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GlennLeDrew
Postmaster

Reged: 06/18/08

Why a too-small iris results in reduced aperture

In a couple of recent threads I've laboured to describe why a too-small iris (or too-large exit pupil) reduces the aperture of a telescope. Time for a picture, the worth of which the old proverb tells us.

A few introductory notes:

1) The exit pupil is a real image of the entrance pupil (nominally the objective) formed by the eyepiece, and located in space behind the eyepiece at the eye point distance. This is where your iris must be located in order to enjoy a full and evenly illuminated field.

2) The eypeice is an optical coupler which places the reduced objective onto the plane of the iris. But this is a two-way street; the eyepiece also projects anything in the plane of the exit pupil onto the plane of the entrance pupil (objective.)

3) As long as the iris is as large or larger than the exit pupil, the full aperture of the objective is utilized.

4) When the iris becomes smaller than the exit pupil, an aperture stop is introduced into the system. The eyepiece projects the iris aperture onto the objective, which is exactly like placing a physical aperture mask of the same size there. If the iris is half the diameter of the exit pupil, its image on the objective is half the diameter of the objective.

Due to the location of the projected aperture stop in the plane of the objective, all light from all parts of the field must necessarily pass through this one and only reduced aperture; the area of the objective outside this contributes absolutely nothing to image formation.

5) When the too-small iris moves about in the larger exit pupil, its image also moves about on the objective. If the iris meets the edge of the exit pupil, its projected image meets the edge of the objective, on the opposite side. If the iris half overlaps the exit pupil (50% clipped), its image is half inside-half outside the opposite objective edge.

As before, no matter where the reduced aperture is projected onto the objective, the area outside the projected iris does not contribute in any way to image formation, anywhere in the field of view.

One last remark, about the bottom panel in the illustration which deals with the too small iris/too-large exit pupil. The shaded region bounded by the red rays lying between objective and exit pupil is the full envelope of light fielded by the eyepiece as it forms an image of one point on the edge of the iris onto its corresponding point onto the objective. And conversely, a point on the objective projected by the eyepiece onto its corresponding point on the exit pupil/iris.

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Chuck Hards
Carpal Tunnel

Reged: 05/03/10

Loc: The Great Basin
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6044403 - 08/24/13 11:30 PM

Well, you were busy today!

Nice illustration, thanks.

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Stacy
Star Partyer

Reged: 09/15/02

Loc: Seattle, WA
Re: Why a too-small iris results in reduced aperture [Re: Chuck Hards]
#6044530 - 08/25/13 01:24 AM

So my new 7x50's are really like 7x30 because of course my iris will never be 7.1mm?

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KennyJ
The British Flash

Reged: 04/27/03

Loc: Lancashire UK
Re: Why a too-small iris results in reduced aperture [Re: Stacy]
#6044621 - 08/25/13 02:57 AM

--- And the greatest benefit of all this is that the reduced effective aperture proportionally increases effective focal ratio, reducing astigmatism and other aberrations and producing much "sharper", "tighter" images than if the system were operating at full aperture.

Kenny

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Stacy
Star Partyer

Reged: 09/15/02

Loc: Seattle, WA
Re: Why a too-small iris results in reduced aperture [Re: KennyJ]
#6044633 - 08/25/13 03:13 AM

Quote:

--- And the greatest benefit of all this is that the reduced effective aperture proportionally increases effective focal ratio, reducing astigmatism and other aberrations and producing much "sharper", "tighter" images than if the system were operating at full aperture.

Kenny

That sounds like a good thing! So all is not lost. That's great because it's not unlike a lot of the other choices and trade-offs we all make with different optical systems. So many different systems and configurations each with it's own best application.

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bremms
Carpal Tunnel

Reged: 08/31/12

Loc: SC
Re: Why a too-small iris results in reduced aperture [Re: Stacy]
#6044960 - 08/25/13 10:53 AM

You can see your exit pupil in a defocused star image. move your eye around a little when you have a 6-5mm exit pupil in the scope. you will see the iris cutting of edges of the light cone. I do have a 7mm exit pupil but the outer mm or has a lot more abberations than the rest. Using a 5-6mm exit pupil with 0.5 diopter astigmatism correction works great for me.

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MessiToM
scholastic sledgehammer

Reged: 12/21/09

Loc: Huntingdon PA
Re: Why a too-small iris results in reduced aperture [Re: bremms]
#6045266 - 08/25/13 02:09 PM

Hmm average exit pupil for a 29 yr old (me)is 6.9mm. My 32mm 2" ep gives me a exit pupil of 7.17mm

Guess Iam missing out on some. BUT maybe this helps with coma near the edge of fov?

I used this calculator http://www.stargazing.net/naa/scopemath.htm

Edited by MessiToM (08/25/13 02:09 PM)

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Chuck Hards
Carpal Tunnel

Reged: 05/03/10

Loc: The Great Basin
Re: Why a too-small iris results in reduced aperture [Re: MessiToM]
#6045278 - 08/25/13 02:15 PM

The beauty of this is that it is easily testable. Install a temporary central obstruction, leaving only a thin annulus around the circumference of the objective. Swap out eyepieces until you end up with one that produces an exit pupil that forms an image.

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GeneT
Ely Kid

Reged: 11/07/08

Loc: South Texas
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6045575 - 08/25/13 05:34 PM

Good job. Nice explanation.

