apparent image size
Sarah Whitney
Does using a larger aperture telescope lend to a sense of more image scale? Or is it purely a function of focal length?
If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm. Is the 150mm f/8 refractor going to seem to have more image scale? Or is the brain tricked by the higher resolution?
Sarah
Al
illustration...
A 3,000 pound automobile can get from zero to 100 mph in about 18 seconds.
The engine is a standard Detroit v-8 developing 220 hp. A unicycle powered
by an 8 hp tricked out lawn mower engine can also go from zero to 100 mph in
about 18 seconds. The top end speed of both vehicles, much like the image
scale of both scopes below, is the same. You wouldn't want to be on a
unicycle doing 100, and you wouldn't want to look through an F/20 60mm (is
there such a thing?) under any kind of power.
Al
"Sarah Whitney" <swhi...@juno.com> wrote in message
news:deudnchFetI...@giganews.com...
Tombo
Forget the analogies Al. The fact is, F20 and F8 are not the same thing.
F- ratio is a ratio of FL to Aperture. An F20
scope will give a narrow FOV whereas F8 will over double the FOV. FOV is
also dependant on the FL of EP.
FOV is a linear relation to F ratio.
Brian Tung
Possibly!
One thing that resonates with what you're suggesting is that most people
find that Jupiter in the telescope at 100x, say, feels very small to
them. Yet at that magnification, Jupiter is looks about 1.25 degrees
across--or about 2-1/2 times as wide as the Full Moon! Most people would
be surprised to hear that the tiny image of Jupiter they see in the
eyepiece is considerably larger than the Moon they see in the sky by the
unaided eye.
Part of the problem is probably that when you're looking in the eyepiece,
there are no obvious points of reference by which to judge size. But I
have always thought that another part of the problem is that Jupiter at
that size just doesn't seem to show as much detail as the Moon does at
1x. The natural reaction is to want to make Jupiter even larger, to make
it easier to see what detail there is, but the root cause is still the
desire to see more detail.
And since a larger aperture permits one to see greater detail on the
planets, it could very well be that the eye is tricked by that greater
detail into thinking that planets look larger through a bigger scope of
the same focal length.
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
Brian Tung
> Forget the analogies Al. The fact is, F20 and F8 are not the same thing.
> F- ratio is a ratio of FL to Aperture. An F20
> scope will give a narrow FOV whereas F8 will over double the FOV. FOV is
> also dependant on the FL of EP.
>
> FOV is a linear relation to F ratio.
I think that is rather an inappropriate way of putting it.
In case it isn't obvious to the original poster, there are two fields of
view: true and apparent. True field of view (TFOV) is the size of the
actual patch of sky that you can see through the eyepiece and telescope.
Apparent field of view (AFOV) is how big that patch of sky *looks* when
magnified by the telescope. Thus, AFOV is, roughly, TFOV times the
magnification. Neither is in a simple relationship to f/ratio.
AFOV is independent of the telescope altogether; you can see that by
looking through the eyepiece when it isn't inserted into any telescope.
You'll generally see the field stop clearly delineating the AFOV.
TFOV, on the other hand, is dependent on the focal length of the telescope
and the field stop opening of the eyepiece. To first order,
field stop opening
TFOV = ------------------------ * 57.3 degrees
telescope focal length
The 57.3 "magic number" comes from the number of degrees in a radian.
(My next Astronomical Games column will explain this formula, among
other things.) Strictly speaking, then, TFOV doesn't depend on the
telescope's focal ratio, either, or the focal length of the eyepiece.
Practically speaking, there may be some dependence on the focal length
of the eyepiece, because shorter focal length eyepieces tend to have
smaller field stops, and longer focal length eyepieces wider field
stops. If a short focal length eyepiece had a very wide field stop,
it would probably have too wide an AFOV and be showing significant
aberrations near the edge of the field. Conversely, a long focal length
eyepiece with a small field stop would be like looking through a soda
straw.
Nonetheless, it is misleading to say that either FOV depends on the
focal ratio of the telescope. To first order, they don't depend on that
at all. Yes, there is some dependence between aperture and focal ratio,
but that is an inverse relationship--focal ratio tends to fall as
aperture increases--and that makes focal length depend less on focal
ratio than it would otherwise (if all telescopes had the same aperture,
for instance).
JS9
scale?
Yes...
With eyepieces providing the same magnification power per inch of aperture,
the viewed image in a larger telescope will appear a larger scale than that
seen through a smaller scope.
Jon Isaacs
For a given eyepiece, FOV is linear in relation to Focal Length of the
telescope, not the focal Focal Ratio.
An 5 inch F10 scope and an 10 inch F5 scope will have the same focal length and
same FOV for any specific eyepiece.
A 10 inch F10 scope will have 1/2 the FOV but twice the magnification with that
same eyepiece.
Jon
Tombo
"Brian Tung" <br...@isi.edu> wrote in message
news:c3a9l6$8ke$1...@zot.isi.edu...
> Tombo wrote:
> > Forget the analogies Al. The fact is, F20 and F8 are not the same
thing.
> > F- ratio is a ratio of FL to Aperture. An F20
> > scope will give a narrow FOV whereas F8 will over double the FOV. FOV
is
> > also dependant on the FL of EP.
> >
> > FOV is a linear relation to F ratio.
>
> I think that is rather an inappropriate way of putting it.
>
> In case it isn't obvious to the original poster, there are two fields of
> view: true and apparent. True field of view (TFOV) is the size of the
> actual patch of sky that you can see through the eyepiece and telescope.
> Apparent field of view (AFOV) is how big that patch of sky *looks* when
> magnified by the telescope. Thus, AFOV is, roughly, TFOV times the
> magnification. Neither is in a simple relationship to f/ratio.
>
> AFOV is independent of the telescope altogether; you can see that by
> looking through the eyepiece when it isn't inserted into any telescope.
