Visibility of Saturn's Encke Gap?
Axel
minimum necessary to see the true Encke Gap (versus the Encke
Minimum), but I think that with Saturn's rings at their widest open
right now smaller scopes actually have more than a prayer of seeing
this elusive feature. I say this because I'm pretty sure I saw the
Encke Gap last Friday in my 8" f/6 Newt at 300x. It was visible only
on the ansae and located at the extreme edge, just before the end of
the A-ring and space. I did not notice the "Encke Minimum", but since
this feature is usually reported as being about halfway out on the
A-ring and the gap I saw was at the extreme edge, I think I saw the
real division.
Am I just delusional or has anyone else seen the Encke Gap recently?
Seeing was quite good with moments of great clarity. Cassini's was
clear all the way around the front of the planet. The Crepe Ring was
prominent at the ansae. The thin shadow of Saturn on the back side of
the rings was also evident.
Ritesh
David Knisely
> I've read a bunch of posts on s.a.a. claiming that a 10" scope is the
> minimum necessary to see the true Encke Gap (versus the Encke
> Minimum)
Well, perhaps not the precise minimum aperture, but 10 inches is a good
place to start. The division is pretty close to the outer edge of the
A-ring (around 0.5 arc seconds or so from the edge), so you would need a
telescope capable of that kind of resolution to have much of a chance to
see it. A ten would be close to that level, but an 8 inch might be just
a hair too small.
> I say this because I'm pretty sure I saw the
> Encke Gap last Friday in my 8" f/6 Newt at 300x.
> Am I just delusional or has anyone else seen the Encke Gap recently?
Well, its hard to say. At 300x, the Division would appear with a
separation of only 2.5 arc minutes from the outer edge of the A-ring to
the eye looking into the eyepiece, so its still going to be pretty
tough. I have glimsed portions at the ring ansae at 353x, but not
consistently (I usually start at around 445x and go up from there when
trying for the division). When I have seen it in my ten inch, the
seeing has always been rock steady, but even then, the division was far
from extremely obvious. The darkening at the outer edge of the A-ring
is sometimes enough to fool people, especially at powers under 350x.
Clear skies to you.
--
David W. Knisely KA0...@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
chrisachroisdk
this thin line within the outer ring? (In smaller apertures) I clearly
saw the same in good seeing (but still waiting for better) with my
CR150HD a few nights ago for the first time (and wondered what I was
seeing) I saw (repeatedly) a fleeting (3-5 seconds) but definite very
thin darker ring within ring A. At 185x with 6.4mm Meade 4000 no
filter, synchronous motor drive to old g.e.m. Was this an illusion or
was it simply the Encke Gap? Or is that too supposed to be beyond my
6" aperture? I had no previous knowledge of the existence of this
ring. Other than hearing the name Encke on the e-groups. Which I
assumed lay within one of the inner rings. I was also able to see
clearly and continuously a light grey/blue ring nearest the planet but
at a distance which I presumed (in my observing ignorance) to be the
Crepe Ring. At first I thought it was chromatic aberration.
As a visual reference I was able to see far more detail and much
sharper views than the recent webcam images of Saturn by Peter
Katreniak. That have been shown on the Yahoo Groups Telescopes UK. The
Cassini Division looked as if it were turned on a lathe and the
variations in density between the rings was very obvious. I was
beginning to see the equatorial, planetary belt split into two with
variations of colour and contrast with greater definition at the
edges. But I'm still learning to 'see' after wasting decades on
telescope making with almost no observing. I have just found the
nearest image to what I saw, on the BBC site:
http://news.bbc.co.uk/1/hi/sci/tech/1377678.stm. Which I found doing a
Google search with <images of saturns rings> while composing this post
and had never seen before. At times I saw more detail on the planet
itself than this image shows. The misty grey/blue inner ring that I
saw does not appear in this image. The minute detail within the rings
themselves (in this image) was not visible to me. I'm still looking
forward to my first night of really good seeing. Any comments?
Chris.B
EDWARD HILLYER
andrea tasselli
Seeing the Encke division with any 6" is simply beyond any physical
law, expecially at 185x. The very best you can see with any 6" is an
edge darkening of the Ring A due to the Encke Minimum at the tip of
the ansae. The image shown in the BBC site is only a weak reproduction
of the actual Hubble Saturn image, which incidentally shows quite
clearly the Crepe ring. I invite you to get a look at the original and
you'll appreciate that the details shown there quite surpass whatever
you can achieve to see in any Earth based scope, much less a CR150HD.
Clear Skies to you
Andrea T.
My Astronomy Pages at:
http://www.geocities.com/andreatax/index.htm
chrisachroisdk
> What you saw was very possible, since even a 4" scope , ZEISS APQ
> 100/1000 made it visible (see reviews at cloudynight.com)
>
Thanks Edward. That's a nice telescope. It's just a shame it's not a 6".<g>
Chris.B
Axel
> Hubble Saturn image, which incidentally shows quite clearly the Crepe ring.
As far as I can tell, the BBC image shows both the Crepe ring and the
Encke Gap quite clearly. It is pretty much what I saw in my 8" Newt
last week, though with much less detail of course.
Ritesh
Craig MacDougal
news:9ba65d5d.03010...@posting.google.com...
> As far as I can tell, the BBC image shows both the Crepe ring and the
> Encke Gap quite clearly. It is pretty much what I saw in my 8" Newt
> last week, though with much less detail of course.
My guess is that you were seeing the Encke Minimum. With my 6" I've never
thought that I could see the "true" Gap (which is called the Keeler Gap in
some circles). However, if the seeing is decent and the rings tilted enough,
I see the Minimum pretty distinctly at 215x. It looks like a thin dark line,
not a broad dimming. (Surely an illusion.)
Now here is the strange part:
sometimes it appears about midway in the A ring, but sometimes it appears
near the outer edge. This can change from one night to the next! I haven't
had the chance to systematically study the effect. It may be an illusion all
inside my skull... it may be that the exact quality of the seeing affects
it... it may even be subtle changes in the shading of the minimum..... I
don't know. I find the effect entertaining, however it's generated.
Clear Skies,
Craig in Tampa
Siegfried Gonzi
> My guess is that you were seeing the Encke Minimum. With my 6" I've never
> thought that I could see the "true" Gap (which is called the Keeler Gap in
> some circles).
Hi:
Some time ago there was an interesting article in Sky&Telescope about
seeing the Keeler Gap (note: Keeler Gap is approved by the IAU and not
just in some circles):
I posted this elsewhere on sci.astro.amateur:
==
It is a little bit more complicated. The following is based on "Beyond
the Dawes Limit: Observing Saturn's Ring Divisions", T. Dobbins and W.
Sheehan, Sky&Telescope, Nov. 2000, p. 117ff:
In 1850 Dawes reported a further division in Ring A. /He used a 6.3-inch
Fraunhofer achromat/. Keeler in 1888 (with a 36-inch refractor on the
Lick Observatory and a magnification of 1000x) confirmed this
observation in the following.
But as life goes: The aforementioned division has always been dubbed
"Encke divison", though, Encke discovered the "Encke Minimum". Even the
IAU keeps the name "Encke Division". In order to honor Keeler the 35 km
wide divison -- which has been discoverd by Voyager -- in the outer A
ring has been dubbed "Keeler Gap".
It should be possible with modern 6" telescopes to see the Encke
divison, provided the observer has Dawes eagle-eyes.
The Encke minimum should be visible in a 5-inch telescopes. A high
magnification is very importan
==
S. Gonzi
Unknown
wrote:
>Hi there Ritesh. You posted:
>> I've read a bunch of posts on s.a.a. claiming that a 10" scope is the
>> minimum necessary to see the true Encke Gap (versus the Encke
>> Minimum)
>
>Well, perhaps not the precise minimum aperture, but 10 inches is a good
>place to start. The division is pretty close to the outer edge of the
>A-ring (around 0.5 arc seconds or so from the edge), so you would need a
>telescope capable of that kind of resolution to have much of a chance to
>see it. A ten would be close to that level, but an 8 inch might be just
>a hair too small.
>
******************************************************************
An 8 inch is at the edge of seeingthe Encke gap. An 8 inch aperture
has about 0.57 arcsecond resolution. A ten inch is at 0.46 arcseconds.
One would need a very very stable atmosphere with an 8 inch scope.
james
*******************************************************************
David Knisely
> It should be possible with modern 6" telescopes to see the Encke
> divison, provided the observer has Dawes eagle-eyes.
Well, while I won't rule out a sighting of the Encke Division (the one
named such by the I.A.U. near the outermost edge of the A-ring) in an 8
inch (much more likely in something 10 inches and larger), I really have
a few problems with claims that the actual division is visible in a 6
inch. The division is, at best, only *0.5* arc seconds inside the outer
edge of the A-ring, so to resolve it from the dark of space would
require a telescope with a resolution very close to that. Using merely
the Rayleigh resolution criteria would mean that the scope would have an
aperture of 10.9 inches (Dawes would be 9.12 inches). Otherwise, the
the division's diffraction structure would tend to cause it to blend
into the darkness of the outer edge of the A-ring and thus not be
visible as a distinct division (a 6 inch would have 0.91" arc Rayleigh
resolution). Its presence might contribute to the darkening of the
outer A-ring which is shown photometrically, and thus may also
contribute to the so-called "Encke Minimum" illusion, but with a 6 inch,
the actual Encke Division will probably not be visible in a 6 inch.
Clear skies to you.
--
Tom Davis
> Well, while I won't rule out a sighting of the Encke Division (the one
> named such by the I.A.U. near the outermost edge of the A-ring) in an 8
Knisely KA0...@navix.net
David,
I'd have to agree with you. To see it with smaller
aperture (or really any size scope), requires perfect
seeing conditions. I will say it is possible, though. I
saw it with my C8, as well as a AP Starfire back in 1989
at Stellafane. The seeing was the best I had ever seen
that one night. Saturn was motionless in the eyepiece.
What I saw, though, I've never seen since. The Encke
division was an India ink, razor thin, black line at the very
outer edge of the ring. Several of us marveled at the view
at 587x in the C8. Magnification was well over 400X
on the AP refractor as well.
I can see it occassionally in my 16" Newtonian, but it
is not visible in either my 7" Mak-Cass or 6" Mak-Newt.
I can see the Encke minima in both. I had an Intes Alter
M603 Mak-Cass that would seem to show the minima
as the division. Since it was only half way between the
Cassini Division and the outer edge of the ring, I knew
it to be the minima. It did look like a sharp black line,
though. This is what I think it is this type of a view that
many mistake for the true Encke Division.
Thanks, Tom Davis
Brian Tung
course!)):
> I can see it occassionally in my 16" Newtonian, but it
> is not visible in either my 7" Mak-Cass or 6" Mak-Newt.
> I can see the Encke minima in both. I had an Intes Alter
> M603 Mak-Cass that would seem to show the minima
> as the division. Since it was only half way between the
> Cassini Division and the outer edge of the ring, I knew
> it to be the minima. It did look like a sharp black line,
> though. This is what I think it is this type of a view that
> many mistake for the true Encke Division.
I agree. I've seen the Encke Minimum lots of times, and of those times,
it appears as a fairly sharp line lots of times--sometimes near the outer
edge of the A ring, sometimes closer to the middle. What distinguishes
its appearance from that of the Encke Division is not its apparent width,
because that can vary quite widely depending on conditions. Rather, in
any small or smallish scope (less than 10 inches, surely), the Encke
Minimum will appear quite a bit darker than the Encke Division (or Gap).