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sopticals
professor emeritus

Reged: 03/28/10

Loc: New Zealand
Re: Why a too-small iris results in reduced aperture [Re: Chuck Hards]
#6046430 - 08/26/13 05:34 AM

Quote:

The beauty of this is that it is easily testable. Install a temporary central obstruction, leaving only a thin annulus around the circumference of the objective. Swap out eyepieces until you end up with one that produces an exit pupil that forms an image.

Very good tip. Must try it sometime.

Stephen.(45deg.S.)

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highertheflyer
professor emeritus

Reged: 07/08/05

Loc: Ft. Worth, Texas
Re: Why a too-small iris results in reduced aperture [Re: sopticals]
#6046685 - 08/26/13 10:23 AM

I have an adjustable iris salvaged from an old camera, with the opening from 50mm fully opened to 3mm fully closed.
Does this iris need to be positioned at the focal point of its primary mirror, or can it be placed in the light cone between the mirror and the eyepiece?
Thx, Jim

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: highertheflyer]
#6046740 - 08/26/13 10:47 AM

Placing an iris at the focus serves only to change the field size. It would be exactly like having an adjustable field stop in the eyepiece.

The ideal place for an iris is at the entrance pupil, or the objective. But then it has to be as large as the objective. One could place the iris somewhere between the objective and the focus. But then when stopping down the light cone, it becomes the entrance pupil. The closer to the focus this iris lies, the worse the vignetting it introduces.

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6047059 - 08/26/13 02:05 PM

Glenn, digital cameras today come almost exclusively with zoom lenses. It seems the zoom element(s) optically act as an iris, which would make them less than ideal imaging devices for astrophotography except that they help stop down the front aperture and thereby reduce all aberations (as well as theoretical resolution, which is not important in snapshots of the sky). So, in a roundabout way, it is beneficial.

As we have seen in a related thread recently, an afocal image seems to magically "wipe out" chromatic aberration in an otherwise poorly corrected system. Of course, the effective relative aperture is very small and the focal ratio is artificially increated to over f/100!), but ti is no wonder that afocal astrophotography has become popular, since even bad telesciopes produce good images due mainly to pupil issues.

My main interest in optics is in the Bath interferometry and the iris inherent in the digital cameras today was a big problem (especially with the edge of the optic being tested) until I obtained a mirrorless digital camera with a fixed f.l. 35 mm film lens. These old lenses a their fastest setting have only the front aperture as the iris and produce completely unvignetted interferograms.

The zoom lens that came with the camera (22-55 mm) has a 7 mm pupil. My fixed f.l. 50 mm "normal" lens set at f/2 has an exit pupil of about 25 mm.

Regards,

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highertheflyer
professor emeritus

Reged: 07/08/05

Loc: Ft. Worth, Texas
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6047193 - 08/26/13 03:18 PM

Quote:

Placing an iris at the focus serves only to change the field size. It would be exactly like having an adjustable field stop in the eyepiece.

The ideal place for an iris is at the entrance pupil, or the objective. But then it has to be as large as the objective. One could place the iris somewhere between the objective and the focus. But then when stopping down the light cone, it becomes the entrance pupil. The closer to the focus this iris lies, the worse the vignetting it introduces.

So may I understand that when a large adjustable iris aperture is placed 50mm's forward of the eyepiece field stop and in the line of the light path towards the objective; that constricting with adjustable's smaller than actual eyepiece field stop diameters, there may be a gain in advantage too?
I have an adjustable iris much smaller than the objective diameter in the junk box, and wish to experiment as you might suggest.
Thanks,
Jim

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MitchAlsup
Pooh-Bah

Reged: 08/31/09

Re: Why a too-small iris results in reduced aperture [Re: highertheflyer]
#6047312 - 08/26/13 04:43 PM

Glenn forgot, also, that the part of the iris that is illuminated (but does not make it to the retina, creates a background glow that diminushes contrast.

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SACK
member

Reged: 08/11/11

Loc: TX
Re: Why a too-small iris results in reduced aperture [Re: MitchAlsup]
#6047756 - 08/26/13 09:30 PM

Hi Mitch,
How does the lit retina create the background glow? How bad is it? I have not noticed before but sounds interesting and good to know for eyepiece selection.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: SACK]
#6047899 - 08/26/13 10:57 PM

Jim,
An iris a mere 50mm in front of the focus would introduce *very* pronounced vignetting. The effect would be more like a very blurred field stop edge.

For example, let's consider an f/5 objective. At 50mm from the focus, the light cone for an image point is 10mm wide. In order to stop down an on-axis image point, the iris opening would have to be less than 10mm wide. Illumination immediately off-axis would fall away steeply, and be zero at not much more than 5mm off axis.

To maintain the same degree of image diminution across the field, the iris must lie at the objective. Otherwise vignetting results, becoming more severe the closer to the focus the iris is located.

Mitch,
I too wonder about the lit portion of the iris introducing a contrast/robbing glow. Is it because of some light is filtering through the iris (which I thought was rather opaque)? If this were the case, then how about normal daytime views as you walk about outdoors? Here the iris is illuminated over its full surface, over an angle approaching 180 degrees, and where the opening is near its smallest; conditions which one would think would be the most conducive to this effect. And of course looking toward the Sun would be the worse case. However, I think the glare here is merely the scatter occurring in the transmissive parts of the eye, not light filtering through the iris.

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Crayfordjon
Vendor - Zerochromat

Reged: 06/17/09

Loc: UK
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048116 - 08/27/13 02:02 AM

An iris that is too small will result is a reduction of aperture, but as the iris is at the eye end of the system and not at the OG, the full Aperture is still used, the smaller iris induced apertures will sweep over the OG as the field angle is scanned. This is how a camers lens works. Your optical diagram is drawn strictly along the optical axis and does not take into consideration the field diameter of he image plane. This is a concept I have been labouring in a recent thread without success.