> You'll generally see the field stop clearly delineating the AFOV.
>
> TFOV, on the other hand, is dependent on the focal length of the telescope
> and the field stop opening of the eyepiece. To first order,
>
> field stop opening
> TFOV = ------------------------ * 57.3 degrees
> telescope focal length
>
>
> focal ratio of the telescope. To first order, they don't depend on that
> at all. Yes, there is some dependence between aperture and focal ratio,
> but that is an inverse relationship--focal ratio tends to fall as
> aperture increases--and that makes focal length depend less on focal
> ratio than it would otherwise (if all telescopes had the same aperture,
> for instance).
>
You are too far over the top. Most people want a straight forward
explanation. Mine is more than
sufficient. Most people are in there right mind and not going to actually
measure the F-stop in the EP.
FOV IS a linear relationship. F10 has TWICE the FOV as F20, so cut the
crap!
Brian Tung
> With eyepieces providing the same magnification power per inch of aperture,
> the viewed image in a larger telescope will appear a larger scale than that
> seen through a smaller scope.
Although that's true, I don't think that's what Sarah was asking. It
stands to reason that at double the magnification, a scope with twice
the aperture will naturally show a larger image scale, because it really
*has* a larger image scale. The question is whether there is a visual
effect of greater image scale even when the magnification is the same.
(At least, that's how I understood the question.)
Tombo
>
> An 5 inch F10 scope and an 10 inch F5 scope will have the same focal
length and
> same FOV for any specific eyepiece.
Refractor or mirror? Can you mix the 2?
That is not what I have been taught. F ratio no matter what determines FOV.
Brian Tung
> You are too far over the top. Most people want a straight forward
> explanation. Mine is more than
> sufficient. Most people are in there right mind and not going to actually
> measure the F-stop in the EP.
>
> FOV IS a linear relationship. F10 has TWICE the FOV as F20, so cut the
> crap!
Sigh. You again?
FOV is a quantity. It isn't a relationship, linear or otherwise. What's
more, it is not in a linear relationship with focal ratio, as you pretend.
Two scopes with the same focal length but different focal ratios have the
same true field of view (with the same eyepiece). Two scopes with the
same focal ratio but different focal lengths have different true fields of
view. Your explanation is not only insufficient, it is plain wrong. I
put it charitably when I said it was inappropriate, but I see no reason
for that charity anymore.
If there's anyone here who needs to cut the crap, it ain't me.
Jon Isaacs
>>
>> An 5 inch F10 scope and an 10 inch F5 scope will have the same focal
>length and
>> same FOV for any specific eyepiece.
>
>Refractor or mirror? Can you mix the 2?
Yes, there is no problem mixing the two.
>That is not what I have been taught. F ratio no matter what determines FOV.
Not quite sure where you picked up this bit of misunderstanding. It may be that
you are confusing Exit pupil with Field of View. The exit pupil is indeed the
inversely proportional to the focal ratio.
But Fratio does not determine the Field of View, rather it is a function of
the eyepiece and the focal length of the telescope involved.
The first order equations are:
The Magnification is the focal length of the telescope divided by the focal
length of the eyepiece.
The True Field of View is the Apparent Fielf of View of the Eyepiece Divided by
the Magnification.
A scope with a 1000mm focal length will provide 40X with a 25mm eyepiece. If
the eyepiece has an AFOV of 60degrees then the TFOV will be 1.5 degrees.
Jon
David Knisely
> Does using a larger aperture telescope lend to a sense of more image scale? Or is it purely a function of focal length?
>
> If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm. Is the 150mm f/8 refractor going to seem to have more image scale? Or is the brain tricked by the higher resolution?
If both telescopes have the same focal length, the image scale at the focal
plane of each will be the same. If you used the same focal length eyepiece in
both scopes, they would each show the same sized image to the eye since the
magnifications would be identical. The resolution and brightness of that
image, of course, will be decidedly different for the two scopes, as the 150mm
will be gathering over six times the light of the 60mm scope, and will have
2.5 times finer resolving ability of the smaller scope. 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 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
Ioannis
news:20040317165318...@mb-m22.aol.com...
[snip]
> Not quite sure where you picked up this bit of misunderstanding. It may be
that
> you are confusing Exit pupil with Field of View. The exit pupil is indeed
the
> inversely proportional to the focal ratio.
>
> But Fratio does not determine the Field of View, rather it is a function
of
> the eyepiece and the focal length of the telescope involved.
>
> The first order equations are:
>
> The Magnification is the focal length of the telescope divided by the
focal
> length of the eyepiece.
>
> The True Field of View is the Apparent Fielf of View of the Eyepiece
Divided by
> the Magnification.
>
> A scope with a 1000mm focal length will provide 40X with a 25mm eyepiece.
If
> the eyepiece has an AFOV of 60degrees then the TFOV will be 1.5 degrees.
Is there any place, like a FAQ that has as many of those tid bits of optical
info, (like FOV, TFOV, EP, Mag, etc) as possible all collected together?
I'd like to have a small telescope optical resource handy on my web-site. I
just saved Brian's article, but I need more.
Thanks,
> Jon
--
Ioannis Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable
james
wrote:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Brian
isn't there a sense of larger scale when going from a larger TFOV to a
more narrower TFOV by reducing the AFOV of the eyepiece? If this
makes sense.
It would seem that looking at a smaller patch of the sky, the object
in the field of view appears bigger? It would seem that with two
eyepieces of same foacl length and different AFOV, the perceived size
is bigger yet the angular magnification remains the same. Is that so?
james
Brian Tung
> Is there any place, like a FAQ that has as many of those tid bits of
> optical info, (like FOV, TFOV, EP, Mag, etc) as possible all collected
> together?
>
> I'd like to have a small telescope optical resource handy on my
> web-site. I just saved Brian's article, but I need more.
