Light path exiting a telescope eyepiece or finder

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Date: 12 Sep 2006 15:21:28
From: James Harris
Subject: Light path exiting a telescope eyepiece or finder

Can anyone explain how light really passes through a telescope???
Opinions I've heard seem contradictory so I'd appreciate some
clarification. In particular (and this is the practical reason for
asking the question) when light exits an eyepiece does it emerge as a
cylinder or as a cone? If a cone is it narrowing - i.e. focussed on a
point after the eyepiece - or is it diverging - i.e. already past the
point of focus?

My local telescope supplier tells me the light is converging but I
doubt the human eye could focus on that. My view is that the light
should emerge as a cylinder (i.e. appearing at infinity) of diameter up
to the size of the pupil of the eye and that the lens of the eye
focusses this on to the retina just as it would when viewing a distant
object. The counterexample he gave is of eye relief where the distance
from the eyepiece matters. I guess there is something in that so am
puzzled. Can anyone shed some light (sic, sorry) on this?

--
Thanks,
James

Date: 12 Sep 2006 17:13:35
From: canopus56
Subject: Re: Light path exiting a telescope eyepiece or finder

James Harris wrote:

> Can anyone explain how light really passes through a telescope???
> <snip> My local telescope supplier tells me the light is
> converging but I doubt the human eye could focus on that.
> My view is that the light should emerge as a cylinder

You are both right. The image of extended objects like the Moon are
not formed only by a single bundle of parallel rays coming down the
telescope tube and out the eyepiece.

Such a cylinder of parallel light rays does exist. You can see it by
focusing a telescope on the Moon and then standing up to two feet
behind the eyepiece. You can still see a dim image of the Moon in the
eyepiece.

But there are two other key parallel beams of light traveling through
telescope that do come to a convergent focus.

Extended objects like the Moon, by definition, have an arcsecond size.
For an extended object like the Moon that size is about 1800
arcseconds. Some of the bundles of parallel beams of light rays come
from center of the object - that is your cylinder of light -

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope1.gif

- but some come from one side of the object at a slight angle of
divergence to the optical axis -

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope4.gif

At the intersection where these bundles of parallel light meet on the
observer side of the eyepiece -

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope5.gif

- is where the virtual image of the extended object forms -

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope7.gif

The distance between this virtual image and the eyepiece is the
eyepiece's eye relief distance.

You can see this effect by again focusing a telescope on the Moon. When
you stand back from the eyepiece there is a dim image formed by the
parallel set of rays travelling directly down optical axis of the
telescope. If you move your eye to the eyepiece at the eye relief
distance, more of the light beams with an angular divergence will enter
your eye pupil. The image will be brighter.

I recommend that you take a few minutes to play with a telescope ray
tracing Javascript applet, put on the web by Professor Mark Peterson of
Mount Holyoke College -

http://www.mtholyoke.edu/~mpeterso/classes/phys301/geomopti/twolenses.html

Using this ray tracing simulater, you can put three bundles of parallel
light through the telescope and angle two of them with respect optical
axis. Put one on the optical axis, a second parallel to the top of the
lens and a third parallel to the bottom of the lens. Do this by -

1) Selecting the "astronomical telescope" link to put a telescope in
the simulator.

2) Use the "Beam" button, to add two beams.

3) Once selected, there are drag "dots" on the beams that allow you to
position and angle them with respect to the optical axis.

4) After you are practiced at using the simulater add a virtual eye and
retina using the "add an 'eye' at the far right" link below the
simulator window. (I found easier to create the desired simulation by
manually adding another lens at the far right using the "Lens" button
and a field stop using the "Aperature" button.)

When using the Mark Peterson lens and telescope simulator, the
following is a screen shot of the type of telescope and eye you want to
try to create:

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope8.jpg

The apparent-field-of-view (AFOV) in the eyepiece is itself an extended
object of sorts.