One has to remember that the Encke Division is tremendously narrow, as
seen from the Earth. I don't recall the exact figure, but it's something
like 300 km, which at Saturn's distance of 1.5 billion km, only subtends
about 0.04 arcseconds. Contrast that with the FWHM of the Airy disc of
an 8-inch telescope (say), which is about 0.6 arcseconds, or about 15
times the size. For that matter, the Cassini Division itself is only
about 0.6 arcseconds across.
What this means is that although either the Encke Minimum or the Division
may appear only as narrow as the Airy disc--as sharp as any linear feature
can look through the telescope--the actual dimming seen at the eyepiece
is substantial with the Minimum (because it's much broader in fact) and
much less with the Division, because the dimming across 0.04 arcseconds
is spread out over 0.6 arcseconds. It certainly isn't going to jump out
at you.
Things are helped considerably with larger aperture, because the Airy
disc of a 16-inch telescope (such as Tom's Newtonian, above) is only half
as wide--about 0.3 arcseconds--so that the darkness of the rings is
spread out only half as much. This makes the dimming of the Division
that much more noticeable. Also, higher magnification helps, up to a
point.
Now, I know that some people will say, "Well, why can't you just let
them enjoy their observation?" I can't argue with that. But when you
post an observation to SAA, you aren't content with merely enjoying your
own observation--you clearly also want others to enjoy it too. Nothing
wrong with that, but do be prepared for some skepticism on any feature
that seems to defy ordinary limits on human visual acuity, to say nothing
of physical law.
Brian Tung <br...@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
Bill Ferris
>I've read a bunch of posts on s.a.a. claiming that a
>10" scope is the minimum necessary to see the true
<pedant mode>
These features are officially recognized by the IAU as the Encke Division and
the Encke Minima. While some refer to the division as the Encke Gap or Keeler
Gap, doing so only leads to confusion about what is being discussed. It's best
to use the official nomenclature when discussing such things.
</pedant mode>
>but I think that with Saturn's rings at their widest
>open right now smaller scopes actually have more than
>a prayer of seeing this elusive feature. I say this
>because I'm pretty sure I saw the Encke Gap last Friday
>in my 8" f/6 Newt at 300x. It was visible only on the
>ansae and located at the extreme edge, just before the
>end of the A-ring and space. I did not notice the
>"Encke Minimum", but since this feature is usually
>reported as being about halfway out on the A-ring and
>the gap I saw was at the extreme edge, I think I saw
>the real division.
The Encke Minima is a broad dusky band, offset just inside of center, within
the A-ring. It is a fairly trivial observing challenge, especially in
comparison to the Encke Division. If you did not see the Encke Minima, then I
doubt you saw the Encke Division.
The Encke Division is located about 80% of the way out in the A-ring, or about
20% in from the outer edge. If you had observed the Encke division, it wouldn't
appear "at the extreme edge" of the A-ring.
>Am I just delusional or has anyone else seen the Encke
>Gap recently?
Far from it. You are in pursuit of the truly difficult observing challenges
available to amateur astronomers. I wish you good hunting.
>Seeing was quite good with moments of great clarity.
>Cassini's was clear all the way around the front of the
>planet. The Crepe Ring was prominent at the ansae.
>The thin shadow of Saturn on the back side of the rings
>was also evident.
Your description of the seeing as "quite good" also leads me to suspect you did
not observe the Encke Division. The seeing would have to be perfect for a
10-inch aperture to have a realistic chance of detecting this feature. Unless
the image was absolutely motionless at 300X in your 8-inch for extended periods
of time, you weren't observing the division.
Those interested in some background on the Encke Division and Encke Minima
should read Jeff Medkeff's monograph, which he posted to SAA in September 1998:
http://groups.google.com/groups?q=Keeler+gap+group:sci.astro.amateur&hl=en
&lr=&ie=UTF-8&oe=UTF-8&selm=3601565a.17942600%40news.goodnet.com&rnum=6
Regards,
Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond
Bill Ferris
>Some time ago there was an interesting article in Sky&Telescope about
>seeing the Keeler Gap (note: Keeler Gap is approved by the IAU and not
>just in some circles):
Actually, official IAU nomenclature does not include, Keeler Gap, as the name
for any feature within Saturn's ring system. The division positioned about 80%
toward the outer edge of the A-ring is officially termed the Encke Division by
the IAU.
Bill Ferris
><pedant mode>
>These features are officially recognized by the IAU as the Encke Division and
>the Encke Minima. While some refer to the division as the Encke Gap or Keeler
>Gap, doing so only leads to confusion about what is being discussed. It's
>best to use the official nomenclature when discussing such things.
></pedant mode>
I love it when I get to correct myself. Re-reading Jeff Medkeff's posting from
Septebmer 1998, I see that the term, Encke Minima, does not have official
standing with the IAU. Apparently, it is a formerly provisional designation
which was not approved by the General Assembly.
Yuri
> 100/1000 made it visible (see reviews at cloudynight.com)
Encke with 4" of good brand? - Why not, but you suppost to be appr. on
Mars!
Encke could be not a problem even with naked eye, being on .... Io
!
A couple of webcom pics from the "sity with no stars" shows appr.
limit for 8" scope (noEnkce) and same planet with 10" - Encke barely
visible.
http://www.ImageHosting.com/images/Yuri/saturns.jpg
Yuri
Axel
> seen from the Earth. I don't recall the exact figure, but it's something
> like 300 km, which at Saturn's distance of 1.5 billion km, only subtends
> about 0.04 arcseconds. Contrast that with the FWHM of the Airy disc of
> an 8-inch telescope (say), which is about 0.6 arcseconds, or about 15
> times the size. For that matter, the Cassini Division itself is only
> about 0.6 arcseconds across.
Well since Cassini's is easily resolvable in even a small telescope,
the typical resolution criteria like Rayleigh etc may not apply to
high-contrast features like ring gaps. Is the 0.6" at the ansae and
with Saturn's rings at their widest open like now? Also, even if
Cassini's is only 0.6" across, from the Hubble images the Encke Gap
looks much wider than just 1/15 of the Cassini Division, so I think
Encke subtends more than 0.04". Perhaps it varies in width.
Also on the Hubble image I don't see a trace of the so-called Encke
Minimum. I only see a uniform brown shading on the A ring and the gap
itself. Now I have seen the Encke Minimum many times at 175x and
higher, but I've always seen it as a broad feature in the middle of
the A ring. I don't doubt that perhaps I was seeing the Minimum again
last week, but this time it was definitely at the extreme edge of the
ring. It did look like a thin line and was only visible in moments of
good seeing, but I can't say that it was "hairline thin".
Ritesh
Tim Povlick
Encke division, Encke Minimum, Keeler gap IAU etc etc. I have a head
ache! I've been following this Saturn ring nomenclature for years now.
If you check the Nasa JPL site on Saturn you will see there is an
Encke gap and a Keeler gap in the A ring. The keeler is so small
there is no hope for an earth based telescope to see it, unless it
has the true corrective optics. The Nasa site also says Encke
Minima is a misnomer used by amateur astronomers. According to
Price in "Observing the Planets" the Encke was seen in a 6"
refractor. One can't make the calculation of arc-seconds versus
resolving power since, as the S&T article points out the gap
has a length to it. Recall a orbital experiment where the
shuttle released about 5 miles of string behind it. Now a little
string should not be visible, but it was easily photographed.
On exceptional nights, and I mean like once in a year exceptional
I swear I've seen it come and go in a TMB-152.
I'd be glad to forward the Nasa sites, later, right now I'm too
busy scopin'.
Tim
Bill Becker
Brings up a question of mine.....is the true Encke Division more easily
obtainable by visual or "photographic" means?
Best regards,
Bill
PS: the word "easily" of course is stretching the imagination a bit.
Yuri" <yp...@attbi.com> wrote in message
news:6ea29974.03010...@posting.google.com...
jeff
absolutly perfect seeing, the kind you can wait years for. I was using an
11" refractor at 520x each time with Saturn near the meridan. Once was on
the night (the date escapes me at the moment) when Saturn occulted a seventh
mag star back in the eighties. As in the description below, it was a razor
thin line and suprisingly easy to see. I have looked for it in vain
innumerable other times. It's just one of those observational quests that
requires perfect conditions, superb optics, and a skilled observer whatever
the size of the scope. Personally, however, I would doubt that it could be
seen in anything smaller than an eight inch.
Jeff Schroeder
"Tom Davis" <tdav...@carolina.rr.com> wrote in message
news:7Z0R9.106246$uJ5.8...@twister.southeast.rr.com...
David Knisely
> According to
> Price in "Observing the Planets" the Encke was seen in a 6"
> refractor.
Price is rather dated now. I guess I had better post this again.
THE RINGS OF SATURN AND THEIR DIVISIONS
(some observational experiments)
Nearly every amateur astronomer has seen Saturn's rings, but not
all are aware of what aperture it takes to view their various details.
The rings themselves can be glimsed in binoculars as an elongation of
the image, and at about 20x, begin to show their true ring form. Even
in a small telescope at 30x, the rings are unmistakable. However, there
are a few details in the rings which can be seen with larger apertures,
such as the C, or "Crepe" ring, and the two prominent divisions, the
Cassini and Encke Divisions. The Crepe ring is the innermost and
darkest of the three main rings, and can be glimsed in fairly small
telescopes as a narrow band across the planet's disk when the ring tilt
is high. However, it often takes a 3.5 inch or larger aperture and
powers over 150x to begin to easily show that dim ring well against the
black background of space. At high tilt angles, I have seen it fairly
easily in a ten inch, although it isn't very bright. In a 12.5 inch
Portaball, the faintly greyish Crepe ring was rather obvious, hugging
the
inside of the B-ring. One problem which newer observers run into is
that with small telescopes, the darker inner band-like edge of the
B-ring is sometimes mistaken for the Crepe ring. The B-ring is the
brightest ring overall, but shows some interesting variations in
brightness across its width. The inner half appears somewhat darker,
and occasionally has shown vague patchyness along the inside of the
leading ansa, with the degree of darkness being slightly different
between the leading and following asae. The A-ring is the outer ring,
and while bright, is somewhat darker than the B-ring.
Cassini's Division, which separates rings A and B is another
subject. It is a black band somewhere around 4220 to 4500 km in overall
width with a diffuse outer edge and a 3000 km wide darker inner
segment. The division can just be glimsed in moderate apertures at
powers of around 65x if the seeing is good, but it best observed at
magnifications over 100x. It was discovered
in a 2.5 inch (63.5mm) telescope, but I often wondered exactly what
minimum aperture is required to actually view it. The division's
angular width varies from about 0.5 arc seconds near superior
conjunction to as large as 0.7 arc seconds near opposition, and its
outermost edge is at best only
2.3 arc seconds from the outer edge of the A-ring (mean opposition).
However, the division can be somewhat smaller in width than the
resolution "limit" for a telescope and still remain detectable, since
the 0.7 arc second width is only about a third of the resolving ability
of the scope which the division was first discovered in.
Three problems crop up when we talk about actually seeing the
Cassini Division. The first is familiarity. Most of us know what it
looks like and how far out it is, so we might tend to "think" we see it
when we actually don't. It would be fairer to have someone who has
never seen the rings do the experiment. The second problem is the ring
contrast between the brighter
B-ring and the somewhat darker A-ring. The A-ring is between 30% and
50% fainter than the B-ring, and has a dropoff in brightness with
distance from Saturn, mostly in its outer half. This tends to reinforce
the perception that there is a dividing line between the A and B rings,
especially at low power and smaller apertures. A third consideration is
the location of the Cassini Division. The 2.3 arc second separation
between its outer edge and the outer rim of the A-ring at mean
opposition means that, from the limits of diffraction optics, at least a
two inch aperture would probably be required to clearly resolve or
separate it from the darkness of space at the outer edge of the ring
system. At smaller apertures, the division would tend to just blend
into the darker color of the A-ring, rather than forming a distinct dark
gap. The division exists of course, but at what point does it become
visible as a curving arc, and not just as a contrast effect?