I have something here that I cooked up a few months ago. It is probably
not going to be interesting to anyone who doesn't already understand the
principles involved, but who knows, maybe someone will get something out
of it. Here it is (fixed-width font required):
T-------x-------M-------x------ p
| | | |
| A | | T = true field of view
| | | M = magnification
x---s | | p = exit pupil
| | | A = apparent field of view
| | | s = eyepiece field stop
| | | F = objective focal length
F---------------x | f = eyepiece focal length
| | | D = aperture
| f | r = focal ratio
| |
x-----------------------D
|
|
|
r
The x's are places where three quantities meet in a T. The rule at any
of these x's is that the two quantities on other end of the crossbar of
the T may be multiplied to yield the third quantity (on the other end of
the upright of the T).
For example, at upper right, we have M, p, and D, meeting in a T. That
means that M times p = D. Consulting the table at right, we see that it
indicates that magnification times exit pupil equals aperture. Which it
does, to first order.
There is one place where the magic factor of 57.3 degrees per radian must
be included--in the upper left, where T x F = s. If T is to be given in
units of degrees, then we must have T x F = s x 57.3.
Stephen Paul
"james" <geo...@washington.edu> wrote in message
news:s8oh50pnsrc5t05pi...@4ax.com...
> isn't there a sense of larger scale when going from a larger TFOV to a
> more narrower TFOV by reducing the AFOV of the eyepiece? If this
> makes sense.
Of course this makes sense. Consider that the moon is always a half degree,
even though by comparison with over-head, it looks huge when it's near the
horizon.
Brian Tung
> isn't there a sense of larger scale when going from a larger TFOV to a
> more narrower TFOV by reducing the AFOV of the eyepiece? If this
> makes sense.
>
> It would seem that looking at a smaller patch of the sky, the object
> in the field of view appears bigger? It would seem that with two
> eyepieces of same foacl length and different AFOV, the perceived size
> is bigger yet the angular magnification remains the same. Is that so?
I think you're probably right, but that would not be an effect of
increasing aperture--the point of interest in the original question.
The apparent FOV is inherent in the eyepiece under most circumstances
and doesn't depend at all on the aperture.
james
wrote:
>James wrote:
^^^^^^^^^^^^^^^^^^^^^^^^^^
Brian
I did fail to mention that aperture remains the same. That explains
why planetary viewing is more prefered with lesser AFOV eyepieces.
Apperture controls angular resolution thus more detail. Angular
magnification is independant of aperture.
james
Brian Tung
> I did fail to mention that aperture remains the same. That explains
> why planetary viewing is more prefered with lesser AFOV eyepieces.
Because the planet looks larger with a smaller AFOV? Maybe. I think
another reason, possibly more important, is that it permits the use of
some "oddball" designs, such as the monocentric, which are impractical
(or perform poorly) with large AFOVs, but yield sufficiently high
contrast near the center of the FOV that planetary observers prefer
them.
Really, they are only oddball in the sense that few amateurs encounter
them. I'm not sure that there's anything inherently oddball about them.
Brian Tung
> T-------x-------M-------x------ p
> | | | |
> | A | | T = true field of view
> | | | M = magnification
> x---s | | p = exit pupil
> | | | A = apparent field of view
> | | | s = eyepiece field stop
> | | | F = objective focal length
> F---------------x | f = eyepiece focal length
> | | | D = aperture
> | f | r = focal ratio
> | |
> x-----------------------D
> |
> |
> |
> r
And if you don't think that doggone "f" in the middle isn't irritating
to my sense of symmetry, you little know me. (It irritated me enough
to mention it here, after all.)
Tombo
John Davis
>some "oddball" designs, such as the monocentric
Oddball? The monocentrics won the AAHA 2003 award for best planetary eyepiece! LOL
Jon Isaacs
Thats right, the whole world is crazy, you are the only one who is sane....
Jon
JS9
> Thats right, the whole world is crazy, you are the only one who is
sane....
He has a point. Sarah just asked a simple question.
Mike Ruskai
>Does using a larger aperture telescope lend to a sense of more image scale? Or is it purely a function of focal length?
>
>If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm. Is the 150mm f/8 refractor going to seem to have >more image scale? Or is the brain tricked by the higher resolution?
Two telescopes of the same focal length with the same eyepiece will give
images of the same size. The one from the larger aperture will simply
look brighter and more detailed. This could possibly create the illusion
of a larger image, I would imagine, but I haven't put it to a test myself
(since I have only the one telescope).
I'm posting this response late, and it duplicates some of what has already
been said, but perhaps not as simply as you desire.
You should ignore the messages by "Tombo", as they are full of errors.
--
- Mike
Remove 'spambegone.net' and reverse to send e-mail.
Mike Ruskai
The reason for this optical illusion (which is really quite incredible -
anyone who hasn't noticed before should make it a point to observe the
full moon as it rises at sunset, then later at midnight) isn't very clear,
but it has nothing to do with field of view.
You can easily see this for yourself by looking through a small aperture
(a hole in paper, or something similar) when the moon is close to the
horizon, and high overhead. The illusion holds based on the moon's
position.
The issue is discussed in Phil Plait's Bad Astronomy book, and the best
answer (in his opinion, and probably mine) is that our perception of the
sky is not semi-spherical. The "dome" is instead quite oblate, so that
objects near the horizon are perceived as further away than objects on the
virtual dome high in the sky. Our brains try to assign actual size to
objects based on perceived distance, so a 30-arc-minute disc represents a
larger object at the more distant horizon than a 30-arc-minute disc high
in the sky, which is closer.
Jon Isaacs
She did ask a simple question.
And there were initially some simple answers.
Unfortunately Tombo posted some very incorrect information that confused the
issue and required detailed explanations to correct.
His further insistance at making the same erronous statements further
complicated this thread.
Jon Isaacs
Tombo
viewing a photo of
a train track disappearing into the distance. If you use a ruler you will
see that the width is the same
along. The perception is however the tracks appear wider when it is closer
in the photo.
