Author Topic: help understanding exit pupil  (Read 210 times)

Tony Patton

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Re: help understanding exit pupil
« Reply #15 on: January 23, 2018, 06:51:50 AM »
Simultaneity is the best mechanism to contemplate. A way to picture this...

Imagine a battery of many search lights covering a patch of ground relatively densely. Each produces a nice cylinder of light. All are aimed so that their beams cross at the same place directly above the central light, and at a height equal to the width of the array of lights. That is, if the array is 100m wide, their beams converge at a height of 100m.

If each beam is 2m wide, when exactly converging all beams will pass through the same 2m wide spot. (Technically, the resulting spot is a bit bigger due to the tilt of the outer beams, but for a real exit pupil this projection effect is not relevant.)

The light bundles for each of the many thousands of effective image points making up the scene all similarly converge at (nominally) the same place, each cylindrical bundle being a continuous 'stream' of photons very like a battery of search lights.

When we view any scene with the unaided eye, if the objects are distant enough (more than, say, 100 iris diameters) so that the light from any one point on them arrives near enough to parallel, we can accurately enough say that we are viewing an image through an exit pupil whose diameter equals the eye's iris diameter. That's because a telescope's exit pupil merges with and effectively replaces the eye's entrance pupil. That is, the exit pupil becomes the eye's entrance pupil. When no optics are actually present, we still have an entrance pupil, and the naked-eye scene is fundamentally no different than for an image delivered by a telescope through an exit pupil of diameter equal to or larger than the iris.

What I'm trying to get at here is that the exit pupil is conceptually extremely simple. It's merely a replacement iris, as far as the retina is concerned. The retina knows nothing of the presence of extraneous optics ahead of the eyeball. All it sees is light coming through a hole, which the eye's lens focuses. The principal requirement is correct eye placement so that the exit pupil is more or less coincident with the iris, so that the image delivered via the eyepiece be seen in full and at the most uniform illumination possible.

artufanchess

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Re: help understanding exit pupil
« Reply #16 on: January 25, 2018, 11:21:50 PM »
OK, I think it all just gelled. The source of my conceptual problem was that I was thinking of the incoming image as arriving as a plane of photons, but of course it's coming in as a curve since all the photons have been traveling for exactly the same length of time from their source. This is really obvious now that I think about. (Doh -- head slap!)

esrescioripp

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Re: help understanding exit pupil
« Reply #17 on: January 26, 2018, 01:52:20 AM »
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People generally choose 50mm eyepieces, and 60mm, too, for their f/10 Schmidt-Cassegrains, and their f/12-f/15 Maksutovs, resulting in exit-pupils of 5mm/6mm for f/10, 4.2mm/5mm for f/12, and 3.3mm/4mm for f/15, and all well within the limits of most people, and regardless of age.

Every serious observer I know regardless of scope type, and including owners of f/11 c14s, and f/10 scts, avoids 50 and 60 mm eyepieces like the plague.
The reason is that optimal views are 1 to 2 mm exit pupil, and no one will opt for 5 mm exit pupil 1 degree fov when it is there to be had at 3.5 mm exit pupil.  They don't even work the math.  They just know it looks better.
I don't take issue with the rest of your detailed presentation but I'd bet 50 and 60 mm eyepieces are not big sellers generally, and most sct owners know better.

smarhurtfranoth

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Re: help understanding exit pupil
« Reply #18 on: January 30, 2018, 08:29:51 AM »
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When outside at night, the human eye becomes dark-adapted over a span of time, and the eye's pupil enlarges, allowing more light to enter, with the maximum diameter ranging from 7-8mm in youth, to 4-5mm in old age. Dark-adapted pupil diameter decreases as we age.

To find the exit-pupil of any eyepiece, you simply divide the eyepiece's focal-length by the f-ratio of the telescope...

Your 25mm Plossl ÷ f/6(of your XT8) = 4.2mm

...or more simply, 25 ÷ 6 = 4.2mm

Your 34mm has an exit-pupil of 5.7mm. If the diameter of your dark-adapted pupil meets or exceeds 5.7mm, then you're good. If not, then the part of the light that exits the eyepiece will fall onto the iris instead of into the pupil; thereby light is wasted, and in the pupil not receiving all of the light gathered by the primary mirror of the telescope.

Take this 2" 50mm eyepiece, for example... http://agenaastro.co...w-eyepiece.html

50 ÷ 6 = 8.3mm

That's a considerably large exit-pupil, and also a very low power. A 50mm would give a low magnification of only 24x with your XT8, but if the diameter of your dark-adapted pupil is less than 8.3mm, then some light will be lost; again, falling onto the iris instead of entering the pupil. It would be like observing with a 6" or 7" telescope even though one has an 8".