To answer this, I used my off-axis variable aperture mask I built
for my ten inch f/5.6 Newtonian to judge double star resolution. This
mask provides me with 94mm, 80mm, 70mm, 60mm, and 50mm clear apertures,
and I put on one additional mask to get 40mm and 30mm apertures. This
way, I could stop down the scope in well-defined steps to see at which
point the division would become invisible. I primarily used 176x and
141x for my tests, although I did try 235x and 101x as well. I did the
tests in October in both 1999 and 2000 on nights when seeing was better
than one arc second. I started with the widest opening 94mm (3.7
inches), which, at 141x and 176x, showed Saturn nicely and the Cassini
Division much of the way around the planet. Even the main belt across
the planet was easily visible, as well as the faint Crepe ring.
Stopping down caused a drop in the brightness and in the ease of detail
visibility, but Cassini's Division could still be seen down to 60mm,
where it was still fairly tracable along a wide arc of each ansa. At
50mm,
the actual division was becoming more difficult and was not very well
shown, detectable mainly at the bend of each ansa. The outer half of
the ring system looked somewhat darker as the division started to blend
in a bit with the A-ring. At 101x, the division was not visible at 50mm
aperture. At 40mm, I could no longer see a clear dark division between
the rings, although
the A and B-rings could still be seen as separate features with
differing brightness and borders. Interestingly enough, even the 40mm
aperture was still showing the main belt on the planet's disk. At 30mm
of aperture, the A and B rings began to merge somewhat, with no clear
signs of any division, and the only visible ring detail being a somewhat
darker outer edge. I tried the
same variable aperture sequence at 235x, but again, at 40mm, Cassini's
division was not visible. To be realistic, while 50mm may allow
"detection" of the division at high ring tilt, in general, 60mm seems to
be about the minimum to clearly and easily show Cassini's Division.
The Encke Division is a considerably more difficult target. It is
a much narrower division, located near the outer edge of the A-ring.
One problem is that again, an albedo feature causes a contrast effect
which can mimic the presence of a true division. The brightness of the
A-ring shows a peak intensity just outside the Cassini division in a
sort of brighter "ringlet". Farther out, the brightness shows a marked
fall off, beginning about halfway out from the inner edge of the A-ring
and continuing to its outer edge. This brightness fall-off combined
with the brighter inner "ringlet" can give the impression that there is
a diffuse division about in the middle of the A-ring, especially at
moderate powers where the image scale is not extremely high. Some
amateurs have referred to this illusionary "feature" as, "the Encke
Minimum", although this name is not official. Many observers prior to
the Voyager probes (including Encke himself) apparently
mistook the brightness falloff/contrast effect as a division and
repeatedly reported it nearly in the middle of the A-ring (a few even
drew 2 divisions there!). Others have had seeing cause doubling effects
which can make ring edges look like additional divisions. The Voyager
and HST images show one significant but narrow division in the A-ring
near its outer edge *not* near
the middle of that ring. That narrow gap has since been "offically"
named the Encke Division by the IAU.
The true Encke division is located about 80 percent of the way from
the outer edge of the Cassini division to the outer edge of the A-ring,
or about 133,706 km from the center of the planet. At mean opposition,
this is only 0.5 arc seconds in from the A-ring's outer edge, so if a
telescope is to be able to separate the gap from the edge of the ring,
it must have at least
this resolving power. To check on the division's true width, I took a
recent Hubble Space Telescope image of the planet as well as a
large-scale Voyager image print of the entire ring system. Using the
known diameter of the visible ring system, I came up with an image
scale, and then determined the approximate width of the division. At
most, the gap appeared to be 400 km
wide, and a few books have even indicated smaller values (325 km from
ASTROPHYSICAL DATA: Planets and Stars, K. R. Lang). In any case, using
the largest value of 400 km, and a 0.5 arc second separation from the
outer edge of the A-ring, I came to the "ball-park" estimate that it
would take a telescope of about 10 inches to have much of a chance of
seeing the division with any certainty. This conclusion is based on the
fact that, if the gap was to be resolved from the outer edge of the
A-ring, the telescope would need to have better than a 0.5 arc second
resolution, and this favors a telescope with an aperture greater than 9
inches. In smaller apertures, the division would bend into the darker
outer portions of the A-ring and would not be visible as a distinct
feature.
Bearing this in mind, in October, I began to look for the
division. I got two or three outstanding nights when it was just
visible with the ten inch at 440x as a fine low contrast and very narrow
arc near the outer edge of each ansae. One evening in particular, I
could glimse it at 353x and see it a good way around each side of the
rings at 440x. However, the fine narrow appearance of this gap and the
powers needed to get the image scale up to where the gap would be
visible makes me think that my estimate of 10 inches minimum aperture
for visiblity was fairly accurate. Magnifications of well over 300x and
very steady seeing are necessary to have much of a
chance of seeing the division and not just the illusion of the "Encke
Minimum". I have some doubts that at apertures significantly under ten
inches would allow viewing of the Encke Gap, but perhaps a quality 8 or
9 inch refractor might be able to show hints of it near maximum ring
tilt.
Siegfried Gonzi
> An 8 inch is at the edge of seeingthe Encke gap. An 8 inch aperture
> has about 0.57 arcsecond resolution. A ten inch is at 0.46 arcseconds.
> One would need a very very stable atmosphere with an 8 inch scope.
Why would you split the Cassini division in a 3" scope?
S. Gonzi
chrisachroisdk
>
> Bill Ferris
Hi,
In my own case in my observations of Saturn. I had no real
preconception of a line being there. Or having any name for a such a
thing whether Encke, Klee, Gap or Minima. These were simply names that
floated past, while browsing the groups.
I repeat, that I saw in my 6" (for a couple of hours) better images
than most of the webcam images on the groups. Including those raised
by this thread! Cassini looked as if were turned on a lathe. Jet black
and plainly visible all the way round without the least effort. I saw
(for some of the time) a thin dark line 1/2 to 3/4 of the way towards
the edge of the outer ring on the left and right. (Lack of suitable
vocabulary here)
I also saw doubling of the equatorial belt. To my eyes it seeemed
that the edges of this "split" belt held incredibly fine detail. I
have since likened this (in my mind) to a very shallow layer of water
over finely figured marble. Or paisley patterns under this same thin
layer of water. Usually disturbed but occasionaly steadying to see
"swirl" patterns on the absolute edge of visibility. This may well
have been a fabrication of my mind's eye. Trying to make sense at the
absolute limit of perceptibility. I have seen no such swirls in
browsing for Saturn images since my telescopic observation. So must
presume they do not exist in reality. The polar darkening was shaded
to perfection and there was an obvious gap between the edge of this
darkening and the equatorial belt. The moons had reduced from their
usual fuzzy appearance to minute pin-pricks, almost too small to see.
Usually they are almost too fuzzy for me to see and "spangle" with
thermal effects. Usually they also appear orange to me, but they had
become whiter. Like incredibly minute stars.
I am quite happy to accept that all of the above was simply
imagination. Or more likely tricks played by my mind on an extremely
difficult observation. I am not on a mission to convert anyone into
belief in my observations. The image WAS small but crystal clear (at
times). But we are all surely aware of the way a planet can "seem" to
grow larger under intense study.
Unfortunately the constant cloud cover since. Has precluded a chance
of another look at Saturn. Since some posters to this thread have said
that 185x is too low a power for these observations. I had better seek
advice on a suitable Barlow next. 185x with the Meade 4000 6.4 is all
I can manage without a Barlow.
I should add that I had earlier removed my Celestron No8 yellow
filter. As it was making the image in my scope very fuzzy. In
comparison with the straight eyepiece alone. I hastily add, that
usually I find the yellow filter beneficial. I again repeat my own
observation. That 'seeing' is bound up closely with chromatic
aberration in these 6" F8 scopes. The false colour all but vanishes
with good seeing and high altitude objects. In bad seeing a mass of
"blue flames" surround bright objects without the yellow filter (and
sometimes with). Small movements of the head can remove the last hint
of blue (and did) in my observations of Saturn. Under these better
seeing conditions. This is so obvious to me. That I wonder why it is
not more frequently commented upon.
I am also quite prepared to accept that what I have called (in my
ignorance of its supposed position) the "Crepe ring". (A pale
grey/blue misty/transparant ring (attached to the inside of the rings
but not reaching the planet)on left and right. It may well have been
an artifact of achromatic aberration. Though a very convincing one to
my eyes.
I know what I (thought I) saw at the telescope that night. I attempt
to convince nobody of the validity of my observations. There must
surely be a well documented history of those who saw detail. At the
limit of detectability with the human eye (Martian Canals for example)
It does not remove the pleasure in the observation. Nor the belief in
the fact that these things were clearly seen by the observer (but with
extreme difficulty and but fleetingly).
And under trying cicumstances. It was probably -8C and I was standing
in the open garden, in thick rhime frost. Only what I was seeing at
the eyepiece kept me out there for so long. I can't wait to catch
Jupiter. I have yet to see it above thirty degrees altitude and the
images are always poor here. Until at least 45 degrees altitude is
reached by an object. Saturn was very high during my observations and
I had the scope where the house wasn't in the way. Which was lucky.
Because my massive tripod was frozen solid to the ground! A few metres
to one side or the other and I wouldn't have been able to make my
observations of Saturn at all.
Finally, I now keep my scope in a unheated shed and this greatly
speeds up the ability to see quality images in my scope. The shed
thermometer showed -6C. I presume it was a couple of degrees colder
outside. I can now be observing in under a minute from making the
decision to go outside. The common habit of keeping the scope indoors
at 20+C and then taking it outside in the winter to -5C. Is bound to
lead to long cool-down times.
Best regards,
Chris.B
Yuri
> Bearing this in mind, in October, I began to look for the
> division. I got two or three outstanding nights when it was just
> visible with the ten inch at 440x as a fine low contrast and very narrow
> arc near the outer edge of each ansae. One evening in particular, I
> could glimse it at 353x and see it a good way around each side of the
> rings at 440x. However, the fine narrow appearance of this gap and the
> powers needed to get the image scale up to where the gap would be
> visible makes me think that my estimate of 10 inches minimum aperture
> for visiblity was fairly accurate. Magnifications of well over 300x and
> very steady seeing are necessary to have much of a
> chance of seeing the division and not just the illusion of the "Encke
> Minimum". I have some doubts that at apertures significantly under ten
> inches would allow viewing of the Encke Gap, but perhaps a quality 8 or
> 9 inch refractor might be able to show hints of it near maximum ring
> tilt.
> David W. Knisely KA0...@navix.net
> Prairie Astronomy Club: http://www.prairieastronomyclub.org
> Hyde Memorial Observatory: http://www.hydeobservatory.info/
Thanks David for most detailed reasonable post!
Yuri
Tim Povlick
> Tim posted:
>
>>According to
>>Price in "Observing the Planets" the Encke was seen in a 6"
>>refractor.
>
>
> Price is rather dated now.
>
Hardly. The copyright is 2000, not much has changed. The history
of observation certainly hasn't, which is what I was quoting.
> I have some doubts that at apertures significantly under ten
> inches would allow viewing of the Encke Gap, but perhaps a quality 8 or
> 9 inch refractor might be able to show hints of it near maximum ring
> tilt.
>
It can be seen in a 6".