I am glad to get the FOV thing straightened out now. Too bad telescope
manufacturers don't
tell us these things.
Tombo
"Jon Isaacs" <joni...@aol.com> wrote in message
news:20040318072550...@mb-m19.aol.com...
Your out ta lunch. Brian Tung started it with his "over the fence"
explanation that
many woudn't bother with
Jon Isaacs
>explanation that
>many woudn't bother with
>
As far as I can tell, the only erronous information posted in this thread was
that the field of view is only a function of the focal ratio and not the focal
length. Clearly wrong and clearly something that needed clarification.
I hope you have that one straight.
Whether or not you were capable of understanding Brian's post, I don't know but
it seems like it went over your head. The fact that you did not bother to
understand it, that was your choice, it does not mean that others were not
interested.
However, this being a technical forum, in depth answers are appropriate and
Brian's knowledgeable posts are one of the strong points of this group.
Jon Isaacs
Jon Isaacs
Tombo
">
> Jon Isaacs
He is mostly entertaining himself and "showing off". You have no right to
judge whether or not I
understand it. I just don't have the time to read all of that like many
others too. You may notice that
Sarah has not returned (at least yet).
Sarah Whitney
>Sarah has not returned (at least yet).
Kind of suprised at the results of my question! When my family used to drive around the
country (USA) as a kid, my brother and I would fight in the backseat a lot. My dad would
get frustrated and say something like "You kids fight, fight fight!! You two would fight
about a pile of cow poop in the road!". SAA is kind of like that, no?
As for my question, I think it really does seem a bit bigger to me in the new 6" f/8. I think
the increased resolution and the fact I'm using smaller AFOV eyepieces than I used to with
a previous 6" f/8 is the culprit.
Sarah
Tombo
". SAA is kind of like that, no?
>
Yes
james
^^^^^^^^^^^^^^^^^^^^^^^^^
Monocentrics have been around since the late 1880's. They fell out of
use oh around WWII time frame. The Steinheil is characterized by a
flint-crown-flint achromatic triplet with the curvatures of the six
surfaces having a common center of radius. About 28 degrees AFOV and
0.6 times the focal length for eye relief. Good toexcellent
preformance in longer focal length scopes, F8, F10, ..., as is all the
designs of that era were. Fast scopes were not as common in the
1800's.
Zeiss and Hastings made tripletts that varied form the
SteinheilMOnocentrtic. Probably modeled after the Abbe triplet used in
the Abbe Ortho. The Hastings triplets are still found in small pocket
magnifiers. They to make good planetary eyepeices.
Brian the real oddball eyepiece that I would liek to see if one still
ever exists is one of Herschel's spherical eyepeice. No eyerelief to
speak of and really only good for F30 and longer scopes. Fifteen
degree field of view. Now that is an oddball eyepiece.
james
james
>Sarah, these guys are all crazy. your answer still remains elusive.
>
^^^^^^^^^^^^^^^^^^^^^^^^^
Sarah
I would trust the information from David Knisely and Brian Tung over
this gentleman. While their explanations can be a bit technical and
difficult to grasp for some beginners, I can say that their knowledge
is reliable and worth considering. They are willing to listen and
answer questions and help anyone along.
In the future if you need answers to questions these two are among
some good sources to trust.
regards
james
Brian Tung
> Brian the real oddball eyepiece that I would liek to see if one still
> ever exists is one of Herschel's spherical eyepeice. No eyerelief to
> speak of and really only good for F30 and longer scopes. Fifteen
> degree field of view. Now that is an oddball eyepiece.
That is an oddball eyepiece--almost literally so!
As I said, I only consider the monocentric an oddball in the sense that
it's just not very common. Many people here on SAA know about it, but
it really isn't in popular use.
james
wrote:
>James wrote:
>> Brian the real oddball eyepiece that I would liek to see if one still
>> ever exists is one of Herschel's spherical eyepeice. No eyerelief to
>> speak of and really only good for F30 and longer scopes. Fifteen
>> degree field of view. Now that is an oddball eyepiece.
>
>That is an oddball eyepiece--almost literally so!
>
>As I said, I only consider the monocentric an oddball in the sense that
>it's just not very common. Many people here on SAA know about it, but
>it really isn't in popular use.
>
^^^^^^^^^^^^^^^^^^^^^^^^^
there are about a half a dozen eyepieces that can fit in that
category.I saw a Galoc sold this week on astromart and odly enough I
know of only one person that has one. Never seen one though.
I have not had a chance to look through one of the newer Monocentrics.
I have heard good reviews though.
james
Stephen Paul
"Brian Tung" <br...@isi.edu> wrote in message
news:c3dn7k$d3v$1...@zot.isi.edu...
> As I said, I only consider the monocentric an oddball in the sense that
> it's just not very common. Many people here on SAA know about it, but
> it really isn't in popular use.
Not at $200+ per pop they're not. That kind of money is generally spent on
well corrected eyepieces for fast light buckets.
Possibly there's just not that many folks who have good 'nuff skies to
justify a high end narrow field "planet only" eyepiece.
Stephen Paul
news:gunaalqrneguyvaxa...@news.west.earthlink.net...
I wrote:
> >Of course this makes sense. Consider that the moon is always a half
degree,
> >even though by comparison with over-head, it looks huge when it's near
the
> >horizon.
>
> The reason for this optical illusion (which is really quite incredible -
> anyone who hasn't noticed before should make it a point to observe the
> full moon as it rises at sunset, then later at midnight) isn't very clear,
> but it has nothing to do with field of view.
If an object at a given magnification appears in a half degree field, it
will "seem" larger than the same object at the same magnification in a field
that is twice as large. I know this to be true, because I've compared the
views of a 13mm Plossl and a 13mm Nagler. The object in the Nagler "looks"
smaller.
It's not the field of view per se, it's the reference. The moon looks larger
next to another object than it does stand alone. Just like anything else.