Peace - Canopus56

Date: 12 Sep 2006 17:05:58
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

James Harris wrote:
> Can anyone explain how light really passes through a telescope???
> Opinions I've heard seem contradictory so I'd appreciate some
> clarification. In particular (and this is the practical reason for
> asking the question) when light exits an eyepiece does it emerge as a
> cylinder or as a cone? If a cone is it narrowing - i.e. focussed on a
> point after the eyepiece - or is it diverging - i.e. already past the
> point of focus?
>
> My local telescope supplier tells me the light is converging but I
> doubt the human eye could focus on that. My view is that the light
> should emerge as a cylinder (i.e. appearing at infinity) of diameter up
> to the size of the pupil of the eye and that the lens of the eye
> focusses this on to the retina just as it would when viewing a distant
> object. The counterexample he gave is of eye relief where the distance
> from the eyepiece matters. I guess there is something in that so am
> puzzled. Can anyone shed some light (sic, sorry) on this?

I hope so. I'm going to explain everything in terms of a refractor,
with lenses for both objective and eyepiece, but a similar explanation
holds for any design.

Suppose you point a telescope at a star. In the ray conception of
light, rays diverge from the star, but the star is so far away that by
the time the light reaches your telescope, the rays that enter your
telescope are as good as parallel. Since your objective is generally
circular, a cylinder of light from the star is what goes in.

The objective then refracts this cylinder into a cone, which converges
to a point at the focal plane. Since there's nothing there to stop the
light, it diverges again in a second, smaller cone, which terminates at
the eyepiece.

The eyepiece refracts this light a second time into a cylinder again,
and it is this cylinder that your eye lens (and cornea) refract a third
time into a third cone, whose point lies, hopefully, on your retina, at
which point your brain processes the signal into a mental picture of the
star.

Now, for a star at the center of the field of view, all the cones and
cylinders have an axis of symmetry that is identical with the axis of
the telescope itself. This is what you typically see in telescope
cutaway diagrams. The situation is somewhat different for stars at the
edge of the field. Suppose that you point your telescope slightly above
the star. In that case, the cylinder of light is a bit askew; when it
reaches the objective, it does so "from below," relative to the scope's
axis, and the light is moving slightly upward.

The objective refracts it into a cone of light, but the cone of light
is also pointed slightly upward, so that it converges to a point on the
focal plane that is slightly higher than before. The diverging cone is
also still headed upward, and it reaches the eyepiece well above center.

The periphery of an eyepiece refracts light more than its center, since
the angles are steeper there. The eyepiece refracts the diverging cone
into a cylinder, as before, but now the cylinder starts from the top of
the eyepiece and is heading *downward*. That is why images in a
refractor (without a star diagonal) are inverted; light entering the
objective from below exits the eyepiece from above.

This means that if you have stars all over the field of view, each one
generates a final cylinder of light emanating from the eyepiece. Each
cylinder starts from a different circular base on the eyepiece--else,
the stars would appear to coincide--but since the ones near the top are
headed downward, and the ones near the bottom are headed upward, they
all converge to a disc. This disc is separated from the eyepiece by a
distance called the *eye relief*. It is at this distance that it is
easiest to fit all the light cylinders into your eye's pupil, so that
one can see *all* the stars (or more generally, the entire field of
view).

This explains why eye position is so critical when using low power. If
you use high power, the exit pupil is small because the cylinders are
so small. The eye relief is the same, but since the cylinders are thin,
you can be too far forward, backward, or off to the side, and the
cylinders still all get into your eye, allowing you to see the entire
image. It is at low power where the cylinders are so wide that any
individual one barely gets into your eye in the first place. Any kind
of misalignment, and many of them simply won't get in, and you see a
black splotch over part of the field of view.

Hope that helped. Let me know if you have any further questions.