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: Why a too-small iris results in reduced aperture [Re: MKV]
#6048129 - 08/27/13 02:15 AM

Here is a good article on the exit pupil and its effects.

Exit Pupil Effects

If the exist pupil is larger than the sensor, be it your eye or a camera, vignetting will result. It's very likely that both Glenn and John are correct, but are talking considering different configurations. It would help if each made an accurate graphic representation of what they are talking about for direct comparison.

regards,

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: MKV]
#6048165 - 08/27/13 03:18 AM

Jon,
As I stated, for the reduced iris condition, as the small iris moves laterally across the larger exit pupil, the reduced aperture at the objective correspondingly sweeps across the objective. And so, yes, the full aperture *can* be utilized, but *never at any one time.* If the iris is 1/2 the exit pupil, never more than 1/2 the objective contributes to image formation. As the iris wanders, that 1/2 objective diameter reduced aperture region also wanders about the objective.

I guess I'll have to prepare a diagram like that in the bottom panel, but for the case where the iris is abutting the edge of the exit pupil (as opposed to being in the center.) Then one would see that the reduced aperture at the objective is still just as small, but offset so as to abut the objective edge.

This in no way means that the objective is ever working at full aperture. Just because one has the freedom to let the iris wander, and thereby utilize different parts of the objective at different times, is not the same as utilizing the full objective at one instant. And so at any one time the instrument is always working at reduced aperture.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048171 - 08/27/13 03:29 AM

Malden,
I've already made a precisely accurate diagram which well illustrates the concept, in the bottom panel. Another version based on the iris having moved to the edge of the exit pupil will be functionally the same, except that the reduced aperture will have moved to the opposite edge of the objective.

The Wiki article you linked to does not get into the detail and the specifics I'm addressing here.

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MKV
Carpal Tunnel

Reged: 01/20/11

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]

Quote:

Jon,
As I stated, for the reduced iris condition, as the small iris moves laterally across the larger exit pupil, the reduced aperture at the objective correspondingly sweeps across the objective. And so, yes, the full aperture *can* be utilized, but *never at any one time.* If the iris is 1/2 the exit pupil, never more than 1/2 the objective contributes to image formation. As the iris wanders, that 1/2 objective diameter reduced aperture region also wanders about the objective.

I guess I'll have to prepare a diagram like that in the bottom panel, but for the case where the iris is abutting the edge of the exit pupil (as opposed to being in the center.) Then one would see that the reduced aperture at the objective is still just as small, but offset so as to abut the objective edge.

This in no way means that the objective is ever working at full aperture. Just because one has the freedom to let the iris wander, and thereby utilize different parts of the objective at different times, is not the same as utilizing the full objective at one instant. And so at any one time the instrument is always working at reduced aperture.

Glenn this is exactly what I see when I use SONY DSC HX-1 camera and a relay scope to capture interferograms. The picture below shows a pupil that cuts off part of an 8-inch f/3 mirror. You can move left and right, as if peeking through a keyhole, and see more of the mirror, but not at the same time. So clearly, I am seeing only a small section of the image, however the image is s till a full-aperture image; its just that you don't see all of ti at once.

regards,

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Crayfordjon
Vendor - Zerochromat

Reged: 06/17/09

Loc: UK
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]

Here is an optical diagram which explains clearly what happens when the iris is smaller than the exit pupil of an eyepiece. The resolving aperture diameter is generated by the diameter of he iris "d", rays entering at an angle from the edge of the OG define the field angle of the focal plane, although the resolving aperture is smaller than the OG aperture"D", the whole of the OG aperture is still used, as is illustrated.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048215 - 08/27/13 05:05 AM

Hi-

I think this original post is overall correct but I would add a few things that are relevant to the discussion. First, I think that talk about entrance and exit pupils should be in the context of object space and image space. In this view, the pupils are defined differently from the way Glenn presents.

Normally, an optical system has a single physical aperture in it that limits its throughput, and the entrance pupil is the image of that stop as seen from object space. The exit pupil is the image of that stop as seen from image space. Since they are both images of the same thing, the two pupils are conjugate - and any other images of that stop are also pupils. But the entrance and exit pupils are defined in different conceptual spaces, and they are formed from entirely different optical components viewing the same physical stop.

This is important because it's not just the size of a pupil that matters, but its location in the corresponding space. It's ironic that Glenn is providing details on the definition of these pupils, because I have been making the same points in discussions of the so called "flashlight test" to measure aperture - the accuracy of which depends critically on the location of the pupil when some stop near the image plane is acting as the aperture stop.

The location of the pupils is similarly important when doing afocal imaging with a digital camera. It's not just the size of the pupils that matters, but their locations. The exit pupil of a normal afocal telescope will have some size and location in the image space behind the eyepiece. Meanwhile, if you look into a digital camera, you will "see" the entrance pupil of that camera as an image of its iris, formed by the lenses in front of that iris. It will have some apparent size and apparent depth into the camera - indicating its location and size in object space as you look into it.

In order to avoid vignetting, the entrance pupil of the camera needs to be big enough *and* close enough to the exit pupil of the telescope. Typical digital camera lenses have the entrance pupil fairly deep inside the lens, behind the front element. In that case, even if the camera entrance pupil is bigger than the telescope exit pupil - if it is too far back the vignetting can be severe.