Take a picture of a tree alone in a field. Take another picture of that tree
with someone standing next to it. In which picture does the tree look large?
Stephen Paul
"Sarah Whitney" <swhi...@juno.com> wrote in message
news:nKOdnTMfTJt...@giganews.com...
> As for my question, I think it really does seem a bit bigger to me in the
new 6" f/8. I think
> the increased resolution and the fact I'm using smaller AFOV eyepieces
than I used to... snip
Almost exactly right.
The increased exit pupil of the larger aperture means a brighter image,
which is more of a factor in making something appear larger, than is the
increased resolution. IMO.
I have a 1540mm focal length 5" F12 Mak and a 1525mm focal length 12.5" F4.8
reflector. With the same eyepiece, the image in the 12.5" scope looks larger
compared to the view in the 5" aperture, even though the magnification and
the field of view remain (virtually) constant. But, it is still only an
optical illusion, just as the moon appears larger on the horizon than it
does overhead, even though its size remains constant.
Stephen Paul
jerry warner
he is expressing which are: "nobody is answering the posters question,
directly".
Everyone has hyjacked the lady's question and are answering each other or
(focusing on and being critical) of everyone else's statements but nobody is
simply
ANSWERING THE LADY'S QUESTION! Its a traffic jam of aesoteric
gobble wobble! Everyone seems to be engaged in 'poetry for poet's' and forgot
there was an orginal question posed in the first place. Its a very offputting
beaurocractic thing which may prove expertise on the part of posters but -
where's the simple straightforward answer to the lady's question?. Now why is
that so difficult! ? It's like rage driving!
Is there a simple answer to the lady's question? The lady must be wondering!
Jerry
Brian Tung
> What he is saying is very common sensical and I agree with THE SENTIMENTS
> he is expressing which are: "nobody is answering the posters question,
> directly".
Easy does it, Jerry. You may wish to see, at the very least, Mike
Ruskai's and my own *direct* response to Sarah, in which we essentially
agree with her own suggestions about why things might appear larger.
Not only did she ask a reasonable question, but she gave it some
thought, apparently.
The rest of what you consider "gobble wobble" came only after some
erroneous information was posted by that "common sensical" poster. I'm
sorry if you didn't enjoy it.
jerry warner
"Does using as larger aperture telescope lend to a "sense" of more image scale"?
The answer is yes. Say you moved from a 6" to a 12" newtonian. Forget the focal length. You were
used to a certain image scale of the Moon with the 6" newt using a 25mm ep. and you suddenly put
the 25mm eyepiece in the new 12" and focus up the Moon and viola! Larger image scale. It's
obvious to everyone but ChingGotchaCook who is busy counting blueberries on Pluto. It's obvious
to you, it's obvious to me, it's
obvious to everyone at NASA! OK?
Now there are "technical" reasons why that would be the case. I wont go into that
here for fear of becoming ROADKILL but you can pull the gyst of those details
out from Brian's post above - rely on Brian (he knows what he's talking about
despite his style at times). Suffice it to say: your 6" newt was probably an f/8 ..
say an old RV6. Focal length 48" inches. Your new 12" Lovely is on the order of
say f/5. Focal length of that 60" inches. You used the same 25mm ep in both cases
so with the 6" newt your magnification was: ~48x. With 12" Lovely your magnificatio
est ~60x, a greater magnification than with the former 6" newt, so you are seeing
more detail spread over a much larger visual scale, and far far brighter. Key word
in your question was 'lend a sense' ... so yes, you do have a great "sense" of
"image scale". In fact you should because: you have a larger image in the same eyepiece! OK?
Having a "sense of image scale" involves several factors. Magnification. FOV.
Brightgness of image. Focal length. Detail seen. Contrast or lack of contrast.
The amount of panning you have to do to take in a whole seen, eg. full moon in
6" newt at 48x vs full moon in 12" f/7 scope at 200x. (are you beginning to get this?) You must
also throw in Recent discoveries in Yucatan Mayan subculture, but you can
forget about that on Tuesdays. Generally speaking the larger the aperture the greater
"sense of image scale" you have, moreover you can "feel" greater image scale potential awaiting
(if the sky would only clear and get steady!) as compared with say a 60mm refractor and a pcoket
full of punny .965 eyepieces. In common sense terms its
the difference between a good large Yukon van and an Urban Metro. You can almost
"feel" the larger scope "holding" more image scale than the smaller scope could. And
you suspicions and feelings are made concrete when viewing at the eyepiece.
Your question is well placed and common sensical.
The brain is not treicked in this matter - far from it! The brain knows very well
when it has more image scale before it than less.
As a child I moved from a 60mm refractor to a 90mm refractor to a 4" Criterion
newtonian, to a 6" f/15 Clark refractor, to a 14" f/6 newtonian to a 16" f/16
cassegrain. All but the last two moves presented me with an increasing sense of increased "image
scale". I judged what I called "image scale" by the amount of panning
I had to do to say keep the full moon in view, or move across it in the case of the
larger scopes ... all using similar 18-32mm eyepieces. Yes. I had a "sense of
increasing" image scale with each growth in scope size (aperture) I encountered.
It was only at the end moving from 14" f/6 to 16" f/16 that I *subjectively* and
rather suddenly "felt" I had come to a wall of some kind, where image scale had
not changed that much ... 14" to 16". Fact is the f/6 14" and the f/16 16" were
telescopes of a wholly different kind used for different purposes. In my young mind
I triedto get each scope to "act" like the other, but could not - some frustration on
my part set it. I finally went back to the 14" f/6 because I liked it image scale better.
