>details, such as on the planets. OTOH, Bell and Douglass
>both would have used "resolution" almost exclusively in
>reference to the ability of an instrument to split double
>stars.
A *very* important distinction, IMO, and well-stated. Without rushing
back to the bookshelf to cite a reference I do recall that Cassini's Division
(0.5 arcseconds) is visible in a 3" refractor which only has a resolution of
only 1.5 (using Dawes' Limit) to 1.6 (using 120/Dmm) arcseconds. This implies
the need for use of the second term, i.e., "definition", to discuss such
extended objects.
>My experience suggests that one can often increase definition
by stopping down during times of poor seeing, but I have not
really noticed that effective resolution is _ever_ helped by
stopping down. Perhaps one of the more experienced double
star observers here can comment on that.
Well, Couteau certainly never mentions it in his book; in fact, he talks
about "long nights of waiting" for the seeing to improve so that new close
pairs can be measured accurately. Obviously, if one wishes to work to the
resolution limit of the full aperture (as in splitting close pairs) than
"stopping down" is the wrong thing to do, regardless of how it might help with
planetary definition.
This has implications for quantifying seeing by using double stars to
determine how close the seeing is (in arcseconds) to one-half the aperture in
use. This methodology may reduce seeing to a number, but the value of that
number in terms of *definition* would seem to be obscure at best, implying that
perhaps a more qualitative evaluation of seeing *using the planetary object to
be viewed* as the benchmark might have more validity - which brings us back to
something like the ALPO 10-step scale for extended objects.
Jeff Beish
You really don't need super computers. Have you ever watched
the Sun set? I have never seen any "cells" in the image of the
setting Sun, only a few distortions of the atmosphere. You would
think that at the altitude the Sun is at right before it sets would be
the very place to detect them.
Astronomical "seeing" estimates will always be just that, a
subjective estimate.
Jeff
On 1 Jan 1998 21:49:39 GMT, cku...@loke.as.arizona.edu (Craig Kulesa)
wrote:
>
>I've been following this thread with much interest, and couldn't resist
>jumping in. Hope I don't make a mess out of things. :-)
>
>Jeff M.'s summary really hits the nail on the head, IMHO. The notion
>of "cells" is phenomenological -- it explains the result you see at
>the telescope but is not a satisfactory physical explanation for the
>I'm sure everyone's tired of this thread, but if anyone's interested in
>physically what contributes to atmospheric seeing, there's a lot of cool
>stuff to talk about there too. :-)
>
>Happy New Year!
>
>
Craig Kulesa
In article <slrn6arfb3....@loke.as.arizona.edu>, Craig wrote:
>Even short exposures of a star on a well-sampled CCD
>array can yield usable measurements of "seeing" by determining the
>diameter of a star's image at half the star's peak brightness.
I should add to the previous sentence that the telescope had better be
seeing-limited and not diffraction-limited. But I also said "can", so
there's a built-in cop-out. :)
Clear skies,
-Craig
Craig Kulesa
In article <68il9v$rgh$1...@gte2.gte.net>, Jeff Beish wrote:
> Have you ever watched
> the Sun set? I have never seen any "cells" in the image of the
> setting Sun, only a few distortions of the atmosphere. You would
> think that at the altitude the Sun is at right before it sets would be
> the very place to detect them.
Don't expect to see a "grid" of "cells" on the setting Sun. That's not
implied here.
Let's suppose the so-called "cell size" is 12 inches. Then... the
wavefront seen by one segment of a mirror is notably different than a
part of the mirror 12 inches away. That's ALL you know. It NOT necessarily
imply pockets of air of a certain size. It does not (in itself) tell you
WHERE the turbulence is in the atmosphere. It does NOT say that the same
wavefront pattern will be there even a tenth of a second later. It simply
describes what happens to a plane wave after traversing a messy atmosphere
at an instant of time. And it is definately NOT the same as saying that
the image of an extended object (like the setting Sun) will look
cell-like.
There's a larger problem though. Looking so close to the horizon,
you reach the confusion limit -- because your line of sight is
not only poking through dozens of miles of upper atmosphere, but also
a LOT of unstable near-ground (< 1 km) air.
Looking so low, you're essentially averaging over such a convoluted mess
that it's darn near impossible to disentangle what's physically
happening in the atmosphere. Forget "cells"... it's just plain
murky down there. :)
There's plenty enough atmospheric turbulence by just looking at points
near the zenith -- and the complications of looking through SO much crud
are greatly reduced. And then... the "typical" few arcseconds of seeing
is not *resolvable* by eye anyways. (Detectable, yes; resolvable, no.)
The best example detecting these things has already been mentioned: a star
observed in a small telescope will move around as the large scale
undulations in the wavefront sampled by the mirror change. At the same
time, a star observed in a big telescope will tend blur out since the
large mirror will sample many undulations of the wavefront at any
particular time. How big is big? And how small is small? Depends on your
site and your weather.
> Astronomical "seeing" estimates will always be just that, a
> subjective estimate.
Definately, if we're talking about eyeballing it. But "always"...??
The most notable exceptions to this would be those folks who are trying to
"undo" the effects of seeing via adaptive optics techniques. That's quite
quantitative. Even short exposures of a star on a well-sampled CCD
array can yield usable measurements of "seeing" by determining the
diameter of a star's image at half the star's peak brightness.
Craig Kulesa
Steward Observatory, University of Arizona
cku...@as.arizona.edu, http://loke.as.arizona.edu/~ckulesa/
--------------------------------------------------------------------------