--
Brian Tung <brian@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.html

Date: 13 Sep 2006 00:30:08
From: Tim Auton
Subject: Re: Light path exiting a telescope eyepiece or finder

James Harris <groupstudy2001@yahoo.co.uk > wrote:

> Can anyone explain how light really passes through a telescope???
> Opinions I've heard seem contradictory so I'd appreciate some
> clarification. In particular (and this is the practical reason for
> asking the question) when light exits an eyepiece does it emerge as a
> cylinder or as a cone? If a cone is it narrowing - i.e. focussed on a
> point after the eyepiece - or is it diverging - i.e. already past the
> point of focus?
>
> My local telescope supplier tells me the light is converging but I
> doubt the human eye could focus on that.

Can you see the whole of your monitor without moving your eyes? Most
people can. Trace the lines from the edges of the monitor to your eye
and you'll see that the human eye can indeed focus a converging light
cone.

Tim

Date: 12 Sep 2006 17:12:04
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

Tim Auton wrote:
> Can you see the whole of your monitor without moving your eyes? Most
> people can. Trace the lines from the edges of the monitor to your eye
> and you'll see that the human eye can indeed focus a converging light
> cone.

Uhh, no. Think about how you see an individual pixel. Light rays
emanate from that pixel and are focused by your eye to a point on your
retina. Light from another pixel diverges from *that* pixel and is
focused to a second point on your retina. And so on. Your eye does
not take light from the entire screen and focus that down to a point;
that would mix the light from the whole screen, which would produce an
indiscriminate blur, if in fact your eyes could do it.

Your eye lenses are convex. They cannot focus light that converges
strongly--certainly not anything that would converge to your retina
without the lens being there. At best, they can focus very weakly
converging light. Fortunately, most objects emit only diverging light;
it takes an optical element, such as a magnifying glass, to produce
converging light. That is why you only see a blur if you look around
the room with a magnifying glass held up to your eye.

--
Brian Tung <brian@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.html

Date: 12 Sep 2006 17:04:27
From: Willie R. Meghar
Subject: Re: Light path exiting a telescope eyepiece or finder

"James Harris" <groupstudy2001@yahoo.co.uk > wrote:

>In particular (and this is the practical reason for
>asking the question) when light exits an eyepiece does it emerge as a
>cylinder or as a cone?

The light exiting the eyepiece converges until reaching the exit
pupil. The diameter of the exit pupil is equal to the telescope's
effective aperture divided by the magnification of the
telescope/eyepiece combination. The light diverges as it moves
further from the eyepiece than the exit pupil.

Willie R. Meghar

Date: 13 Sep 2006 07:05:57
From: John Carruthers
Subject: Re: Light path exiting a telescope eyepiece or finder

Date: 13 Sep 2006 17:13:13
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

John Carruthers wrote:
> Hi james, check it out yourself, try focussing on the moon then hold a
> piece of frosted glass or tracing paper behind the eyepiece. Does the
> image diameter increase as you move the screen further from the
> eyepiece ?

No image is formed on the frosted glass, if the telescope is in focus;
a telescope is an afocal system. What you see instead is a convolution
of pencils of light formed over the entire illuminated surface of the
Moon. There will essentially be no detail visible there.

--
Brian Tung <brian@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.html

Date: 13 Sep 2006 22:00:56
From: OG
Subject: Re: Light path exiting a telescope eyepiece or finder

"John Carruthers" <joncarruthers@hotmail.com > wrote in message
news:1158156357.243654.265600@h48g2000cwc.googlegroups.com...
>
> James Harris wrote:
>> Can anyone explain how light really passes through a telescope???
>> Opinions I've heard seem contradictory so I'd appreciate some
>> clarification. In particular (and this is the practical reason for
>> asking the question) when light exits an eyepiece does it emerge as a
>> cylinder or as a cone? If a cone is it narrowing - i.e. focussed on a
>> point after the eyepiece - or is it diverging - i.e. already past the
>> point of focus?
>>
>
> Hi james, check it out yourself, try focussing on the moon then hold a
> piece of frosted glass or tracing paper behind the eyepiece. Does the
> image diameter increase as you move the screen further from the
> eyepiece ?
> jc

James
The light from the distant object should be emerging in a parallel sided
cylinder when the telescope is correctly focussed at a distant object.