Tim
David Knisely
> Hardly. The copyright is 2000, not much has changed. The history
> of observation certainly hasn't, which is what I was quoting.
>
I have the 1994 issue. It is "dated" in that the material of historical
reference is just that: historical and not based on recent
observations. This historical material does *not* represent what is
being seen today, nor does it use the current IAU nomenclature. The
drawings done of the planet prior to spacecraft exploration and imaging
have shown things like no divisions at all, a division in the middle of
the A-ring, and sometimes even *two* or even *three* divisions there.
However, the Voyager images taken in 1980/81 and those taken by the HST
from its first one in August of 1990 to today do *not* show prominent
divisions in the A-ring as drawn by some of the early observers
(including Encke, who also put a broad diffuse feature smack-dab in the
middle of the A-ring). All that has been shown by these images for the
past 23 years has been the narrow division near the outermost edge of
the A-ring where Saturn's tiny moon Pan orbits within the rings. The
notable observer who got things close to what is currently seen was
Keeler, who did record in a drawing a single narrow division close to
the current position of the one shown in spacecraft images using a 36
inch refractor at Lick Observatory in 1888.
The fine division near the outermost edge of the A-ring is *now*
known as the Encke Division (whether it was first observed by Encke or
not). It is a very narrow gap (less than 400 km wide) which sits 3074
km inside the outer edge of the A-ring, and is *at mean opposition* no
farther than 0.495 arc seconds inside the outermost edge of the ring.
This *requires* an aperture with that 0.495 arc second resolving ability
to separate it from the dark outermost edge of the ring. It might be
marginally-possible in a *good* 8 inch at the most favorable opposition
(a ten or twelve inch aperture would be more believable), but would not
be visible in a six. Sorry, but there is no way to make a roughly 0.5
arc second separation resolve using the 0.76 arc second resolution of a
6 inch despite the power or the quality of the instrument used. What is
being seen with smaller apertures is the "illusion" of the so-called
"Encke Minimum" and not the actual division now bearing Encke's
name.
Tim Povlick
> observations. This historical material does *not* represent what is
> being seen today, nor does it use the current IAU nomenclature. The
I think you misunderstood my response, the historic material is
just that, accurate historic material. One wouldn't use it that
as current scientific fact. On this I'm sure we agree.
> The fine division near the outermost edge of the A-ring is *now*
> known as the Encke Division (whether it was first observed by Encke or
> not). It is a very narrow gap (less than 400 km wide) which sits 3074
> km inside the outer edge of the A-ring, and is *at mean opposition* no
> farther than 0.495 arc seconds inside the outermost edge of the ring.
There is also the Keeler gap out there.
> This *requires* an aperture with that 0.495 arc second resolving ability
> to separate it from the dark outermost edge of the ring. It might be
> marginally-possible in a *good* 8 inch at the most favorable opposition
> (a ten or twelve inch aperture would be more believable), but would not
> be visible in a six. Sorry, but there is no way to make a roughly 0.5
> arc second separation resolve using the 0.76 arc second resolution of a
> 6 inch despite the power or the quality of the instrument used. What is
> being seen with smaller apertures is the "illusion" of the so-called
> "Encke Minimum" and not the actual division now bearing Encke's
> name.
I don't agree since this one is over looking the 'length' of the
structures in favor of the width. If the object were a 0.495
arc-second circle I'd agree. The S&T article on Saturn pointed
this out. Also in a previous post I mentioned the experiment NASA
did with the tether strings. A small diameter tether was dragged
behind the shuttle and was clearly visible from earth, naked eye.
It shouldn't have been. Thanks for the informtive post & 73's
Tim
eclipse
the 14th using an MN61 at 225X (8mm Radian, Meade 2X 140 Barlow, CG5 Mount)
I know I saw the Division and not the Minima. At the time, I didn't know
the division existed and had to do some research to identify what it was, a
hairline very near the outer portion of the A Ring.
The seeing on the 13th was 10/10, the best I have ever seen and Encke was
solid. On the 14th it was fading in and out on the order of every 3-4
seconds. I observe from near Georgian Bay 44 Deg Lat and I don't expect to
see it again.
Best Regards,
Richard Jordan
"Tim Povlick" <potent...@san.rr.com> wrote in message
news:IAqR9.215507$%k2.59...@twister.socal.rr.com...
chrisachroisdk
> The fine division near the outermost edge of the A-ring is *now*
> known as the Encke Division (whether it was first observed by Encke or
> not). It is a very narrow gap (less than 400 km wide) which sits 3074
> km inside the outer edge of the A-ring, and is *at mean opposition* no
> farther than 0.495 arc seconds inside the outermost edge of the ring.
> This *requires* an aperture with that 0.495 arc second resolving ability
> to separate it from the dark outermost edge of the ring. It might be
> marginally-possible in a *good* 8 inch at the most favorable opposition
> (a ten or twelve inch aperture would be more believable), but would not
> be visible in a six. Sorry, but there is no way to make a roughly 0.5
> arc second separation resolve using the 0.76 arc second resolution of a
> 6 inch despite the power or the quality of the instrument used. What is
> being seen with smaller apertures is the "illusion" of the so-called
> "Encke Minimum" and not the actual division now bearing Encke's
> name.
> --
> David W. Knisely
Well David,
You've certainly convinced me! I promise never to mention
anything I'm not supposed see, ever again. Having just spent 4 hours
at -20C (-4F) at the telescope (in 4" of snow) watching Saturn crawl
up to 55 degrees altitude. I can safely report that I had some
difficulty seeing the equatorial belt for some of the time from 7-11pm
when it finally clouded over. (Thin high cloud belts passed at
intervals all evening). The polar shading was anything but obvious.
Though Cassini's Division was easily visible all the way round at 60x,
120x and 185x. I saw no hint of the Encke gap, minima or any optical
illusions in A. Though the difference in the density of the belts A &
B was quite obvious. Jupiter was pale and uninteresting with only two
fuzzy belts and blobby spangled moons. One of which appeared from
behind Jupiter during the evening. When I gave up at 11pm, Jupiter had
reached only 30 degrees.
My only explanation for my last observation of Saturn was a truly
remarkable set of seeing conditions. In my absence of serious
observing practice over the last few years. I obviously forgot how
difficult it usually is to see these features. But when blatantly
presented with them all. I, in my naivety, assumed they were
commonplace observations (in reasonable seeing conditions)for a 6"
refractor. An instrument that I have owned for only 5 months. During
which time I have been teasing every last drop of fuzziness from
Saturn and Jupiter at all sorts of anti-social hours, with very little
success. Only briefly have I had seeing conditions that suggested the
remarkable possibilities of this telescope. Though thermal effects
were always present. Until the Saturn observation.
So my first cold winter night with the 6", viewing Saturn when it was
high in the sky. Must remain as one of those treasured memories. Until
I am (hopefully) favoured again with similar conditions. But I promise
not report it. In case it flies in the face of physical optics. <G>
Chris.B
Richard P. Johnson
Tim Povlick wrote:
>
> I don't agree since this one is over looking the 'length' of the
> structures in favor of the width. If the object were a 0.495
> arc-second circle I'd agree. The S&T article on Saturn pointed
> this out. Also in a previous post I mentioned the experiment NASA
> did with the tether strings. A small diameter tether was dragged
> behind the shuttle and was clearly visible from earth, naked eye.
> It shouldn't have been. Thanks for the informtive post & 73's
>
> Tim
I have to agree with Dave on this one. He's talking about resolving TWO lines
separated by .495", the gap and the outside edge of the ring. This is more
like seeing a sub resolution black line against a white background. Easy to
see even though well below the theoretical resolving power. This is done all
the time with lunar rilles for instance. But now add a second line parallel
to the first separated by less than the resolving power of the scope and
you'll see a broad gray feature as the two merge together. This is the
problem seeing Encke's gap with a 6" scope. It isn't that you couldn't see
the gap if it were well separated from other features. It is that it is at
best .495" from another black feature, the edge of the A ring. That is the
problem.
Also it is the rare human eye that can separate two objects like these at less
than 3 minutes of arc separation. I sure can't. At 300x Encke's division is
less than 2.5' from edge of the A ring as seen by the eye in the eyepiece.
Yet most reports of seeing it in 6" scopes cite even lower power than this.
To me this indicates they are seeing a form of the minimum not the actual
gap. Again note we are talking about separating two things not seeing one
narrow object.
The eye/brain looks for patterns it is familiar with and will often invent one
if none is seen. Many a seasoned observer has had this happen more than
once. I've easily seen this gap at the edge of the A ring in my 6" f/12 scope
then gotten out my 10" f/8 and seen that it was my brain showing me the gap
that wasn't really there, just the minimum. For some reason it looks linear
in the 6" near the edge but is nearly centered and broad in the 10" only
minutes later. I gave up on seeing the gap in the 6" long ago, I was fooled
too many times. I have seen it twice in the 10" f/8 however and it was far
closer to the edge of the A ring than it ever appeared to be in the 6" and I
thought it was close to the edge those times! Even at 500x I barely could
separate it from the edge of the A ring.
As a side note the minimum never appears linear in my 6" f/4 even though it
has excellent Optical Craftsman optics. It is always seen as a broad
feature. I'm guessing the far larger secondary is to blame, even if that does
make the airy disk smaller than it would otherwise be. It is a 35% by
diameter obstruction while the secondaries on the 6" f/12 and 10" f/8 are both
12.5%, 0.75" and 1.25" respectively.
Rick
Wayne Hoffman
wrote:
>I saw the Encke Division on the early morning of Sept.13, 2001 and again on
>the 14th using an MN61 at 225X (8mm Radian, Meade 2X 140 Barlow, CG5 Mount)
>I know I saw the Division and not the Minima.
No, Richard, you didn't. No matter what you *think* you saw, theory
says you couldn't see it, so you didn't. Period. Case closed. (:>
Wayne Hoffman
http://home.pacbell.net/w6wlr/
33° 49' 17" N, 117° 56' 40" W
"Don't Look Down"
David Knisely
> You've certainly convinced me! I promise never to mention
> anything I'm not supposed see, ever again.
Well, I wouldn't go quite that far, since there are a number of things
in amateur astronomy which some people think are impossible to see in a
given aperture, but are in reality quite possible given the right
circumstances. However, the point is that people do sometimes see
things that are not quite what they seem to be at first glance. The
Encke "Minimum" effect has repeatedly fooled a number of people
(including, apparently, Encke himself), as it can be seen in a 6 inch at
low to moderate power. It is simply a bit of a contrast effect caused
by the slightly brighter inner "ringlet" just beyond the outermost edge
of the Cassini Division, coupled with the darkness "falloff" of the
A-ring as you look closer and closer to the ring's outer edge. The
whole thing at times looks a little like there is a faint division near
the middle or just beyond the middle of the A-ring, even though there
really isn't one within range of the instrument. I was fooled a number
of times years ago by this effect, but with larger apertures and more
observing experience, I came to understand what was happening.
I have viewed Saturn in telescopes with apertures from 2 to 30
inches, but the only apertures I have seen the "IAU" Encke Division with
are those ten inches and larger. I have seen the Encke "minimum" effect
in my 8 inch f/7 Newtonian, but never the actual division itself. Even
in the ten inch on nights of superb seeing, the division was far from
outstandingly obvious, although it was visible as the exceedingly fine
long arc near the outer edge of the A-ring that it truly is. Even the
slightest seeing tremors tended to make it totally vanish, so even on
nights when Saturn looks pretty good, it can often remain invisible.