It was a subjective thing with me at the time given my own general purpose needs
(with telescopes) at the time. For one thing, I preferred wide angle viewing with
lots or panning to see as much in a night's viewing as possible. Which brings up
the last common sense aspect of "image scale" you are referring to -
Things actually being seen in acute sharp focus so as to form a whole crisp image,
and the larger the crisp image across the field of the eyepiece being used, the
greater our sense of "image scale" ... because part of our sense of image is
'having a sharp crisp image which is an "image". Correct? Have a very large
crisp full moon to pan across delivers that larger sense of image scale you are
asking about, and desiring.
Example: I already said I had a f/16 16" cassegrain and a 14" f/6 newtonina.
Obviously, the cassegrain had more potential for magnification. In fact I could
crank up that cassegrain on the moon until all I could see was faint dark grainy
blurs of lunar detail. Not much of an image! That kind of an "image" does not
lend itself to having image scale, in your sense of the asking. Because there is
no "image". All there is is massive blurry half dark light with hints of detail. In order
for there to be a subjective sense of image "scale" there must first be a detectable
sense of image. If at 500x, every aspect of the image before you was contrasty
and crisp, then you would be more inclined to catagorise that scope as having
"great image scale" ........ because there is a useful image to begin with. This helps
to point out the subjective nature of 'having a sense of image scale' and what will
satisfy that objective, and what will not.
I hope this addresses some of the issues you raised. Sorry for any mispelllings.
Jerry
eyepiece and
David Knisely
> What he is saying is very common sensical and I agree with THE SENTIMENTS
> he is expressing which are: "nobody is answering the posters question,
> directly".
I answered it directly.
> Is there a simple answer to the lady's question? The lady must be wondering!
Yes there is, and I gave it:
>> Does using a larger aperture telescope lend to a sense of more image scale? Or is it purely a function of focal length?
>> If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm. Is the 150mm f/8 refractor going to seem to have more image scale? Or is the brain tricked by the higher resolution?
>
>
> If both telescopes have the same focal length, the image scale at the focal plane of each will be the same. If you used the same focal length eyepiece in both scopes, they would each show the same sized image to the eye since the magnifications would be identical. The resolution and brightness of that image, of course, will be decidedly different for the two scopes, as the 150mm will be gathering over six times the light of the 60mm scope, and will have 2.5 times finer
--
David W. Knisely KA0...@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
Jon Isaacs
>"Does using as larger aperture telescope lend to a "sense" of more image
>scale"?
>
>The answer is yes. Say you moved from a 6" to a 12" newtonian. Forget the
>focal length. You were
>used to a certain image scale of the Moon with the 6" newt using a 25mm ep.
>and you suddenly put
>the 25mm eyepiece in the new 12" and focus up the Moon and viola! Larger
>image scale.
I find it odd that you complain about other folks not directly answering the
original question and then you answer the wrong question.
Sarah, the original poster, is clearly aware of the focal length issue, that
was included in the original question, so one cannot just forget it.
Here is the original post:
>Does using a larger aperture telescope lend to a sense of more image scale?
>Or is it purely a function of focal length?
>
>If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm.
>Is the 150mm f/8 refractor going to seem to have more image scale? Or is
>the brain tricked by the higher resolution?
>Sarah
As we know, the image scale is indeed merely a function of focal length.
Jon
Tombo
> Jerry Warner wrote:
> > What he is saying is very common sensical and I agree with THE
SENTIMENTS
> > he is expressing which are: "nobody is answering the posters question,
> > directly".
>
> Easy does it, Jerry. You may wish to see, at the very least, Mike
> Ruskai's and my own *direct* response to Sarah, in which we essentially
> agree with her own suggestions about why things might appear larger.
> Not only did she ask a reasonable question, but she gave it some
> thought, apparently.
>
> The rest of what you consider "gobble wobble" came only after some
> erroneous information was posted by that "common sensical" poster. I'm
> sorry if you didn't enjoy it.
>
Seems that some have some here can be equally "voiciferous" and still have
respect. It's ok for some
but not for others. What a bunch!
Tombo
Funny how no one ever asks them directly inspite of all the smoke and
mirrors. People want answers from the hip,
not the sky!
BllFs6
>mirrors. People want answers from the hip,
>not the sky!
Well....Id say tombos answers come from a stinky body part that is generally
covered by 2 layers of clothing and is often massaged with toilet paper....(and
hint...it aint the hip!)
Blll
Tombo
"BllFs6" <bll...@aol.com> wrote in message
news:20040319131822...@mb-m27.aol.com...
I said people! Not the likes of you, whatever you are. Obviously your name
is foreign around here which
makes you a lurker, a troll, a nobody, and various other words that take
some form of the 7 words you can't say on
the radio or TV!
jerry warner
Jerry
jerry warner
Jerry
jerry warner
aperture telescope lend to a sense of more image scale?". The lady was asking
for an accounting of the "subjective" exprience of image scale based on anything
technical experts here at saa could offer - unfortunately she gave examples of 60mm f.20 vs 150mm f.8 scopes and we know 60mm f/20 scopes do not exist in the
real world of amateur astronomy! One poster commented and pointed this out. Her example is therefore obviously a poor place to start in explaining the SUBJECTIVE
apperception of image scale, as amateurs use varying scopes and apertures and
come to some realisation of something occuring which they then call a differing
"sense of image scale" across different aperture scopes.
Fact is, the woman is referring to something more than image scale.
I pointed out this is a complex phenominon, ie her "sense of image scale", whereas
you stuck to her unrealistic example and explaind her "perceptions" in terms of ...
focal length.
Her and other's perceptions, ie "sense of image scale" are not strictly confined
to the matter of Focal Length. I'm sorry but you are wrong.
Your answer did not deal with dissimilar focal lengths at vary apertures (and focal
lenths), which I believe is what the woman was driving at in spite of the specific example she chose.
You yourself must have sensed she was asking for more because you yourself swung
the issue to "brightness and resolution"?
I try to read between the lines with some people's posts. I try to get to what people
are thinking and asking in terms of my common experience which may relate to
what people are asking. Others here tend to answer only a narrow slice of what
people are asking, or not asking.