John
I'm not sure what 'image' you would get using the method described - I
suspect the nearest to an image you could bring to focus would be that of
the Objective behind the eyepiece.

Date: 14 Sep 2006 05:03:50
From: Helpful person
Subject: Re: Light path exiting a telescope eyepiece or finder

Date: 14 Sep 2006 07:12:09
From: Jonathan Silverlight
Subject: Re: Light path exiting a telescope eyepiece or finder

In message <eea6qp$niq$1@praesepe.isi.edu >, Brian Tung <brian@isi.edu>
writes
>John Carruthers wrote:
>> Hi james, check it out yourself, try focussing on the moon then hold a
>> piece of frosted glass or tracing paper behind the eyepiece. Does the
>> image diameter increase as you move the screen further from the
>> eyepiece ?
>
>No image is formed on the frosted glass, if the telescope is in focus;
>a telescope is an afocal system. What you see instead is a convolution
>of pencils of light formed over the entire illuminated surface of the
>Moon. There will essentially be no detail visible there.
>

I think there's some confusion here. John Carruthers is describing what
amounts to a perfectly standard method of taking astronomical photos,
using eyepiece projection. It is a real image in the optical sense.
ISTR that the eye does not focus on this real image. It normally sees a
cylinder of light, effectively an object at infinity, but it is possible
to focus on a closer point using light that is not parallel.

Date: 14 Sep 2006 09:33:24
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

Jonathan Silverlight wrote:
> I think there's some confusion here. John Carruthers is describing what
> amounts to a perfectly standard method of taking astronomical photos,
> using eyepiece projection. It is a real image in the optical sense.

It's possible, but I think it's pretty unlikely. He said to focus the
telescope on the Moon *first*, and *then* put a piece of frosted glass
behind the eyepiece. That is not what you do when you do positive
projection. Besides, the original question seemed definitely to involve
visual use--not imaging.

With a refractor, when you do positive projection, you must rack the
focuser further outward, so that the outgoing light is not collimated,
but rather converges to a point on the image plane of the camera or CCD.
You must focus on the Moon with the camera/CCD already in place (unless
you have some focusing gimcrack that you use first), and without a lens.
If you focus the telescope on the Moon first, you'll have to take the
image afocally.

It is a common misconception that there is an image at a distance behind
the eyepiece equal to the eye relief. There is no image there; that is
simply where all the parallel cylinders of light meet in a spot that is
conveniently placed for your eye to capture them.

--
Brian Tung <brian@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.html

Date: 14 Sep 2006 13:13:49
From: Willie R. Meghar
Subject: Re: Light path exiting a telescope eyepiece or finder

(Brian Tung) wrote:

>It is a common misconception that there is an image at a distance behind
>the eyepiece equal to the eye relief. There is no image there; that is
>simply where all the parallel cylinders of light meet in a spot that is
>conveniently placed for your eye to capture them.

Hi Brian,
The above statement clearly implies that the exit pupil (located at a
distance behind the eyepiece equal to the eye relief) is *not* an
image of the entrance pupil.

Perhaps you meant to say:

"It is a common misconception that there is an image *of the object
upon which the telescope is focused* at a distance behind the eyepiece
equal to the eye relief . . . etc."

Either your statement is inaccurate or my understanding of entrance
and exit pupils is in need of repair.

Willie R. Meghar
http://meghar.blogspot.com/

Date: 14 Sep 2006 12:38:21
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

I (Brian Tung) wrote:
> It is a common misconception that there is an image at a distance behind
> the eyepiece equal to the eye relief. There is no image there; that is
> simply where all the parallel cylinders of light meet in a spot that is
> conveniently placed for your eye to capture them.