The location and narrowness of the IAU Encke Division and the physics
requirements for its separation or "resolution" from the darkness of
space at the edge of the A-ring puts limits on the smallest aperture the
division can be viewed with as well. If the division was more
separated from things, it might be visible in a smaller scope, but the
effects of diffraction will tend to make it blend into the background of
the outer A-ring and its outer edge, preventing it from being seen as a
distinct feature in something like a 6 inch.
I think that, since Keeler properly drew the division in the right
location and Encke did not, the division should have rightly been called
the Keeler Division, but that wasn't what happened (he got a rather
minor division beyond the Encke Division named after him which is far
too small to be viewed from Earth-based instruments). The old
descriptions of the Encke "division" varied widely, which hinted at the
possibility of it being somthing which was not a real physical gap. The
"nail in the coffin" to Encke's "division" was the Voyager image which
showed only the narrow tiny gap close to the edge of the A-ring as the
only significant division in the rings other than Cassini's.
> So my first cold winter night with the 6", viewing Saturn when it was
> high in the sky. Must remain as one of those treasured memories. Until
> I am (hopefully) favoured again with similar conditions. But I promise
> not report it. In case it flies in the face of physical optics.
Go ahead and report what you see. You have seen the spectacle which is
Saturn, something few of those on this Earth today have ever seen, so
you should continue to appreciate and treasure that view everytime you
get a chance to see it. Clear skies to you.
Brian Tung
> Well since Cassini's is easily resolvable in even a small telescope,
> the typical resolution criteria like Rayleigh etc may not apply to
> high-contrast features like ring gaps. Is the 0.6" at the ansae and
> with Saturn's rings at their widest open like now? Also, even if
> Cassini's is only 0.6" across, from the Hubble images the Encke Gap
> looks much wider than just 1/15 of the Cassini Division, so I think
> Encke subtends more than 0.04". Perhaps it varies in width.
I think I'm not getting my point across. I didn't mention Rayleigh at
all. That doesn't mean that the Airy disc has no effect, however.
Even if it's a linear feature, it still gets smeared by the diffraction
pattern. The thinner the division, the less "darkness" there is to
spread around, and the less pronounced the dimming at the eyepiece.
Both divisions may look equally "sharp," but the Encke Division will be
much lower in contrast than the Cassini Division.
> Also on the Hubble image I don't see a trace of the so-called Encke
> Minimum. I only see a uniform brown shading on the A ring and the gap
> itself. Now I have seen the Encke Minimum many times at 175x and
> higher, but I've always seen it as a broad feature in the middle of
> the A ring. I don't doubt that perhaps I was seeing the Minimum again
> last week, but this time it was definitely at the extreme edge of the
> ring. It did look like a thin line and was only visible in moments of
> good seeing, but I can't say that it was "hairline thin".
It's certainly hard for me to see the Minimum as broad unless I use very
high magnification.
eclipse
Thank you for your thoughtful response and thank you to the others who took
the trouble to refer to some interesting theory and although I can
understand why someone would take such a position, an unqualified rejection
of a sincerely reported observation of a difficult phenomenon isn't helpful
to anyone IMO. This is SAA however and I knew I would take some lumps.
The reason I say I did not see the Minimum is that the line I saw was very
sharply defined and very narrow, like a groove on an LP recording viewed at
a distance. This line was as close to the outer edge of the A ring as I
would think could be possible.
Best regards,
Richard Jordan
"David Knisely" <KA0...@navix.net> wrote in message
news:3E17D888...@navix.net...
TommyBoy
to see it. Would that clarify things at all? Would a thin dark line
near the outer edge of the A-ring prove anything?
Should we trust an undoctored pic but not our eyes?
Wayne Hoffman <w6wlr@REMOVE_THIS_SPAM_FILTERyahoo.com> wrote in message news:<n6af1vke0eh6mvs97...@4ax.com>...
Darren Hennig
Yes, this latest apparition of Saturn is very conducive to some amazing
observational opportunities - especially for those like you who actually
LOOK and spend less time on SAA chat rooms... ;-)
I'd love to see your report sometime - drop me a line!
Darren Hennig
Uh - wrong. Theory is just that - theory. I have been getting some of the
best lifetime views through my two refractors this fall/winter season. We're
talking conditions such as what Richard has gone through - nearly 0°F - dry
and pretty cold. Here in Canada, with those kind of cold, stable and dry air
masses aloft, the seeing and transparency would rank amoungst the world's
best - I'd bet my equipment on it! No, it's not necessarily Mauna Kea, the
Aussie outback, Arizona, Florida or Chile, but the conditions are DAMNED
fine, thank you very much. When the conditions get THAT good, it is
unbelieveable how much detail can be seen, regardless of the aperture used.
For myself, I have been getting stunning Saturn sessions myself in Alberta
this fall. I had resolved the A ring so well in my 4" refractor, that it was
seen as a grooved-like [similar to a phonograph record] pattern. Right on
the verge of seeing Encke. I don't know why this was so, but I saw this
detail on more than one evening, and even have a CCD photo taken by Mssr.
Legault aroudn 6h AFTER my report was posted on one of the Yahoo! groups
[dated, so that's proof I didn't look at the photo first].
For Jupiter, I saw Callisto cross the disk about 3 days ago, in a 4"
instrument. I also saw Io once about 2 weeks ago, in my 78mm scope, also
crossing the disk. Even with 3" of aperture, I'm seeing color variations in
the bands, turbulent wake from the GRS, the white oval in the SEB, and so
on. BTW, this is even at 122x, no less!
Io is around 1.1" across, yet I resolved it in my 78mm scope, at 122x, when
"theory" says I shouldn't be able to resolve below about 1.5". Whatever. The
feature I saw was right at my visual threshold, and I had plenty of
planetary references to use as guides. The grey-brown spot I saw under
perfect seeing moved faster than the planetary features, and after referring
my notes during the 3h session, the speck I saw was where Io should be. No,
I don't exactly know why this was exactly possible, but it WAS Io. Theory
states... to heck with the textbooks. Go out and catch a perfect night of
seeing with ANY instrument with very good to excellent optics, and then tell
me what you see. Sorry that you're not...
Some people like Richard and myself are seeing BEYOND DAWES. I don't fully
understand this yet, and I have now decided to start a Yahoo! group
dedicated to trying to unravel this type of mystery. It's posturing and
negative attitudes like this that quells people from reporting this type of
thing more, for fear of being labelled a flak or idiot or
overly-imaginative. What's really sad about this whole affair on SAA,
especially, is that fewer people actually LOOK through their scopes to catch
something wonderful, and spend more of their time knocking others' reports,
or engaging in "tech-talk". Visual observation and technique has gone, for
many, sour in light of CCD imaging, and the like.
This Yahoo! group I refer to will be by private admittance, and those
wishing to participate in it are welcomed so long as they have open-minded,
constructive contributions to make - posturing and negativity can be left at
the door.
Anyway - what do you have to say about my Io observations? Ridiculous?
Imaginative? whatever...
Darren.
Darren Hennig
I just began the exceeding Dawes group on Yahoo! I'd like to invited a few
here who appear to be the types I would like to see on this group - Richard
Johnson, Brian, and Dave Knisely all come to mind right now, amoungst
others. Those here who are interested, the link is below.
Here's the link:
http://groups.yahoo.com/group/exceedingdawes
What I hope to accomplish with this is a scientific approach to the
discussions, and a sharing of observing reports that seem to be beyond the
usual "threshold". I want to draw from Physics, physiology, psychology, and
more to try and ascertain why at times visual observers can exceed this
threshold.
Any interested parties please feel free to join in. My ulitmate goal here to
to perhaps try collectively to refine theory, and have a good set of
reference materials on the subject. I think everyone in the hobby would
benefit.
Darren Hennig.
Chief Moderator, Exceeding Dawes Group
Cover2Cover
Why not allow others the ability to see what is being said?
Don
Wayne Hoffman
<dhen...@teluaplanet.net> wrote:
>Sorry Wayne.
>
>Uh - wrong. Theory is just that - theory. I have been getting some of
the
>best lifetime views through my two refractors this fall/winter season.
<snipping>
>Anyway - what do you have to say about my Io observations? Ridiculous?
>Imaginative? whatever...
You need to re-read my message a bit closer, Darren - especially note
the "smiley." I added this in the hope that readers would realise I
was "yanking a few chains." More to the point, I *agree* with
you (and others) who maintain that Enke can be glimpsed in smaller
apertures than "theory" allows.
I have no doubt you made the observations you describe. Good on
you, and keep it up...
BTW, I tried to reply via direct E-mail but the message bounced.
David Knisely
> Uh - wrong. Theory is just that - theory.
Well, it isn't exactly "theory" as the diffraction effects which prevent
some things from being observed are very well documented. However, the
observational results do mirror what diffraction theory does tell us.
The key here is to think of the Encke Division and the outermost edge of
the A-ring as two roughly parallel lines. Diffraction effects will blur
these lines slightly, especially when their actual physical angular
width is less than the resolution of the instrument. In fact, a dark
line will have a "diffraction pattern" similar to that of a point
source, except that it is "stretched out" in one direction. This may
not necessarily result in a very narrow darker line not being visible
against a white background, but if two of these lines are sitting right
next to each other (closer than the resolution limit of the aperture),
their diffraction patterns causes them to blend together, forming a
diffuse band rather than two separate lines (see AMATEUR ASTRONOMER'S
HANDBOOK, by J.B. Sidgwick (c. 1980 Dover), p. 43-51). With the "IAU"
Encke division, this diffraction pattern interaction effect causes the
division to blend into the darkness at the edge of the A-ring unless the
aperture of the Instrument is large enough to be able to clearly
separate them. Two angular resolution limits (in arc seconds) which
might be used here are Dawes limit (4.56/D, where D is in inches), and
the Rayleigh limit (5.45/D). If we set these limits to about 0.5 arc
seconds (the mean oppositional separation of the Encke Division from the
outer edge of the A-ring), we get a required aperture of 9.12 inches
(Dawes) and 10.9 inches (Rayleigh). The exact limit is a little blurred
here, but it does indicate that a larger aperture than 6 inches would
probably be needed to actually pull out the narrow Encke Division from
the darkness at the edge of the A-ring.
> Io is around 1.1" across, yet I resolved it in my 78mm scope, at 122x, when
> "theory" says I shouldn't be able to resolve below about 1.5".
I'm not certain which theory you are referring to. The radius of the
diffraction disk of a point source like a star is *very* roughly about
half of the Rayleigh limit (or the diameter would be about equal to that
limit, which for a 4 inch is 1.36 arc seconds). For brighter objects it
would be a bit greater than this, and for fainter ones, it would be a
bit smaller. Io's angular size varies from about 1.27 arc seconds to
0.79 arc seconds, and with the approximate size of a point source
diffraction pattern, you would indeed see a "disk" of Io at a high
enough power, although it would probably be the diffraction disk which
would be giving the majority of the effect. 122x is just enough power
to see the diffraction disk of a star, so I am not surprised that you
saw a Io in the form of a disk.
> Some people like Richard and myself are seeing BEYOND DAWES.
Dawes is a limit based on double stars alone, and even then, is only
approximately valid (it isn't strictly valid for some extended detail).
There are a number of details in extended objects which *are* beyond
Dawes limit and yet are visible, so it is a more complex situation than
you might expect. A good discussion of what is visible under carefully
controlled experimental conditions can be found in AMATEUR ASTRONOMER'S
HANDBOOK, so it might be helpful to consult take a look at what
professionals have found observationally. Clear skies to you.
Thierry Legault
"Darren Hennig" <dhen...@teluaplanet.net> a écrit dans le message news:
j3ZR9.103730$Zv4.7...@news2.telusplanet.net...