Clear skies to you -
Jerry
jerry warner
Jerry
Brian Tung
> I refer you to the first sentence in the woman's question. "Does
> using a larger aperture telescope lend to a sense of more image scale?"
> image scale based on anything technical experts here at saa could offer
> we know 60mm f/20 scopes do not exist in the real world of amateur
> astronomy!
Actually, I'm not sure it was unfortunate at all. It seemed to me that
she chose those numbers precisely because she wanted to eliminate the
one factor that would directly affect the actual image scale presented
by any single eyepiece: the focal length of the objective. Notice that
60 x 20 and 150 x 8 are both equal to 1200 mm.
By making sure that the two hypothetical telescopes had the same focal
length, she could focus her question on the *subjective* effect of larger
image scale, even when the actual image scale was the same. That's why I
answered her question in the way I did, and I suspect the same reasoning
was going on in the heads of at least a few of the other responders.
Incidentally, the 60 mm f/20 does exist in amateur astronomy--just not
in consumer telescopes. My first homemade telescope was a 30 mm f/30.
In any event, the question involved hypothetical telescopes, so the
fact that they don't exist is not fatal to the question. One could
easily imagine making one (or actually make it, for that matter).
W. Snell
> Does using a larger aperture telescope lend to a sense of more image scale? Or is it purely a function of focal length?
>
> If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm. Is the 150mm f/8 refractor going to seem to have more image scale? Or is the brain tricked by the higher resolution?
Maybe you could try masking down the 150mm f/8 to 60mm f/20. While
you are looking through the scope remove the mask (or have an
assistant do so) and immediately make a comparison. Try this at a
variety of magnifications. The larger aperture image will be brighter
and should show more detail; maybe that will make an object seem
closer and therefore seem larger.
Jon Isaacs
>larger
>aperture telescope lend to a sense of more image scale?". The lady was
>asking
>for an accounting of the "subjective" exprience of image scale based on
>exprience of image scale based on anything technical experts here at saa could
offer - unfortunately she gave examples of 60mm f.20 vs 150mm f.8 scopes and
we know 60mm f/20 scopes do not exist >in the real world of amateur astronomy!
I think you underestimate the woman.
It is pretty clear she choose those numbers because she already understood the
importance of focal length and wanted to provide an example of two scopes with
identical focal lengths but different apertures.
>
>I try to read between the lines with some people's posts.
We all do. But the queston was clear:
""Does using a larger aperture telescope lend to a sense of more image scale?
Or is it purely a function of focal length? ""
The OP was obviously aware of the effects of changing focal length. Thats why
the suggestion to "forget the focal length" missed the target.
jon
David Knisely
> I refer you to the first sentence in the woman's question. "Does using a larger
> aperture telescope lend to a sense of more image scale?".
Actually, what she wanted to know was too subjective, so all that can be
stated are the facts (which I did state). If both instruments of identical
focal length are viewing something like the moon at low power with the same
eyepiece, the "sense" would be that the moon would be the same size and with
about the same amount of detail, since we aren't pushing the instruments
towards their full resolving capabilities. The only difference would be image
brightness.
> unfortunately she gave examples of 60mm f.20 vs 150mm f.8 scopes and we know 60mm f/20 scopes do not exist in the
> real world of amateur astronomy!
Sure they do: a 60mm f/5 and a 4x Powermate. Why someone would want to do
this is not clear, but she at least had some idea about the image scale being
dependent on focal length.
> I pointed out this is a complex phenominon, ie her "sense of image scale", whereas
> you stuck to her unrealistic example and explaind her "perceptions" in terms of ...
> focal length.
I did no such thing. I used her example and stated the facts about what
happens at equal powers (ie: the same eyepiece used in two telescopes with
equal focal lengths). The "sense of image scale" was not defined by her. If
she meant that the scale "seems" larger, I don't see it being any different as
long as the initial conditions are the same. The only time the scale would
"seem" larger is if the power was increased in one scope over the other. More
power would be supported more fully by the larger aperture, so you could
achieve more useful image scales with the larger aperture (assuming the
seeing supported it), but in terms of true image scale at the focal plane,
that is only dependent on the focal lenght. Clear skies to you.
james
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I have and I have found their information solid and trustworthy. I
tend to believe that if they are truly interested in learning they
take time to understand. Answers from the hip kind of reminds of the
busy old man that really does not take time to answer questions that
are anoying. Kind of give an a simple answer and send them on their
way.
james
Rush
The woman, the lady, she, her, her name is Sarah.
I find it funny that all these men talk about each other by name but
refer to Sarah as I indicated above.
You guys haven't gotten out that much, have you?
Just kidding around, but if I thought I knew the answer to Sarah's
question, I would refer to Sarah by her name, kinda like the guys do.
If Brian referred to Tombo as he, we would be saying he who, who he,
you taking 'bout me?
I am 52 years old and over the past 25 to 30 years I have come to
treat the opposite sex as equally as I can and I still get into
trouble every now and then.
Hope this stirs up some thought.
Rush
jerry warner
Brian Tung wrote:
> Jerry Warner wrote:
> > I refer you to the first sentence in the woman's question. "Does
> > using a larger aperture telescope lend to a sense of more image scale?"
> > The lady was asking for an accounting of the "subjective" exprience of
> > image scale based on anything technical experts here at saa could offer
> > unfortunately she gave examples of 60mm f.20 vs 150mm f.8 scopes and
> > we know 60mm f/20 scopes do not exist in the real world of amateur
> > astronomy!
>
> Actually, I'm not sure it was unfortunate at all. It seemed to me that
> she chose those numbers precisely because she wanted to eliminate the
> one factor that would directly affect the actual image scale presented
> by any single eyepiece: the focal length of the objective. Notice that
> 60 x 20 and 150 x 8 are both equal to 1200 mm.