If the entrance pupil of the camera is smaller than the telescope exit pupil, then now the iris of the camera is the aperture stop of the overall system, and the entrance pupil has a size and location unrelated to the telescope objective. The entrance pupil is the image of that iris as seen from object space, looking through the objective, the eyepiece, and the front elements of the camera lens ahead of the iris.

When I see afocal images taken through a telescope, the vignetting is a mixture of the small size of the entrance pupil of the camera, combined with its distance from the eyepiece, limiting the result. If you can't get the camera lens physically close enough to the eyepiece then there will be vignetting. If the camera lens aperture is too small, then the aperture of the system is reduced and the location of the entrance pupil is unknown without a ray trace.

Similar problems show in Foucault test images, where it is difficult to get the entrance pupil of the camera close enough to the knife edge to get a full view of the mirror.

In all these discussions, the physical size of the front lens of the camera isn't what's important, but instead its the stated aperture of the lens - i.e. its entrance pupil diameter - combined with the location of that entrance pupil (which itself is an image) in image space behind the eyepiece, or knife edge.

Frank

Edited by freestar8n (08/27/13 05:12 AM)

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: Crayfordjon]
#6048219 - 08/27/13 05:25 AM

Although your diagram shows that the full aperture is "used" - different parts of the aperture are used for different points in the field. This means the objective is acting more like a field stop than an entrance pupil. This is in fact the case for a Galilean telescope, where the human iris is the aperture stop of the system. The field becomes vignetted and the objective acts as the field stop.

In terms of both the entrance pupil diameter and diffraction performance, the full objective aperture is not being used. The aperture is only helping to reduce vignetting across the field.

Since the entrance pupil is much smaller than the objective, the f/ratio of the system is much greater than would appear using the objective aperture as the entrance pupil diameter. The objective has no connection, in size or location, to the entrance pupil of the overall system - and it's the entrance pupil diameter that determines throughput, diffraction performance, and the effective speed of the system. Since the system is slower, it improves the aberration performance by being stopped down.

Frank

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048249 - 08/27/13 06:36 AM

Jon,
Your diagram does not address the matter of the reduced iris. You've made the iris large enough to accommodate the full light bundle from the objective.

Execute another version, but 'pinch' down the observer's iris to 1/2 the diameter as made here. You will find that the central 1/2 of the objective is now contributing to image formation.

And by the way, the label, "half field angle" is not correct, as it's referring to the semi-angle of the light cone for an image point, which is a reflection of the t/ratio. The two sets of rays you draw come to a common focus at the field center. And so the top set are really paraxial, or parallel to the optical axis, being rays from the on-axis image point. Rays which emanate from any angle off axis do not come to focus on the optical axis.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048256 - 08/27/13 06:45 AM

Frank,
You raise valid points, of course. But it would be preferable that the fundamentals are assimilated before drilling down into that kind of detail. If the very concept of the reduced aperture is not fully understood first, further confusion can only follow.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048270 - 08/27/13 07:08 AM

I think that in many situations the details of how pupils are defined are unimportant - but when it comes to aperture reduction and so forth - more rigorous adherence to the formalism has clear benefit. That's why the formalism exists in the first place. In order to talk about the entrance pupil of a system, you need to establish what is acting as the physical stop, and then determine the entrance pupil diameter and location, you need to do a ray trace of that stop through the optical elements in front of it. Once you establish the resulting image of that stop, it has a size and location - in object space - and *that* is the entrance pupil.

When a small camera lens, or human iris, is placed some distance behind the eyepiece, its size *and* location both determine the size and location of the entrance pupil. If the limiting aperture is not at the location of the exit pupil of the telescope, then the corresponding entrance pupil of the system will not be conjugate with the objective - and the objective will act as a cross between a field stop and an aperture stop.

There is nothing hard to understand about where the pupils end up - because it is just a ray trace. And I think it is all much clearer if you stick to the formalism that has been established - because it becomes critical in discussions like these.

You have to ask - what is acting as the physical aperture stop, what is the resulting image of it, where is it, and how big is it. If the aperture stop is not at a location conjugate with the objective, as happens with an afocal digital camera, these details become essential.

Frank

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048324 - 08/27/13 08:08 AM

Frank,
To help in the obtaining of an understanding of the *basics*, I'm limiting to the ideal case of a system whose entrance pupil is always the objective. That is, we're not concerned with restrictors between objective and field stop for any and all field angles. Furthermore, we're assuming there is no impediment to getting the iris in the plane of the exit pupil.

Would you agree that this is a valid approach to follow in the quest for an understanding of the *fundamentals*? Until this has been achieved, is it not wise to defer the plunging into the subtler details regarding internal stops and longitudinal displacement of iris and exit pupil?

I gather you're still skeptical about the flashlight test, or even its superior variant, the beam-expanded laser test. If so, we two have a seemingly intractable and fundamental disconnect in communication on the matter of pupils. Can you imagine the poor neophyte's head scratching when straying from the more important basics too soon?

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]

To show how a small iris, as it moves around within the confines of the larger exit pupil, continues to project as the same-diameter reduced aperture, I've overlaid additional light ray bundles to the bottom panel in my original illustration. This was the quicker expedient, and I beg your forgiveness for the clutter.

IMPORTANT NOTE: I have not moved the eye downward the required distance of 1/2 the iris diameter. You are unfortunately asked to imagine this instead.

As noted, the eye/iris is located centrally--on the optical axis--within the larger exit pupil. The envelopes of light bundles for image points from the top edge (blue), middle (white) and bottom edge (red) of the FOV are drawn as lines only. Because the iris is centered, all light bundles which contribute to image formation on the retina pass through the central part of the objective.