> Some people like Richard and myself are seeing BEYOND DAWES. I don't fully
> understand this yet, and I have now decided to start a Yahoo! group
> dedicated to trying to unravel this type of mystery
Keep cool Darren! ;-)
Theory works, but only if it is applied within its limits :-)
Dawes was a double star observer. His law works for double
stars of equal magnitude, and *only* for that. Moreover, it
is a broad limit (finding a limit of 1" for a given telescopes
does not imply that at 0.99" the double star cannot be
separated at all and that at 1.01" the separation of obvious).
A planetary detail has nothing to see with a double star.
So the Dawes law does not works at all, and it has been
known for a long time that linear details that are much
thinner that the Dawes limit can be detected if the contrast
is sufficient. The Cassini division (0.7 arcsec wide) can be
seen through a 3" telescope even if the Dawes limit for
this aperture is over 1.5 arcsec.
Moreover, the separation of two details must not be confused
with the detection of a single detail, for example a dark or
bright spot on a planet or on the Moon. In this case, the main
parameter is the contrast of the detail. If the contrast is
sufficient, a given telescope can show 'details' of arbitrarily
small size. The best example is stars: we see them
with telescopes or even with the naked eye although their
size is a fraction of a milliarcsecond! (But in this case, can
these things be considered as 'resolved'?)
Jean Dragesco in his book High Resolution Astrophotography
devotes a chapter to this question ('the relation between the
theoretical resolving power and the visibility of fine, isolated
details') and reports many experiments about the detection
by eye of telegraph wires in front of a bright sky only
several arcsec thin.
On the other hand, diffraction effects exist and must not be
forgotten, they can sometimes be confused with real details.
In conclusion, the 'resolution' ability of a given telescope
cannot be calculated in general because each detail, with
its shape and contrast, is a particular case.
Regards
Thierry Legault
http://perso.club-internet.fr/legault/
Brian Tung
> Thank you for your thoughtful response and thank you to the others who took
> the trouble to refer to some interesting theory and although I can
> understand why someone would take such a position, an unqualified rejection
> of a sincerely reported observation of a difficult phenomenon isn't helpful
> to anyone IMO. This is SAA however and I knew I would take some lumps.
I didn't see any unqualified rejections, except for a joke one from
Wayne Hoffman.
In your report, I notice that you say you knew it was the Division and
not the Minimum. However, you don't say why. How did you establish
that it was the Division? I think you can understand that if you don't
go through your reasoning, then some skepticism is warranted.
eclipse
I have to admit that I didn't "get" Wayne's joke. I saw the smiley, but I
thought it was there to soften what might be considered to be a harsh
assessment of what I said.
I was short in my first post because my wife was calling me for supper and I
was about to be done with the Internet for the day. And to be very frank, I
thought I was going to get some extreme flack no matter what I said. I am
and was aware of the optical theory. I'm gratified to see that this thread
has generated some interesting information and am especially pleased that
Wayne was making a joke.
I say it wasn't the Minimum because the line I saw was very close to the
outer edge of the A ring and it was very sharp, like a groove of a
phonograph record held at a distance. I expect the Minimum to be further
into the A ring and to be a more diffuse feature. What I saw was, to use
that overused term, razor sharp. BTW, given my local conditions, I don't
expect to see this feature again.
I am for sure not one of the great observers (only been at it for 9 years)
and I have been wrong about what I have seen in the past (wishful thinking
and averted imagination) However, in the case of Encke, or more correctly
what I sincerely believe to have been Encke, at the time, I didn't know the
feature existed. My imagination is good, but it's not that good <G>
I heard Stephen O'Meara last summer at Starfest. He advises"believe what
you see". I take it to heart, but I only report what I saw and my
interpretation of it. This is complicated business. I think that's at
least a part of why we like it so much.
Best regards,
Richard Jordan
"Brian Tung" <br...@zot.isi.edu> wrote in message
news:ava6qu$pjg$1...@zot.isi.edu...
Darren Hennig
you, more as a general comment.
Thierry:
I woudl certainly love it if you were to join our group - I am certain that
your experience [esp. in high resolution planet work] would be of tremendous
help in this project.
Darren.
Darren Hennig
This image of Io was JUST near my visual threshold for the evening. What was
interesting is that it was pretty faint against the bright planet
background, and this would account for its non-Airy appearance at 122x.
What is even more astounding is that I had plenty of light left to see
Ganymede as either a disk-like object or with its associated Airy pattern -
well, oddly enough that night, there was NO airy pattern discernable, and
Ganymede, once it egressed from the shadow of Jupiter, was seen as a disk. I
noted that even the stars themselves in the FOV were right on the verge of
breaking into an Airy pattern, but not Ganymede [quite yet]. Amazing... This
may also explain why Io's pattern was not seen. My feelings on it are that
the diffraction ring, if available, was lost due to contrast effects against
the EQ zone's brightness.
What I did see on the disk moved relative to the planet's other features, so
I know it was NOT an atmospheric feature of the planet. Further, having the
planet, the GRS wake detail, the white SEB oval, and finally Ganymede after
egress as reference points, I was able to determine that what I saw was
indeed Io. I did this to make certain that I wasn't "imagining" things. My
notes were conferred against two programs: Meridian and my accurate SkyChart
III program. In ALL cases where I timed [synched to Atomic Time in Boulder]
the positions, Io was right there!
It was a once-in-a-decade night, but then, I'm on a roll this fall/winter
season so far!
Cheers - feel free to join the Exceeding Dawes group, if you wish.
Darren.
Bratislav
> > I have some doubts that at apertures significantly under ten
> > inches would allow viewing of the Encke Gap, but perhaps a quality 8 or
> > 9 inch refractor might be able to show hints of it near maximum ring
> > tilt.
> >
>
> It can be seen in a 6".
I'd sure appreciate a reference here, Tim.
Many people (including Thomas Back) have reported seeing it in a 6",
yet once actually seeing it in a MUCH larger scope (Thomas saw it in his
20" Zambuto Stamaster) have admitted that 6" is indeed way too small.
I have tried many times with 8" or less and NEVER suceeded. That includes
night of phenomenal seeing when Saturn remained tack sharp in my Mak at 750x.
So unless you provide a credible reference, your claim stays on a far side
of a fantasyland.
Anything less than 10", _possibly_ a high quality 9" APO, and at less than
500X, I remain deeply skeptical.
Bratislav
Stephen Paul
"eclipse" <ecl...@inetsonic.com> wrote in message
news:JaXR9.12748$i%.2500350@localhost...
> Hi David and others on this thread:
>
> Thank you for your thoughtful response and thank you to the others who
took
> the trouble to refer to some interesting theory and although I can
> understand why someone would take such a position, an unqualified
rejection
> of a sincerely reported observation of a difficult phenomenon isn't
helpful
> to anyone IMO. This is SAA however and I knew I would take some lumps.
>
> The reason I say I did not see the Minimum is that the line I saw was very
> sharply defined and very narrow, like a groove on an LP recording viewed
at
> a distance. This line was as close to the outer edge of the A ring as I
> would think could be possible.
>
> Best regards,
>
> Richard Jordan
Hi,
Without knowledge of its actual appearance as seen in photographs, I once
believed I was seeing the Encke division. What I saw were two Cassini "like"
divisions. One of course was Cassini, the other a mystery at the outer edge
of the A ring, which turns out to be too large to be the Encke division.
Some time has past and I have made other observations that again revealed
this "second" Cassini like feature.
Here are some thoughts I have had about this "feature" of late.
In photographs one can plainly see that the A ring has two predominantly
high contrast features, the Encke division, and a narrow white ring just
outside the Cassini division (the area between which one presumably finds
the Encke "minima"). If one assumes that the Encke gap provides an outer
edge on the area of the minima, and the contrast change at the white ring an
inner edge, then possibly using enough aperture to create very bright
images, this darker area of the A ring might be the source of the illusion
of a second Cassini like feature.
I am uncertain if this is what is meant by "seeing the Encke minima". What I
see, appears to my eye as dark as the Cassini division. Are those who
suggest that we are seeing the minima, suggesting the minima appears as a
high contrast black gap?
Stephen Paul
Shirley, MA
Richard P. Johnson
Brian Tung wrote:
> Richard Jordan wrote:
>
>
> In your report, I notice that you say you knew it was the Division and
> not the Minimum. However, you don't say why. How did you establish
> that it was the Division? I think you can understand that if you don't
> go through your reasoning, then some skepticism is warranted.
>
> Brian Tung <br...@isi.edu>
No way I can say for absolute certainty that was what I saw. I'd have to pick up
the scope, put it and me in a rocket while looking through the eyepiece and
travel a lot closer looking the whole time <g> My word choice was poor, sorry.
What I was seeing was so very very different from the minimum, so much finer, so
much closer to the edge than I'd ever seen the minimum it appeared as something
very different. The first time my brain goes "Oh that's the real gap."
Unfortunately, Lowell probably said the same thing about a few "canals". Also I
could see a bright inner edge to the A ring followed by a broad lower albedo band
that held pretty constant right up to the very fine gap I'm thinking likely was
the true gap. I've used this scope on Saturn 40 years now and only saw
conditions sufficient for this with the rings at near max tilt twice in those
years. I see a minimum type effect that is quite constant in the 10 f/8 scope
whenever seeing is well above normal and the rings are tilted well for this and
it is quite a constant effect of a broad dimmer albedo ring which can double at
times but never has a fine black ring developed in this area. In the 6" f/4 I on
similar nights at 300x (400x - 500x in the 10") the smaller scope shows the dim
region to be almost hairlike and only slightly off center. It's quite odd how
the minimum differs between these too scopes. I should also mention the 10" f/5
with a 31% obstruction (large as it was designed to illuminate a full 35 mm
frame) also only shows the hairline minimum but closer to the edge of the A ring
at 400x. It is as if the gap between the outer edge of the A ring to the minimum
is about equal to the resolution my eye is capable of. In other words the effect
is in my eye more than the scope. Boosting the power beyond this softens the
image to the point that the hairline ring goes away. Again indicating to me the
eye/brain is trying to make sense of something a bit beyond what it is capable of
doing and this is how it interprets what is being seen. On average nights all
this is moot. I tried last night after the clouds cleared but seeing wasn't good
enough to repeat anything and it is cloudy tonight.
There are so many darned variables when the eye/brain is being used and it can be
fooled so darned easy trying to prove anything this way is pretty shaky.
Overprocessing of digital images can do the same. When working at or beyond
normal limits makes me want to change the beer slogan More aperture -- less
atmosphere. But I like breathing.
My fascination on how the eye/brain screws up came after a demonstration by Don
Taylor. He set up a back illuminated Mars that we all viewed through our scopes
at several hundred yards. It was about 18" of arc I believe. And did look very
much like Mars. We drew what we saw. While the basic albedo features were the
same we saw different faint albedo features. But the kicker was when we saw
"Mars" close up. It contained only very unmartian features. All sorts of
oddball items existed on his Mars. They all looked very very different from what
we drew. It was quite amazing how different the items were from what we saw
through the rather poor seeing across the field. There were line drawings of
comets, Saturn. Constellations, and the like. We blended these into normal
Martian features. Quite a laugher but also sobers you up!
Rick
http://www.hydeobservatory.info/
http://www.prairieastronomyclub.org/
Richard P. Johnson
"Richard P. Johnson" wrote:
> In the 6" f/4 I on
> similar nights at 300x (400x - 500x in the 10") the smaller scope shows the dim
> region to be almost hairlike and only slightly off center.
That should read 6" f/12.
Rick
Siegfried Gonzi
> Some people like Richard and myself are seeing BEYOND DAWES.