Why speculate? Ask her why she chose those numbers. Given the unrealism of
60x20 I thought she chose them out of thin air. I took the fact they both
equaled
1200mm to be an accident. It matters only if she intended the identity:
1200mm.
We will never know - she has moved to the Canary Islands!
>
>
> By making sure that the two hypothetical telescopes had the same focal
> length, she could focus her question on the *subjective* effect of larger
> image scale, even when the actual image scale was the same. That's why I
> answered her question in the way I did, and I suspect the same reasoning
> was going on in the heads of at least a few of the other responders.
>
> Incidentally, the 60 mm f/20 does exist in amateur astronomy--just not
> in consumer telescopes.
Ive never seen one. Ever.
> My first homemade telescope was a 30 mm f/30.
> In any event, the question involved hypothetical telescopes, so the
> fact that they don't exist is not fatal to the question. One could
> easily imagine making one (or actually make it, for that matter).
I agree, but I dont think she was asking from the level of sophistiation you
presume, or else she would not have had to ask, in the first place!
jerry warner
Jon Isaacs wrote:
> >I refer you to the first sentence in the woman's question. "Does using a
> >larger
> >aperture telescope lend to a sense of more image scale?". The lady was
> >asking
> >for an accounting of the "subjective" exprience of image scale based on
>
> >exprience of image scale based on anything technical experts here at saa could
> offer - unfortunately she gave examples of 60mm f.20 vs 150mm f.8 scopes and
> we know 60mm f/20 scopes do not exist >in the real world of amateur astronomy!
>
> I think you underestimate the woman.
>
>
I have no idea who she is. If she was more sophisticated than I thought then why
was she asking at all?
> It is pretty clear she choose those numbers because she already understood the
> importance of focal length
Then why was she asking?
> and wanted to provide an example of two scopes with
> identical focal lengths but different apertures.
Then her question was about focal lengths ~ image scale, and she should have
said so - especially if she is this sophisticated. I did note she did not chime in
after all the side chatter began. Maybe you are correct and she already knew the
answer and was just soaking it all up. Maybe she was a troll? Maybe she was
Shawn Grant Esq. !
>
>
> >
> >I try to read between the lines with some people's posts.
>
> We all do. But the queston was clear:
OK. Her question used the word "sense of". Thats subjective. If hers was a
technical question then why couch it is subjectivism. I mean: was hers a cell phone
call at the Green Light or a Technical Question? I did have a 14 year old kid walk
up to me last summer and ask: "where is Mars?" then "Where is the Big Dipper?"
then "where is the Moon tonight". She grinned and shrugged at each answer. Turned
out she had a brand new phD in Astrophysics from MIT !
>
>
> ""Does using a larger aperture telescope lend to a sense of more image scale?
> Or is it purely a function of focal length? ""
>
> The OP was obviously aware of the effects of changing focal length.
Then why did she ask in plain Englitzsch! "Or is it purely a function of
focal length"?
> Thats why the suggestion to "forget the focal length" missed the target.
I take no responsibility for 'Mombo on the Ghombo in Clavius', if thats what you
are
referring to. HI!
Thanks,.
Jerry
>
>
> jon
jerry warner
David Knisely wrote:
> Jerry Warner wrote:
>
> > I refer you to the first sentence in the woman's question. "Does using a larger
> > aperture telescope lend to a sense of more image scale?".
>
> Actually, what she wanted to know was too subjective,
That's exactly what I thought too.
> so all that can be
> stated are the facts (which I did state). If both instruments of identical
> focal length are viewing something like the moon at low power with the same
> eyepiece, the "sense" would be that the moon would be the same size and with
> about the same amount of detail, since we aren't pushing the instruments
> towards their full resolving capabilities. The only difference would be image
> brightness.
>
> > unfortunately she gave examples of 60mm f.20 vs 150mm f.8 scopes and we know 60mm f/20 scopes do not exist in the
> > real world of amateur astronomy!
>
> Sure they do: a 60mm f/5 and a 4x Powermate.
David, that's a Jackalope Scope ... and snooker pool. Thats a poor man's Yiddish
telescope! Stop Hockim ne inde Chinukh!
> Why someone would want to do
> this is not clear, but she at least had some idea about the image scale being
> dependent on focal length.
Related to; not dependent. Thats how I took her level of understanding.
>
>
> > I pointed out this is a complex phenominon, ie her "sense of image scale", whereas
> > you stuck to her unrealistic example and explaind her "perceptions" in terms of ...
> > focal length.
>
> I did no such thing. I used her example and stated the facts about what
> happens at equal powers (ie: the same eyepiece used in two telescopes with
> equal focal lengths).
do did I.
> The "sense of image scale" was not defined by her.
No because she was asking for us to define it (explain it) for her -
> If
> she meant that the scale "seems" larger, I don't see it being any different as
> long as the initial conditions are the same. The only time the scale would
> "seem" larger is if the power was increased in one scope over the other.
and that is exactly where I took her because that is what I thought she was driving
at based on her common experience using a telescope, probably looking at extended
objects like the Moon.
> More
> power would be supported more fully by the larger aperture, so you could
> achieve more useful image scales with the larger aperture
exactly what I pointed out to her -
> (assuming the
> seeing supported it), but in terms of true image scale at the focal plane,
here it is again! She was not asking for "true image scale at the focal plane"
but "sense of image scale ..."
>
> that is only dependent on the focal lenght.
David, I took her lack of participation after a while as a sign of her being overwhelmed
with technicalities and attitudes. Maybe I am wrong. Maybe she slipped into a
dream state and went forth to find and buy a 60mm f/60 refractor from Ancortes?
But I was concerned she and her question had been lost, to rhetorical warming.
Jerry
BllFs6
>presume, or else she would not have had to ask, in the first place!
So.....
You assuming she's stupid....and the other guy is assuming she's reasonable
smart and sophisticated (and can ACTUALLY do math...)
So, I ask....who gave the more helpful and respectable replies to her question
then?
Blll