For reference, note the three relevant diameters indicated (drawn inside the eyeball circle); the centered iris diameter and location as drawn, the iris position when offset to the bottom edge of the exit pupil, and the full exit pupil diameter actually produced by the optics.

Added in solid fills of the same color scheme as before are the light bundles which would be accommodated when the iris is moved downward until it just 'touches' the edge of the exit pupil. The portion of the objective now utilized is at the top edge.

But note this important fact; the view angle has not changed. The field of view is centered on the very same point, and does so always. As will be stressed again, we are merely sampling a smaller portion of the full envelope of light the instrument can field.

Let's say the exit pupil is 6mm and the iris is 3mm. The system is thus stopped down to half its actual diameter, and the image is 1/4 as bright. If the objective were, say, 60mm in diameter, it's now working at 30mm. As long as the iris wanders about within the confines of the exit pupil, there will be no pupil clipping, and so the image brightness remains constant. While this is going on, the image of the iris projected onto the objective moves around within the confines of the objective, always as a 30mm diameter aperture.

Once the iris crosses the edge of the exit pupil, its image crosses the edge of the objective, the resulting pupil clipping causing image diminution.

Here's how to see all this in operation. On a piece of paper or cardboard, cut out a hole no larger than 1/2 the objective diameter. (A smaller hole makes for a more dramatic demonstration!) Place the sheet in front of the objective. While looking at the exit pupil, move the hole about in front of the objective. Note how the reduced exit pupil moves about, just like a miniature version of what's going on up at the front end. This can be easier to see if you first make a reference surface from a piece of tracing paper which has a circle drawn upon it equal to the unobstructed exit pupil diameter, held motionless in the plane of the exit pupil. And a long focal length eyepiece, making for a larger exit pupil, makes things easier to see as well.

As you know, the hole in the card is simply an aperture mask, like those used for stopping down scopes to yield sharper planetary views. Just as the smaller, offset objective mask results in a correspondingly smaller exit pupil similarly offset, so does a small iris at the exit pupil do the very same in reverse. At all times--in both situations--one is merely sampling a smaller portion of the full light bundle the scope is capable of accommodating. In terms of image brightness and resolving power (neglecting for the moment the matter of increased aberrations which can be introduced by the peripheral areas of optical elements), neither the optical system nor your eye cares a whit what portion of the entrance/exit pupil is being utilized.

Let's imagine that on a bright day your scope is producing an 8mm exit pupil. You install an objective mask which produces a 2mm exit pupil. Let's say your daytime-restricted iris is also 2mm. To see anything like a full-brightness image, you must locate your iris pretty darn precisely (to within 1/2mm anyway). If the objective mask was placed at the objective's edge, the reduced exit pupil would be offset to the edge of the 'native' 8mm exit pupil.

Now we remove the mask, and the exit pupil becomes 8mm. Does the image get any brighter? No, bacause your 2mm iris is still stopping down the system. If your eye meanwhile remained perfectly motionless, the very same portion of the objective which was used with the mask in place is still being used now. You could hold your hand up in front any other part of the objective and it will have absolutely zero impact on the view. Moving your iris about within the confines of the 8mm exit pupil only results in a different portion of the objective contributing to image formation at any one moment.

In this example, with mask in place or your iris by itself stopping down the system, the aperture is reduced to 1/4, the working f/ratio of the objective increases by a factor of 4, the image becomes 16X dimmer (compared to the full exit pupil), and for the on-axis iris at least, all telescope system aberrations are reduced. This holds true wherever the iris falls within the exit pupil. Whether it's the eye or an afocal camera behind the eyepiece, these are inviolable facts, as long as the iris can be brought to the plane of the exit pupil.

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dan_h
Carpal Tunnel

Reged: 12/10/07

Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048410 - 08/27/13 09:04 AM

Quote:

There is nothing hard to understand about where the pupils end up - because it is just a ray trace.

If that were the case, this thread would not exist.

dan

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048430 - 08/27/13 09:19 AM

I think it should be pretty obvious to anyone, using simple descriptions and few details, that viewing through an eyepiece with an iris smaller than the exit pupil, will result in a loss of throughput - and an effective loss of aperture. But there is confusion in the subsequent discussion about all the aperture "being used" - and the challenges of afocal imaging with a digital camera.

Your original post here doesn't come across as intended to be simplified - it sounds like it is meant as a formal description of pupils and how they work. In that regard I think it is good to point out, for example, that the exit pupil is not defined as "an image of the entrance pupil." Both pupils are images of the physical aperture stop, as seen from different spaces, and as formed by the subset of elements in the path to the aperture. That is a very different description from yours, it is consistent with the formal definition, and it elucidates concepts critical to a non-confused discussion of what happens in afocal camera imaging.

The flashlight test is definitely a bad way to measure aperture - in general - precisely because it assumes the entrance pupil is near the objective. This is not the case when you have a limiting stop in an unusual location - particularly near the focal plane - as happens with e.g. binoviewers or using a camera afocally.

As an aside - remember that you did change the flashlight test to the "collimated laser" test - due to its inadequacies. I thought we had made at least that much progress on the topic - but now I see people talking about the flashlight test again - so there was some ground lost. In many topics such as these, a clear understanding of pupils and how they work is essential and can't be glossed over, which is why I was somewhat surprised you took on the task to give a tutorial on pupils.

For both the flashlight test and afocal imaging, the location of the pupil is critical, and the simplifying assumptions that make it easier to visualize will prevent an actual understanding of how it all works.

I can see maintaining a simplified version of this topic in a beginner section, but not in this forum where more advanced topics come up all the time - and there is generally a desire to adhere to formalism and nomenclature.