Sorry Darren, but this proposition is stupid.
You should rather take into account, that there exists experiments,
which tested experimentally how our eyes can resolve threads. This was
in former days were visually observing planets was on the hot spot. The
Dawes limit is for splitting doubles; you can infer it from the
modulation transfer function.
I will try to find the reference for the aforementioned (I red it in a
book dedicated to planetary observing).
S. Gonzi
ValeryD
> Sorry Wayne.
>
> Uh - wrong. Theory is just that - theory. I have been getting some of the
> best lifetime views through my two refractors this fall/winter season. We're
> talking conditions such as what Richard has gone through - nearly 0°F - dry
> and pretty cold. Here in Canada, with those kind of cold, stable and dry air
> masses aloft, the seeing and transparency would rank amoungst the world's
> best - I'd bet my equipment on it! No, it's not necessarily Mauna Kea, the
> Aussie outback, Arizona, Florida or Chile, but the conditions are DAMNED
> fine, thank you very much. When the conditions get THAT good, it is
> unbelieveable how much detail can be seen, regardless of the aperture used.
>
> For myself, I have been getting stunning Saturn sessions myself in Alberta
> this fall. I had resolved the A ring so well in my 4" refractor, that it was
> seen as a grooved-like [similar to a phonograph record] pattern.
You, obviously, have a powerful image processor build-in in.... your tongue!
V.D.
ValeryD
> Io is around 1.1" across, yet I resolved it in my 78mm scope, at 122x, when
> "theory" says I shouldn't be able to resolve below about 1.5". Whatever. The
> feature I saw was right at my visual threshold, and I had plenty of
> planetary references to use as guides. The grey-brown spot I saw under
> perfect seeing moved faster than the planetary features, and after referring
> my notes during the 3h session, the speck I saw was where Io should be. No,
> I don't exactly know why this was exactly possible, but it WAS Io. Theory
> states... to heck with the textbooks. Go out and catch a perfect night of
> seeing with ANY instrument with very good to excellent optics, and then tell
> me what you see. Sorry that you're not...
FYI, the resolving power of 78mm scope in a green light is about 1.8" and
this is for two star-like sources on a black background. The Airy disk size
of such scope in green light has 3.6"(!) diameter. How you can "resolve" Io's
1.2" disk? Explain, but without fantasies, please. Ha ha ha!
V.D.
David Knisely
> FYI, the resolving power of 78mm scope in a green light is about 1.8" and
> this is for two star-like sources on a black background. The Airy disk size
> of such scope in green light has 3.6"(!) diameter. How you can "resolve" Io's
> 1.2" disk? Explain, but without fantasies, please. Ha ha ha!
>
The resolution of a telescope must be clearly defined here. One
standard is the Rayleigh Criteria which for visible light (ie: 5500
Angstroms) is approximately d = 5.45/D, where d is in arc seconds and D
is the aperture in inches. This is the distance from the center of a
point-source diffraction pattern (ie: the center of the diffraction disk
generated by a star) and the center of the first dark ring of the
diffraction pattern. The radius of the visible Airy disk is somewhat
dependent on the brightness of the source, but to a reasonable
approximation, it is about half of the Rayleigh Criteria. Thus the
*diameter* of the visible disk of a point source is about the same as
the Rayleigh Criteria. For a 78mm scope, the Rayleigh Criteria is about
1.8 arc seconds. This puts the size of the Airy disk slightly larger
than that of Io, so Io is clearly not resolved in the Rayleigh sense.
To actually resolve the disk when Io is at its greatest angular size
would probably require an aperture of 4 inches or larger. Clear skies
Brian Tung
> Uh - wrong. Theory is just that - theory.
Setting aside the observation that Wayne was just joking, I have to say
that this gives precisely the wrong idea about what theory really is--at
least, optical theory.
One thing it is not is purely academic. Optical theory places real and
*practical* constraints on what image information is presented at the
eyepiece, even under perfect circumstances. However, because the
constraints are not always easily digested, they are often "processed"
somewhat to yield simpler limits, such as Dawes's Limit. Although these
limits have their basis in theory, they are not themselves theoretical.
They are, at least in part, empirical. But because they are often
confused with theory, this gives many people the idea that theoretical
limits can be exceeded, given the proper observer at the proper time.
If one keeps this in mind, then feats such as "exceeding Dawes's Limit"
can be understood simply as discovering that empirical rules of thumb
are occasionally wrong. It's interesting to consider why this might
happen under any particular set of circumstances, but there's no reason
to think that anything fundamentally new or unusual is going on. I hope
that isn't disheartening or dissuades you from having these discussions.
> Io is around 1.1" across, yet I resolved it in my 78mm scope, at 122x,
> when "theory" says I shouldn't be able to resolve below about 1.5".
> Whatever.
Optical "theory" says no such thing. I think that if you understood
what optical theory actually says about resolution and contrast, you
would not be so quick to dismiss it with a "Whatever." Do not fall into
the trap that many people do: when they observe a contradiction between
what they see and what they understand of theory, they quickly assume
the theory is at fault, not their understanding of the theory. It could
be either, but it's worthwhile to check both before deciding.
> This Yahoo! group I refer to will be by private admittance, and those
> wishing to participate in it are welcomed so long as they have open-minded,
> constructive contributions to make - posturing and negativity can be left at
> the door.
Sorry, but I cannot sympathize with such a constraint. First of all,
decisions about what is posturing and negative will be made by a possibly
biased party, and secondly, what you say above about theory could easily
be interpreted as posturing and negative, since you haven't made a proper
study of it. Discussion about these points *must* be open or else you
risk closing out some valid viewpoints--however stridently presented.
That's why I favor moderation only for the purposes of keeping out spam
and the like.
> Anyway - what do you have to say about my Io observations? Ridiculous?
> Imaginative? whatever...
Actually: very good, but hardly out of the ordinary, and certainly it
doesn't contravene theory.
Brian Tung
> Resolved means strictly that you were able to see Io as a disc that
> was visibly larger than that of a similar brightness field star. You
> can see an object without being able to resolve it.
I submit that "resolved" is difficult to interpret for objects whose
spatial frequency is close to the limiting resolution. To my eye, in
my 5-inch scope (FWHM = about 1 arcsecond), Ganymede and Callisto are
noticeably larger than Io and Europa, and Io and Europa are, in turn,
noticeably larger than field stars. (Not tremendously so, of course!)
Sol Robbins
My ability to to equate numbers and how they relate to what I see is
not as developed as some who have posted here.
I have taken to drawing the planets during this apparition. I usually
e-mail my Jupiter drawings to the ALPO Jupiter co-ordinator. In one, I
caught Io about 1/8 of Jupiter's diameter in from the preceeding side.
Even though I did'nt note in my observation, I did draw it.
Anyway, Io was cuaght in my drawing by the ALPO Jupiter co-ordinator.
Optical theories aside, I was asked to see if I could observe an Io
transit across Jupiter's entire disc. Reason being is that Jupiter is
dimmer than last year. Photometers have been used to measure disc
brightness band by band, zone by zone, across Jupiter's CM. Last I
heard, it seems that the Equatorial Zone is about 5% dimmer than last
year.
Even though this doe not add to the topic in this thread, I hope that
some of you may find this of interest.
Thanks, Sol Robbins
ar...@mercury.kherson.ua (ValeryD) wrote in message news:<5c4a4ee7.0301...@posting.google.com>...
ValeryD
> For a 78mm scope, the Rayleigh Criteria is about
> 1.8 arc seconds. This puts the size of the Airy disk slightly larger
> than that of Io,
Dave, for 78mm scope angular size of Airy disk in yellow-green is 3.6".
Please, do not argue with this obvuous fact.
V.D.
Brian Tung
> For a 78mm scope, the Rayleigh Criteria is about
> 1.8 arc seconds. This puts the size of the Airy disk slightly larger
> than that of Io,
Valery D. wrote:
> Dave, for 78mm scope angular size of Airy disk in yellow-green is 3.6".
> Please, do not argue with this obvuous fact.
I'm pretty sure David says elsewhere in his post that he is using FWHM,
not zero-to-zero.
David Knisely
> Dave, for 78mm scope angular size of Airy disk in yellow-green is 3.6".
> Please, do not argue with this obvuous fact.
>
I thought we had hashed this out via private e-mail. Why are you
bringing it up now on the group?
You used term "Airy disk" incorrectly. 3.6" arc is the approximate
diameter of the *first minimum* of the diffraction pattern (to the point
of zero intensity in the middle of the first dark ring between the Airy
disk and the first bright diffraction ring). The *radius* of the
diffraction pattern to the first minimum is r = 5.45/D where D is the
aperture in inches and r is in arc seconds (visible light at 5500
Angstroms), or for metric units: r = 138.4/D where D is in
millimeters (this is also known as 'the Rayleigh criteria' for
resolution). For a 78mm aperture, r = 1.77 arc seconds which means that
the *diameter* of the first dark ring of the *diffraction pattern* is
3.54 arc seconds. However, while some sources define the "diffraction
disk" radius as the radius of the diffraction pattern to the first
minimum, this is *not* the Airy disk. The Airy disk is the *visible*
disk we see with our eyes when looking at a point source. The intensity
of the diffraction pattern falls below our visual threshold *before* we
get to the minimum intensity point (the middle of the first dark ring)
between the Airy Disk and the first diffraction ring. Thus, the Airy
Disk is *always* somewhat smaller than the radius of the so-called
"diffraction
disk". The brighter the point source is, the bigger the Airy disk tends
to seem, and the fainter the point source, the smaller the disk
appears. Thus, the precise diameter of the Airy disk is *not* well
defined mathematially. Its approximate radius on average is roughly
half of the radius of the diffraction pattern to the first minimum, but
for bright sources it can be 75% to 85% the size of the Rayleigh
criteria and for faint sources it can be close to 1/3rd of the Rayleigh
criteria, so you can't just throw out the figure "3.6" arc" for the Airy
disk as an "obvious fact" when it isn't. Some observers put the radius
of the Airy disk as very nearly equal to the Dawes limit figure of
4.56/D, which agrees with some micrometer measurements for bright stars,
but again, the precise value is not precisely determined. If the
angular diameter of Io significantly exceeded that of the angular
diameter of the Airy Disk for a star of a similar brightness to Io, then
we would say that the true disk of Io was "resolved". This would
probably be the case for an aperture of 100mm or larger when Jupiter is
at opposition, but I do agree that the gentleman's claim of actually
resolving Io with a 78mm is incorrect.
David Knisely
ASTRONOMER'S HANDBOOK, here is some basic information about "exceeding"
Dawes limit.
EXCEEDING (or not exceeding) DAWES LIMIT
There are some isolated details visible telescopically in extended
objects which have a relatively simple diffraction structure and thus
may sometimes be seen when their angular size is smaller than Dawes
Limit (ie: dots, lines ect.). However, more complex or adjacent detail
can require *more* than Dawes Limit to pick them out, so the resolution
of detail is a more complex question and sometimes cannot be addressed
by just quoting the empirical double-star limit cited by Dawes. Below
are some common definitions and observational examples of what can be
seen with some relatively simple detail in experimental setups.
*** DEFINITIONS ***
DIFFRACTION PATTERN: The tiny "bulls-eye" pattern of a small disk of
light surrounded by concentric faint rings, created by the interference
of light at the focus of a telescope when imaging a distant point
source. Usually, telescope powers of greater than 30x per inch (1.18x
per mm) of aperture are needed to clearly show this pattern. The
effects which bring about the
appearance of the diffraction pattern often impose limits on the ability
of an instrument to separate double stars, as well as on the amount of
detail which a given aperture can reveal.