Willie R. Meghar wrote:
> The above statement clearly implies that the exit pupil (located at a
> distance behind the eyepiece equal to the eye relief) is *not* an
> image of the entrance pupil.

Well, it isn't. The exit pupil is not an image of anything, when the
telescope is used afocally.

> Either your statement is inaccurate or my understanding of entrance
> and exit pupils is in need of repair.

If your concept of the exit pupil is as an image of the entrance pupil,
then yes, I think it is need of repair. The entrance pupil is not an
object or an image, so you can hardly create an image of it. It is
merely a boundary, typically circular, that all of the light entering a
telescope goes through. Correspondingly, the exit pupil is a boundary,
typically circular, that all of the light *exiting* the telescope goes
through.

For our purposes, one can only create an image of an object, or another
image. When used visually, a telescope is pointed at a distant object.
A real image of that object is created at the common focal plane of both
objective and eyepiece. All the light from any single point of the
object ends up at the corresponding single point on the real image. It
then goes through the image--there being nothing in the telescope to
stop it--and is refracted again by the eyepiece into parallel rays,
which are then focused by your eye's lens and cornea into a third
corresponding single point of a real image on your retina.

You may want to take a look at the image cited by Stephen Tonkin earlier
in this thread. I haven't seen it myself, but based on Stephen's
description, I think it will reveal what I'm talking about better,
perhaps, than mere words.

--
Brian Tung <brian@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.html

Date: 14 Sep 2006 20:29:40
From: Willie R. Meghar
Subject: Re: Light path exiting a telescope eyepiece or finder

(Brian Tung) wrote:

>If your concept of the exit pupil is as an image of the entrance pupil,
>then yes, I think it is need of repair.

Hi Brian,
Your words have failed to convince me that the concept of looking upon
an exit pupil as the image of an entrance pupil is in need of repair.

I'll attempt to better explain this concept (which is *not* contrary
the rest of your own view).

I have made changes in a telescope's entrance pupil and seen a
corresponding change in the exit pupil. For example, when a hexagonal
mask is placed over a telescope's entrance pupil, effectively changing
the shape of the entrance pupil, the exit pupil will undergo a similar
change and take on a hexagonal shape.

I have placed a transparent ruler across the entrance pupil and
determined a telescope's effective aperture by reading the markings of
that ruler as seen in the exit pupil. (The ruler's "image" (for lack
of a better word;-) in the exit pupil can be magnified using a
magnifying glass.)

I have moved my eye from side to side while examining an exit pupil,
including exit pupils that have had "images" (for lack of a better
word;-) of objects such as rulers and similar devices that were
intentionally placed at the entrance pupil in them. In all cases the
exit pupil, along with the "image" of whatever object was placed
across the entrance pupil displayed parallax just as an object placed
at the same location as the exit pupil would have displayed parallax.

The outer edge of a telescope's entrance pupil is a *real, physical
object*. Whatever shape that edge may have, the exit pupil will also
have.

You can modify an actual telescope's entrance pupil in any way and see
for yourself if the exit pupil undergoes a similar change. Place an
"X" on a refractor's objective and you'll see an "x" in the exit
pupil.

I think the reason for our disagreement is that you think that your
view of what an exit pupil is *excludes* the possibility that an exit
pupil is *also* an image of the entrance pupil. I don't believe your
explanation excludes this possibility.

I've not disagreed with *most* of your explanation concerning what an
exit pupil is. (Thus there's no need to respond by defending the rest
of your view). I have disagreed with the specific statement that an
exit pupil is not an image of anything.

I no longer have the posting that gives the URL you made reference to;
but unless the information on that site provides evidence that the
exit pupil is *not* an image of the entrance pupil, it will do nothing
in resolving our differences. It matters not at all if the site
provides an accurate and correct interpretation of the exit pupil
*unless* it also shows that an exit pupil *cannot be* the image of the
entrance pupil.

I don't recall were I first heard mention of the exit pupil being an
image of the entrance pupil (I did not originate the concept); but my
own experimentation has subsequently convinced me of its correctness.