Anyway - basic concepts of pupils and how they work should be Optics 101 - and are normally in the introductory material of an optics text. I don't know why you consider it to be an advanced topic in the first place.

Frank

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: dan_h]
#6048464 - 08/27/13 09:38 AM

Quote:

If that were the case, this thread would not exist.

My point is that even with the complete definition of pupils and how they work, determining their sizes and locations is a matter of simple ray tracing and linear imaging. It's not like it introduces quantum optics or even aberration theory. It's a simple definition and a ray trace - in both directions - from what is determined to be the acting aperture stop.

Frank

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048489 - 08/27/13 09:55 AM

Frank, while a bit off topic, the flashlight test makes absolutely no assumptions whatsoever about where a restricting stop might lie in the system. That's the beauty of it. It simply defines the full envelope of on-axis light that can enter the objective and make it out the eyepiece. No matter where a restrictor might lie.

Parallel light into the eyepiece and parallel light out the objective traces and delineates the one and same envelope of light coming into the objective and which makes it out the eyepiece.

I introduced the laser refinement only so as to ensure the tiniest point of light at the common eyepiece/objective focus, which ensures the sharpest shadow edge, even for a restrictor which might lie not at all far from the focus. For most optical systems, as long as the flashlight source delivers a reasonably sharp and unambiguously measurable circle of light out the objective, it can be taken as sufficiently reliable.

The sharpness of the shadow is what is an indicator of validity.

I can see that a new thread, illustrated, is what's required.

This flashlight test issue between you and me is the equivalent of this current thread between Jon Wall and myself.

That most everyone is remaining on the sidelines while we 'titans' clash (in a good way!) tells me that such topics as these are not fully understood by the vast majority of amateurs. Hence my desire to aducate, but hopefully in a way resembling measured steps. If I diverge from canonical definitions, that's a result of my own ignorance. And due to the kinds of simplifications and stripping down to essentials I'm striving for.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048520 - 08/27/13 10:15 AM

I don't consider the flashlight test entirely off topic - it's actually pertinent. And I fully support an opportunity to educate about the concept of pupils. But I recommend sticking to the formalism more - not just to be fancy - but without it, you can come to very wrong conclusions.

If you recall, it was trivial for me to place a small aperture near the focus of an sct and measure the entrance pupil with the flashlight test - and get a very real looking measurement that was completely wrong. There was nothing subtle about it - it was a great looking, and completely wrong result. The way to understand why the test failed so demonstrably was to draw a simple diagram showing the size and location of the entrance pupil - which emphasized the need for a very accurately collimated beam - which then led to the collimated laser test. In order to make the flashlight beam collimated, you have to hold it far from the eyepiece - in which case very little light gets through - again because of the flux received by that small "entrance pupil" (the exit pupil of the eyepiece in reverse). So when the flashlight is properly placed, the image with large aperture scope is much fainter - which again leads to the need for a laser.

If you place a flashlight near an eyepiece the beam will not be collimated and it will be divergent - which will generate a large error in the shadow size if the entrance pupil is far from the objective. This is optics/pupils 101, and I strongly encourage you to read up on it - both as it relates to the flashlight test, and to afocal camera imaging.

The definition, size, and location of the pupil is critical when it is created by an aperture stop that isn't at a "normal" location conjugate to some primary element. This happens all the time and should be handled properly - and it really isn't hard to do if you assume knowledge of ray tracing - which I believe is pretty safe in this forum.

Frank

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Jon Isaacs
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Loc: San Diego and Boulevard, CA
Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048552 - 08/27/13 10:35 AM

Quote:

I think it should be pretty obvious to anyone, using simple descriptions and few details, that viewing through an eyepiece with an iris smaller than the exit pupil, will result in a loss of throughput - and an effective loss of aperture.

It's not so clear to everyone and historically this has been an issue that has been discussed with some heat. In particular, the issue of the resolution loss caused by an aperture masked by the iris. Several years ago Alan French was in a heated discussion on this very topic in the Astromart forums.

In the current CN binocular forum, there are those who gained understanding from Alan's careful explanation of this particular issue though he was one against many..

'nuff said.

Jon

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: Jon Isaacs]
#6048625 - 08/27/13 11:19 AM

OK - well it does seem odd to me consider how basic it is compared to much more advanced topics discussed here. I'm all for elucidating it especially if people are interested - but if this is a known topic of contention, then I think it is even more important to treat it more rigorously. I don't know about the discussion you are referring to - so I can't comment on the treatment it gave on the topic.

I'll try to put some diagrams together, but at the same time - the basic concepts of pupils are treated in most any text on optics - but people seem to regard the entrance pupil as some very obvious lens or physical hole - when it often isn't so simple.

Separately - the questions about coupling a camera to a telescope afocally cannot be discussed properly without explicitly dealing with the location of the camera pupil relative to the eyepiece. Since that topic is intertwined this the one regarding the more basic aperture reduction caused by a small human pupil - I think it makes sense not to oversimplify the discussion.

Frank

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Jon Isaacs
Postmaster

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048640 - 08/27/13 11:34 AM

Quote:

Separately - the questions about coupling a camera to a telescope afocally cannot be discussed properly without explicitly dealing with the location of the camera pupil relative to the eyepiece. Since that topic is intertwined this the one regarding the more basic aperture reduction caused by a small human pupil - I think it makes sense not to oversimplify the discussion.

This discussion did start out addressing aperture masking and the human eye.