RAYLEIGH LIMIT: a resolution limit based on the wave nature of light and
the diffraction structure of a point-source image. At a wavelength of
5500 Angstroms: R = 5.45/D, where R is the radius of the diffraction
pattern (arc seconds) to the first minimum (middle of the first dark
ring between the bright central disk and the first bright ring), and D
is the aperture of the telescope in inches. Metric form: R = 138.4/D,
where D is the aperture in millimeters. R is sometimes referred to as
the radius of the "diffraction disk", but is *not* the same as the
"Airy" disk. It can be a somewhat useful figure for comparing the
resolution of differing apertures, but cannot always be strictly applied
to things like unequal double stars or to the viewing of detail in
extended objects.
DAWES LIMIT: An empirical limit based on the resolution of double stars
(and applicable mainly to double stars): r = 4.56/D, where D is the
aperture in inches (Metric form: r = 115.3/D where D is the aperture in
millimeters). This limit is approximate only, as it varies slightly
with the brightness of the two stars. It also cannot be strictly
applied to unequal double stars or to the viewing of detail in extended
objects.
AIRY DISK: The visible disk of light formed by a telescope from a point
source such as a distant star when viewed at very high power. The disk
is sometimes referred to as, "the Spurious Disk" in some texts. The
Airy disk dominates the central portion of the diffraction pattern, and
is often
surrounded by one or more faint diffraction rings. The precise radius
of the Airy disk varies by exactly how bright the point source is, but
it is always somewhat smaller than the Rayleigh Limit. Bright stars
will often produce an Airy disk radius which is up to 85% of the
Rayleigh limit, while faint stars can produce disks that are noticably
smaller. A fair approximation overall
for the radius of the Airy Disk is about half of the Rayleigh limit.
OBSERVATIONAL EXAMPLES OF RESOLUTION OF CERTAIN EXTENDED DETAIL
(from AMATEUR ASTRONOMER'S HANDBOOK, by J.B. Sidgwick, p.49-50)
("r" is Dawes Limit)
(NOTE: those specifically interested in the minimum aperture necessary
to resolve the Cassini and Encke divisions from their adjacent ring
edges should pay close attention to "C" below.)
A. BLACK SPOT ON A WHITE BACKGROUND:
i. W.H. Pickering, while testing his 10 inch reflector under good seeing
conditions in Jamaica, found that a circular spot became visible if its
angular diameter exceeded 0.20" arc (r/2.3).
ii. Experiments by W.H. Steavenson showed that r/3 is a fair average
figure.
iii. The naked-eye visiblity of sunspots whose diameter is less than r
is well known.
B. SINGLE DARK LINE ON A WHITE BACKGROUND:
i. Cassini Division, width of about 0.5" arc was discovered with about a
2.5 inch aperture (r/3.5).
ii. Experiments by W.H. Steavenson yielded about r/5 as an average
figure.
iii. During testing of an 11 inch refractor at Harvard, a human hair
was visible against a light-toned ground at a distance of nearly 1/4 of
a mile where it subtended 0.029" arc (r/14).
iv. W.H. Pickering glimsed a dark line 0.03" wide with his 10 inch
(r/15).
C. PARALLEL LINES ON A LIGHT BACKGROUND:
i. W.H. Pickering: minimum separation for resolution with a 10 inch
reflector was 0.63" arc (1.4r).
ii. A similar performance was given by the Arequipa 15 inch, which
resolved a pair of parallel lines when their separation was increased
past 0.42" arc (1.4r), in good seeing; slight atmospheric deterioration
immediately raised the threshold to about 2r. At less than 0.42" arc,
the lines appeared as a grey band of width about 1.5 times their
separation.
iii. Resolution of the lines at 12" arc with a 0.4 inch Object Glass
(1.1r).
Clear skies to you.
ValeryD
> The brighter the point source is, the bigger the Airy disk tends
> to seem, and the fainter the point source, the smaller the disk
> appears. Thus, the precise diameter of the Airy disk is *not* well
> defined mathematially.
Does it mean, that as fainter will be planetary disk (say, Io), as better
we will resolve it with a given scope? According to your rule, this
should be the case. Try to use a simple filter and see. The results, I
can assure you, will not back up your rule.
V.D.
Martin Brown
David Knisely wrote:
> 3.54 arc seconds. However, while some sources define the "diffraction
> disk" radius as the radius of the diffraction pattern to the first
> minimum, this is *not* the Airy disk. The Airy disk is the *visible*
> disk we see with our eyes when looking at a point source. The intensity
> of the diffraction pattern falls below our visual threshold *before* we
> get to the minimum intensity point (the middle of the first dark ring)
> between the Airy Disk and the first diffraction ring. Thus, the Airy
> Disk is *always* somewhat smaller than the radius of the so-called
> "diffraction
> disk". The brighter the point source is, the bigger the Airy disk tends
> to seem, and the fainter the point source, the smaller the disk
> appears. Thus, the precise diameter of the Airy disk is *not* well
> defined mathematially.
The Airy disk is perfectly well defined mathematically but our eyes
perception of it is not.
One reason why quantitative measures like FWHM are usually to be preferred.
The problem arises from the way the human eye preceives the point spread
function since anything below our threshold of vision is black. This is the
easy case for a star against a uniform black background.
The other thing to remember is that in the case of a darker feature against a
bright planet surface you are really up against it since the criterion for
visibility is a just noticable difference between the blur of all the
surrounding bright bits of planet onto the tiny darker region. And it is
harder to see a small difference between two bright regions than between dark
sky and the same small difference.
Regards,
Martin Brown
Paul Schlyter
ValeryD <ar...@mercury.kherson.ua> wrote:
>David Knisely <KA0...@navix.net> wrote in message news:<3E1B3847...@navix.net>...
>
>
>> The brighter the point source is, the bigger the Airy disk tends
>> to seem, and the fainter the point source, the smaller the disk
>> appears. Thus, the precise diameter of the Airy disk is *not* well
>> defined mathematially.
However, the first minimum in the interference pattern *is* well
defined mathematically.
>Does it mean, that as fainter will be planetary disk (say, Io), as better
>we will resolve it with a given scope? According to your rule, this
>should be the case. Try to use a simple filter and see. The results, I
>can assure you, will not back up your rule.
That's not what he said. But if you view a point light source (e.g. a star)
against a dark background, then if the star is brighter, you'll also see
fainter parts of the Airy disk, and thus it will appear somewhat bigger.
But if the "background" isn't dark, then this doesn't apply. That's the
case when viewing extended objects such as planetary surfaces: each
point then has a "background" of adjacent points, which aren't dark.
--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se
WWW: http://hem.passagen.se/pausch/index.html
http://home.tiscali.se/~pausch/
ValeryD
So, all, who can understand these things, should conclude - Darren's
statements about resolving Io's disk with 78mm scope just a plane lie
purposed for promoting of magical properties of SV's achromats.
Again - powerful image OVERprocessor in his tongue. :-)
V.D.
Brian Tung
> So, all, who can understand these things, should conclude - Darren's
> statements about resolving Io's disk with 78mm scope just a plane lie
> purposed for promoting of magical properties of SV's achromats.
No, I do not think it is a lie--I'm sure he believes he sees what he says
he sees--but I also think that the word "resolve" has limited significance
when you're talking about what is essentially a featureless disc (for most
amateur telescopes, anyway). It may very well be possible to see that the
convolved disc is somewhat larger than it is for a field star; that can,
in fact, be distinguished in a 5-inch SCT.
It is not as though there is a distinct breakpoint where the edge of Io's
disc would suddenly be resolved. That much should be obvious, since the
size of the Airy disc (diffraction disc, whatever terminology you want to
use) varies continuously with aperture.
David Knisely
> The Airy disk is perfectly well defined mathematically
Well, not quite in the same way that the Rayleigh/diffraction disk is.
The "threshold" of the eye is rather vague (varies depending on eye
sensitivity, the glare in the system, ect.), and, as I stated, the
visible Airy disk varies in apparent size depending on the brightness of
the star. This is not the case for the diameter of the "diffraction
disk" which is defined by the location of the first minimum in the
diffraction pattern and does not change with star brightness.
Micrometer measurments of the spurious disk of bright stars by Pickering
(10 inch telescope) yielded a radii of 0.46" arc, similar in magnitude
to Dawes Limit as well as the wavelength divided by the aperture, so
these are values which again are slightly smaller than the radius of the
diffraction pattern to the first minimum. Clear skies to you.
Martin Brown
Brian Tung wrote:
> Valery D. wrote:
> > So, all, who can understand these things, should conclude - Darren's
> > statements about resolving Io's disk with 78mm scope just a plane lie
> > purposed for promoting of magical properties of SV's achromats.
>
> No, I do not think it is a lie--I'm sure he believes he sees what he says
> he sees--but I also think that the word "resolve" has limited significance
> when you're talking about what is essentially a featureless disc
Although I am inclined usually to give the benefit of the doubt. In this case
I do smell a rat.
"Resolved" as used by Darren at best meant glimpsed.
Checking the respective albedoes shows that Io (0.61) should normally be
brighter than Jupiters mean (0.44) and would not appear as "a gray-brown spot
on Jupiters disk" but as a very slightly brighter dot.
> It is not as though there is a distinct breakpoint where the edge of Io's
> disc would suddenly be resolved. That much should be obvious, since the
> size of the Airy disc (diffraction disc, whatever terminology you want to
> use) varies continuously with aperture.
Although convention would normally dictate that you would only claim to have
resolved something when you can see clearly that it is broader than the point
spread function of the instrument in use.
Regards,
Martin Brown
David Knisely
> So, all, who can understand these things, should conclude - Darren's
> statements about resolving Io's disk with 78mm scope just a plane lie
> purposed for promoting of magical properties of SV's achromats.
A "lie" is much too strong a word to be used here. Darren may have seen
the Airy disk of Io, and from its behaviour (no diffraction rings) may
have mistakenly concluded that he was seeing Io's disk itself. It is a
mistake many amateurs make, and strong personal feelings may make a
person defend what they saw as truth without actually lying about it.
Gerhard Niklasch
(8" f/5 Newtonian, 2.5x Powermate, Takahashi LE-7.5 for 333x,
for which the seeing was _almost_ good enough :) - I did want
the power to be large enough to _see_ the two disks convolved
with the PSF, which worked well enough, and yes, I do know how
spatial convolutions work. :)
I was struck by an interesting optical illusion. Almost during
the entire occultation, and alternately observing with each eye
in turn, and tilting my head sideways this way and that, I could
not rid myself of the impression that the northern one of the
two tiny disks was passing in front of the southern one (which
it wasn't!), and that I could discern its edge "in front of"
Ganymede outlined as a dark arc displaced to the S. Curious...
After watching the two satellites separate again, I'm coming up
with the following hypothesis: While Ganymede is brighter overall,
Io beats it in terms of surface brightness, and the more concentrated,
brighter convolved image on the N together with the retinal contrast
enhancement machinery may have created the illusory dark curving
"edge" in just the wrong place.
Now I hope to stay awake long enough to see both moon's shadows
and then the two satellites themselves in transit... (through
rapidly changing 7/8 cirrus and at some -13 deg C; and moving
myself, eyepieces and chair into the warm room and back outside
in alternating 15-minute intervals ;).
By the way, for the first time this observing season, I found
it striking to consciously perceive the inclination of Jupiter's
equatorial plane (= orbital plane of the Galilean satellites)
against the Earth's, by virtue of the whole line-up travelling
through the Dob's field of view at such a noticeable angle between
its own axis and the direction of motion.
Clear skies,
Gerhard