Any outlook that contradicts the view I mentioned will need to explain
the above experimental results without the use if images. It will
need to explain how some phenomena can display all the properties of
an image without being an image.

Your previous posting has failed to provide anything to convince me
that an exit pupil *is not* the image of the entrance pupil. If this
reply fails to convince you that an exit pupil *is* the image of the
entrance pupil, then we may have to simply agree to disagree, place
this issue aside, and look forward to some future topic in which we
can be in agreement!

Willie R. Meghar
http://meghar.blogspot.com/

Date: 15 Sep 2006 09:23:24
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

Willie R. Meghar wrote:
> Your words have failed to convince me that the concept of looking upon
> an exit pupil as the image of an entrance pupil is in need of repair.

Hmm, OK, I know I'm posting entirely too many responses, but I'm going
to recant everything I said (for the moment, at least). There are a few
things that don't make sense here--your assertions, elementary ray
traces, Paul's comment, and my observations with the ruler, all of which
seem to disagree on one point or another. I'm going off to the woodshed
(metaphorically speaking) and working this thing out.

I'll let you know how it comes out.

--
Brian Tung <brian@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.html

Date: 14 Sep 2006 23:22:21
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

Willie R. Meghar wrote:
> Your words have failed to convince me that the concept of looking upon
> an exit pupil as the image of an entrance pupil is in need of repair.

That my words have failed to convince you is my fault, but the fact
remains that your concept is in need of repair. :)

> I have made changes in a telescope's entrance pupil and seen a
> corresponding change in the exit pupil. For example, when a hexagonal
> mask is placed over a telescope's entrance pupil, effectively changing
> the shape of the entrance pupil, the exit pupil will undergo a similar
> change and take on a hexagonal shape.

That is in accordance with the laws of optics; nonetheless, neither of
the pupils is an image. Just because a change in the entrance pupil
produces the corresponding change in the exit pupil does not make the
latter an image of the former.

> I have placed a transparent ruler across the entrance pupil and
> determined a telescope's effective aperture by reading the markings of
> that ruler as seen in the exit pupil. (The ruler's "image" (for lack
> of a better word;-) in the exit pupil can be magnified using a
> magnifying glass.)

Yes. However, what you are observing is not a real image represented
by the exit pupil. You can verify this as follows. Point the telescope
at a varied background. Now step back so that you see the exit pupil
sharply. Notice that what you see of the background "through" the exit
pupil changes as you move your eye from side to side. If the exit pupil
were an actual image, what you see would stay the same.

By moving your eye around sufficiently, you can eventually capture the
entire field of view, which is what you could get at a single glance if
you were to put your eye's pupil right *at* the exit pupil.

Now, when you put a ruler right up against the objective, an image of
that *is* formed. I hesitate to say it's an image of the entrance
pupil--I'd call it the image of the ruler--but to the extent that there
is something physically at the entrance pupil, an image of that is
formed. However, that image is *not* formed at the exit pupil, whose
distance from the eyepiece is equal to the eye relief. Rather, the
image of the ruler is formed at a distance equal to the *focal length*
of the eyepiece.

For instance, if you use a 6 mm Radian, which has a focal length of
6 mm (of course) but an eye relief of 20 mm, the image of the ruler is
actually formed at 6 mm from the eyepiece, but the exit pupil is at
20 mm. The exit pupil is merely the imaginary opening through which
you can see the image of the ruler, which is between the eyepiece and
the exit pupil. In some cases, the eye relief is approximately equal
to the focal length, and then the image appears right at or near the
exit pupil--but that is just coincidence. There is no particular
reason that the image of the ruler needs to be right at the exit pupil.
It might be either in front of or in back of it, depending on whether
the eye relief is greater than or less than the focal length. When the
two distances are unequal, you can detect the difference by noting that
the portion of the ruler you see changes ever so slightly as you move
your eye from side to side.