Jon

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MitchAlsup
Pooh-Bah

Reged: 08/31/09

Re: Why a too-small iris results in reduced aperture [Re: SACK]
#6048672 - 08/27/13 11:57 AM

Quote:

Hi Mitch,
How does the lit retina create the background glow? How bad is it? I have not noticed before but sounds interesting and good to know for eyepiece selection.

Light bounces off the iris and then illuminates the cornea from behind causing what looks a lot like skyglow.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: Jon Isaacs]
#6048675 - 08/27/13 11:59 AM

OK. Then - if the eye pupil is physically located at the exit pupil of the eyepiece, and if the eye pupil is smaller than the original exit pupil - is there anyone here who still thinks the effective aperture, for collecting light or for diffraction resolution, has not been reduced?

Note that this is separate from the full aperture "being used" to reduce vignetting as you look around the scene or something.

Is there anyone here who thinks the full aperture and speed of the lens is being used in this situation, for on-axis image quality and brightness?

If so - can you describe why that would be?

Frank

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6048691 - 08/27/13 12:07 PM

Quote:

Jon,
Your diagram does not address the matter of the reduced iris. You've made the iris large enough to accommodate the full light bundle from the objective.

Oh - I just realized something much more fundamental about that diagram. All the parallel input rays on the left should be emerging parallel on output. The diagram is incorrect and combines the role of the eyepiece with the role of the eye.

In the corrected diagram, the large parallel input bundle would become a smaller, but still too big, parallel output bundle that slams into the human iris and only lets the central part through - with a corresponding loss of light.

Does anyone disagree with that?

Frank

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Ravenous
sage

Reged: 11/14/09

Loc: UK
Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6048728 - 08/27/13 12:24 PM

Quote:

In the corrected diagram, the large parallel input bundle would become a smaller, but still too big, parallel output bundle that slams into the human iris and only lets the central part through - with a corresponding loss of light.

Does anyone disagree with that?

Frank

I suspect you're right - I wasn't able to understand where the extra lines at the eyepiece came from. I didn't want to ask for an explanation, as things were getting too argumentative anyway...

But I think Glenn's original post is long overdue. Newbies reading around here may get misled by some of the other posts.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: Ravenous]
#6048970 - 08/27/13 02:20 PM

Heh heh. I missed that aspect in Jon's diagram, where the emergent rays out the eyepiece, which formerly passed through the objective edge zone, are not parallel to the central rays. I was concentrating too much on the action ahead of the eyepiece.

This cardinal mistake (to make nothing of the assertion of full aperture use in spite of a restricting iris) by an experienced telescope maker is ample enough reason to cover this whole business.

Frank,
For the flashlight aperture test, from word one I always stressed the importance of placing the flashlight no nearer to the eyepiece than about 10 eyepiece focal lengths, to assure reasonably parallel light going into the exit pupil. This ensures that the tiny image of the light is formed near enough to the focus to make for a reliable result.

And as I mentioned in our previous discourse, if one has any doubt as to parallelism of emergent light, just measure the illuminated circle near the objective and at least a full focal length beyond this. If the two measures are the same, the result is very accurate.

That's the crux of the matter. If the circle is sharp, and is known to be parallel, and emerges sufficiently parallel to the optical axis, its diameter truly is the working aperture. Parallel light straight into the eyepiece, parallel light out the objective; that's all you need. No requirement to beetle one's brow about pupil or stop locations.

I will be cooking up a fuller treatment on this!

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Nils Olof Carlin
Pooh-Bah

Reged: 07/26/04

Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6049033 - 08/27/13 03:03 PM

You can use an oversized exit pupil, if it gives you a wider true field - it may not in practice, you should check.

If too large, the obstructed part (if any) of the exit pupil will be wide enough to be bothersome by blocking much of the eye pupil. Not least when you observe the sun or moon, and your eye pupil is much smaller than it is during deep sky observing. A low-power EP good for deep sky may be useless here.

The eye is a globe more or less, rotating in its "socket" - this means that the iris will move sideways when you shift your gaze (it is very roughly 1/2" in front of the center of rotation). Matching the exit pupil accurately to the iris will mean that even the smallest shift of gaze will give you a (noticeable) loss of light, until you have re-positioned your eye.

Choosing exit pupils that are at least 1 or 2 mm larger or smaller than your dark-adapted pupil will make observing easier.

Nils Olof

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Mark Harry
Vendor

Reged: 09/05/05

Loc: Northeast USA
Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6049127 - 08/27/13 03:49 PM

I think the exit/entrance pupil has been hammered into threads concerning singlet aperture telescopes a bit unfairly in the past referred threads, and a bit too often.
There is a balance that has to be realized between aperture, and light gathering that has been glossed over; and is backed up with what is exactly detectable by eye, vs resolution on reduced aperture.
Specifically it's all touted as theoretically possible, but what is actually visible. And by eyeball, each orb has varying degrees of acuity.
******
I repeat the question- How many of the "titans" have actually made a singlet telescope that actually has achro-description performance??????????????????????????????????????????????????????????????????
Or who has equated definitions with achro performance between what is worse/better than described conditions/capabilities in what the human eye perceives???
----
If they haven't, there should be an obvious silence.
The disgusting post of an enlarged image of the -TIP- of the antenna (full of lateral color) doesn't mention the central part of the image where the lateral color is -NILL-.
Such biased posting in the mask of a forum should be discouraged, or banned; and shows a lack of fundamental understanding of human oriented vision, and a total lack of consideration of civilized man to bolster an ego oriented bent on making their fellow man look stupid.
(been said before- if it works for those experimenting, who are -YOU to say what the definition of visual acuity should be?????)
Wake up; moderators.

Mark

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Boot
wildly diverse musical tastes

Reged: 06/04/07

Loc: The Outernet
Re: Why a too-small iris results in reduced aperture [Re: Mark Harry]
#6049149 - 08/27/13 03:57 PM

Well said, Mark.

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Mark Harry
Vendor

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Loc: Northeast USA
Re: Why a too-small iris results in reduced aperture [Re: Boot]
#6049268 - 08/27/13 04:48 PM

Thanks, Joseph.
M.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: Mark Harry]
#6050022 - 08/28/13 02:53 AM

Mark,
This thread was motivated by an evinced lack of understanding that a restricting iris reduces the working aperture commensurately. This was confirmed by a post made here, in which a supporting diagram was badly in error.

Furthermore, my issue was with the unknown, or at least uncertain, variables involved with the images used to support claims of performance on the Hypo. A reasonable deduction based on what scant evidence was provided had the actual working aperture reduced to but a fraction, which is known to reduce aberrations. I wanted to see images which assuredly used at least near to the full available aperture.

Someone could easily post images taken with a stopped down achromat which could be taken for apochromatic performance. Would this reflect actual performance? No.

And so the aim to provide at least the basis for understanding the relationship between pupils in a telescope. For all parties.

Finally, this thread has not been concerned with flagellating any particular optical system, being content to confine itself to optical concepts.

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Mark Harry
Vendor

Reged: 09/05/05

Loc: Northeast USA
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6050234 - 08/28/13 08:40 AM

I want it to be understood;
Glen did -NOT- post the pic I mentioned in my previous post!
Mark

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Crayfordjon
Vendor - Zerochromat

Reged: 06/17/09

Loc: UK
Re: Why a too-small iris results in reduced aperture [Re: Mark Harry]
#6050527 - 08/28/13 11:44 AM

Freestart8, the fault is mine, I omitted to draw in the full aperture of the exit pupil dia, the iris dia shewn is for the human iris which is typically around 2mm, whereas the exit pupil is 10mm. The diagram in not in error, but was drawn hastily, I feel that some of the more veherment critics are all to ready to knock people off their pedestals, an occupation all too familiar with those who are intellectually challenged.

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Ravenous
sage

Reged: 11/14/09

Loc: UK
Re: Why a too-small iris results in reduced aperture [Re: Crayfordjon]
#6050551 - 08/28/13 11:55 AM

Are you saying those 45 (ish) degree lines coming from out of the eyepiece represent the user's view of the edge of the field?

Not vehemently critical I hope, I am just trying to understand just how useful certain optical configurations might be.

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: Ravenous]
#6050604 - 08/28/13 12:24 PM

Jon,
in your diagram, two groups of rays, one from on-axis and the other from the OG's edge, both pass through *the same point on the optical axis*. This means these two ray bundles are from the same point in object space, namely on the optical axis. And so the ray bundle from the edge zone on the OG must emerge out the eyepiece *parallel to the optical axis*. Not at the angle shown.

In order for your illustration to be correct, the edge zone ray bundle must intercept the focal plane *below* the on-axis focal point. Then it would correspond to an edge-of-field image point, whose rays would emerge out the eyepiece at an angle equaling the apparent semi-field angle.

I belabour this point not because I'm intellectually challenged , but because the diagram can all too easily give the impression that all image points over the full field come to the one focus point on the optical axis. We know this is impossible, and I'm sure you'll agree that we must avoid any pitfalls which militate against a full and correct understanding.

I know, because in spite of great care in my illustrations (and text), I'm still not getting through to all. If there were even little mistakes, confusion would only be all the more profound.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Why a too-small iris results in reduced aperture [Re: Ravenous]
#6050796 - 08/28/13 02:15 PM

Quote:

But I think Glenn's original post is long overdue. Newbies reading around here may get misled by some of the other posts.

I'm realizing there is a whole context here that I don't know about - and the thread is in fact at a much more basic level than I thought. In that case I'm not even sure the word "pupil" is needed - just show a picture of light hitting the eye - and not going in. I don't have any idea where the misunderstanding lies, or how to address it - assuming there is in fact a disagreement.

This very scenario is described succinctly in the military standardization handbook on optical design, which can be downloaded free. It is summed up in one paragraph in section 4.4.2.4 - and it includes the subtlety described by Nils above, where it is in fact beneficial to have a pupil that is a bit oversized . So there is benefit in having it slightly oversized, to make sure the eye pupil is always fully illuminated - but the throughput is always limited by the eye pupil.

Frank

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GlennLeDrew
Postmaster

Reged: 06/18/08

Re: Why a too-small iris results in reduced aperture [Re: freestar8n]
#6050970 - 08/28/13 04:12 PM

Frank,
The primary thrust here is to provide some understanding of the way in which the eyepiece behaves as an optical coupler, whereby it projects upon the objective an image of the iris, and vice versa. And along the way to touch upon other aspects of performance that result. In such a diverse community of people, one can't overlook the simple, basic fundamentals.

I suspect that if you polled a hundred experienced observers and ATMs who've not looked here (and maybe a few who have!), the number who could describe (or even be peripherally aware of) how the eyepiece can project an image of the iris onto the objective might well be counted on two hands, perhaps even one. Well, that's my inner cynic peeling out.

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p1taylor
professor emeritus

Reged: 02/19/08

Loc: West Midlands UK
Re: Why a too-small iris results in reduced aperture [Re: GlennLeDrew]
#6052867 - 08/29/13 04:00 PM

this is a silly carrion to me what deferens dos it make, as long as you see what you are looking at.

peter

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