> Your previous posting has failed to provide anything to convince me
> that an exit pupil *is not* the image of the entrance pupil. If this
> reply fails to convince you that an exit pupil *is* the image of the
> entrance pupil, then we may have to simply agree to disagree, place
> this issue aside, and look forward to some future topic in which we
> can be in agreement!

As I say, if my words aren't convincing to you, then my words are at
fault. However, despite their not being convincing to you, they *are*
correct. If you still desire convincing, I recommend that you do some
ray tracing with pencil and paper and see if that doesn't help. If
not, then--as you say--I am willing to put the matter aside.

--
Brian Tung <brian@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.html

Date: 14 Sep 2006 23:57:07
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

I (Brian Tung) wrote:
> Yes. However, what you are observing is not a real image represented
> by the exit pupil. You can verify this as follows. Point the telescope
> at a varied background. Now step back so that you see the exit pupil
> sharply. Notice that what you see of the background "through" the exit
> pupil changes as you move your eye from side to side. If the exit pupil
> were an actual image, what you see would stay the same.

In the interests of full accuracy, I should add that this isn't always
true in general. For instance, because the real image formed by a
telescope is only illuminated by the objective, you don't see the entire
image equally illuminated, and some of it may be vignetted altogether.
However, it is true in this particular case. If the exit pupil were
the image, then as long as you could see the exit pupil, it would show
the same details--which it doesn't.

> As I say, if my words aren't convincing to you, then my words are at
> fault. However, despite their not being convincing to you, they *are*
> correct. If you still desire convincing, I recommend that you do some
> ray tracing with pencil and paper and see if that doesn't help. If
> not, then--as you say--I am willing to put the matter aside.

Heh. I should probably qualify that. They're correct as far as they
go. There's a lot that I've simplified for the sake of brevity. I
haven't simplified anything that essentially invalidates what I've
written, though.

--
Brian Tung <brian@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.html

Date: 15 Sep 2006 07:43:18
From: Paul Schlyter
Subject: Re: Light path exiting a telescope eyepiece or finder

In article <eedis3$39c$1@praesepe.isi.edu >, Brian Tung <brian@isi.edu> wrote:
>I (Brian Tung) wrote:
>> Yes. However, what you are observing is not a real image represented
>> by the exit pupil. You can verify this as follows. Point the telescope
>> at a varied background. Now step back so that you see the exit pupil
>> sharply. Notice that what you see of the background "through" the exit
>> pupil changes as you move your eye from side to side. If the exit pupil
>> were an actual image, what you see would stay the same.
>
>In the interests of full accuracy, I should add that this isn't always
>true in general. For instance, because the real image formed by a
>telescope is only illuminated by the objective, you don't see the entire
>image equally illuminated, and some of it may be vignetted altogether.
>However, it is true in this particular case. If the exit pupil were
>the image, then as long as you could see the exit pupil, it would show
>the same details--which it doesn't.

The exit pupil is the image of the objective, formed by the eyepiece.

>> As I say, if my words aren't convincing to you, then my words are at
>> fault. However, despite their not being convincing to you, they *are*
>> correct. If you still desire convincing, I recommend that you do some
>> ray tracing with pencil and paper and see if that doesn't help. If
>> not, then--as you say--I am willing to put the matter aside.
>
>Heh. I should probably qualify that. They're correct as far as they
>go. There's a lot that I've simplified for the sake of brevity. I
>haven't simplified anything that essentially invalidates what I've
>written, though.
>
>--
>Brian Tung <brian@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.html

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

Date: 15 Sep 2006 09:50:32
From: Brian Tung
Subject: Re: Light path exiting a telescope eyepiece or finder

Willie R. Meghar wrote:
> Your words have failed to convince me that the concept of looking upon
> an exit pupil as the image of an entrance pupil is in need of repair.

All right, I abjure. I think. There are still facets of my
observations that bother me, such as my results with the ruler, which I
will for now chalk up to some aberration (maybe spherical aberration of
the exit pupil? but that doesn't quite make sense either), or the
relationship between eye relief and focal length. Those can wait. At
any rate, I hereby take back all those nasty things I said about you,
and I also acknowledge your right to say all kinds of nasty things about
me. :)

--
Brian Tung <brian@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.html

Date: 15 Sep 2006 17:27:24
From: Willie R. Meghar
Subject: Re: Light path exiting a telescope eyepiece or finder

(Brian Tung) wrote:

>All right, I abjure. I think. There are still facets of my
>observations that bother me, such as my results with the ruler, which I
>will for now chalk up to some aberration (maybe spherical aberration of
>the exit pupil? but that doesn't quite make sense either), or the
>relationship between eye relief and focal length. Those can wait. At
>any rate, I hereby take back all those nasty things I said about you,
>and I also acknowledge your right to say all kinds of nasty things about
>me. :)

Hi Brian,
Back in the days . . . Chris 1011 might have chimed in and possibly
corrected both of us!

It's been a while since I've played around (or experimented) with exit
pupils, rulers, etc. (Most of the time I prefer to study things that
reside far away on the other side of the eyepiece:-) I'm not certain
concerning an explanation of your experimental results. If we were
neighbors we could get together in a manner in which we could both see
the exact experimental set up as well as the results -- though that
*might* just mystify both of us! Then what? We could end up taking
the telescopes off their optical benches and looking upward ;-)

What nasty things? We had a mutual disagreement. It's not like you
condemned anyone and everyone who uses a pseudonym;-)

Willie R. Meghar
http://meghar.blogspot.com/

Date: 13 Sep 2006 22:40:19
From: sftonkin
Subject: Re: Light path exiting a telescope eyepiece or finder

Brian Tung <brian@isi.edu > wrote:
[snip excellent explanation]

Just to expand on this with a diagram, the light from a single star
would follow the path shown in the diagram at the top of:
http://www.astunit.com/tutorials/telescope.htm

If you now imagine a star below the optical axis, its rays would form a
pattern symmetrical with that shown in the diagram, the line of
symmetry being the optical axis (grey horizontal line). You will see
that the bundles of rays from the two stars cross each other on the eye
side of the eyepiece. The smallest circle through which they all pass
is the position of the exit pupil. (It is in the position of the image
of the objective formed by the eyepiece.)

Date: 14 Sep 2006 22:15:18
From: canopus56
Subject: Re: Light path exiting a telescope eyepiece or finder

canopus56 wrote:

To correct text:
<snip >
> At the intersection where these bundles of parallel light meet on the
> observer side of the eyepiece -
> http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope5.gif
> - is where the virtual image of the extended object forms -
> http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope7.gif
> The distance between this virtual image and the eyepiece is the
> eyepiece's eye relief distance.

should read -

At the intersection where these bundles of parallel light meet on the
observer side of the eyepiece -

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope5.gif

- is where the _real_ image of the extended object forms -

http://members.csolutions.net/fisherka/astronote/astromath/Telesim/Telescope7.gif

The distance between this _real_ image and the eyepiece is the
eyepiece's eye relief distance.

- Canopus56

Date: 15 Sep 2006 14:00:00
From: James Harris
Subject: Re: Light path exiting a telescope eyepiece or finder

James Harris wrote:
> Can anyone explain how light really passes through a telescope???

As the OP I'd like to say thanks for the explanations and the web
links. That was exactly the kind of info I was looking for but hadn't
been able to find. My mental image was missing the off-axis rays of
light and how they pass through the optics. I now know I cannot simply
put a 45degree mirror to my finderscope (what I wanted to do) and why I
cannot place a camera lens too far away from an eyepiece. I think I can
also understand the difference between afocal and prime focus
photography.

I can't pretend I understand it all yet but I'm getting there. It is
surprisingly challenging to understand the whole picture! - as I think
some other posts have confirmed.

Thanks for the help.