However, some don't mind wasting a little light, if they can have that very low power in its place.

People generally choose 50mm eyepieces, and 60mm, too, for their f/10 Schmidt-Cassegrains, and their f/12-f/15 Maksutovs, resulting in exit-pupils of 5mm/6mm for f/10, 4.2mm/5mm for f/12, and 3.3mm/4mm for f/15, and all well within the limits of most people, and regardless of age.

As one goes up in magnification, like with your 10mm(120x), the smaller the exit-pupil becomes(1.7mm), and everybody's happy.

It doesn't get to 8mm, even in small children and infants.
The range of variation, to the ends of the curves, for humans with dark adapted pupils, is 2 to over 8mm.
98% fall in the 3 to 7mm range.

wetrerede

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Re: help understanding exit pupil
« Reply #19 on: January 31, 2018, 07:53:17 AM »
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The reason is that optimal views are 1 to 2 mm exit pupil, and no one will opt for 5 mm exit pupil 1 degree fov when it is there to be had at 3.5 mm exit pupil. They don't even work the math. They just know it looks better.
I don't take issue with the rest of your detailed presentation but I'd bet 50 and 60 mm eyepieces are not big sellers generally, and most sct owners know better.
But of course there are exceptions that have everything to do with exit pupil.
A couple of years ago my introduction to larger aperture was via the 16" f10 SCT club scope mounted in a domed observatory at our Oklahoma dark site. Disappointed at being unable to see Barnard 33, the Horsehead Nebula, even with all that aperture and an H-Beta filter in place I asked for advice. David Knisely suggested the 3.0mm exit pupil I was using was too small to allow the filter to work as well as it could. So I got a 63mm Konig that produced a 6.3mm exit pupil, tried again and Bob's Y'er Uncle. There it was plain as day.
"Not big sellers generally" - absolutely correct, but useful or even critical at times.
I keep the eyepiece now though I never use the SCT any more, preferring the much brighter 16" f4.5 Dob, for its useful contribution to my understanding of very large exit pupils in my scopes.

fefeldarsro

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Re: help understanding exit pupil
« Reply #20 on: January 31, 2018, 09:15:17 AM »
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People generally choose 50mm eyepieces, and 60mm, too, for their f/10 Schmidt-Cassegrains, and their f/12-f/15 Maksutovs, resulting in exit-pupils of 5mm/6mm for f/10, 4.2mm/5mm for f/12, and 3.3mm/4mm for f/15, and all well within the limits of most people, and regardless of age.


Every serious observer I know regardless of scope type, and including owners of f/11 c14s, and f/10 scts, avoids 50 and 60 mm eyepieces like the plague.
The reason is that optimal views are 1 to 2 mm exit pupil, and no one will opt for 5 mm exit pupil 1 degree fov when it is there to be had at 3.5 mm exit pupil. They don't even work the math. They just know it looks better.
I don't take issue with the rest of your detailed presentation but I'd bet 50 and 60 mm eyepieces are not big sellers generally, and most sct owners know better.

There is no overall optimal exit pupil. There are optimal exit pupils for a given telescope, observer, conditions and object. That can range from 8 mm for large, diffuse objects, under dark skies with an observer whose eye dilates to 8 mm to well under 1 mm for bright planetary nebulae, the planets and double stars.

In general, when you reduce the magnification, the field of view increases in proportion. But at some point you start to run into the limitation of the eyepiece format. Using TeleVue eyepieces as a guide, in the 2 inch size, the 31 mm Nagler offers 90% of the maximum possible TFoV, the 41 mm Panoptic offers the maximum possible TFoV. Moving to the 55 mm Plossl provides that same TFoV as the 41 mm Panoptic.

In an F/10 SCT, these correspond to a 3.1 mm exit pupil, a 4.1 mm exit pupil and a 5.5 mm exit pupil. For most objects, the higher magnifications are a better choice but there are objects that are better viewed at large exit pupils, that's why eyepieces like the 55 mm Plossl exist.

However, these are generally large objects and because of the restricted field of view, F/10 scopes, particularly large aperture ones, are not well suited for viewing large objects. If one wants to view larger objects, view with bright exit pupils, swapping telescopes is a more effective strategy than swapping eyepieces, the SCT is not the right tool for that job.

While there are SCT owners who do use 55mm Plossls, were 68 degree and 82 degree eyepieces available and affordable with a 60 mm focal length, I am quite sure you would find them popular with the SCT owners, that give them more of an equal footing with then Newtonian.

Jon

David Felkel

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Re: help understanding exit pupil
« Reply #21 on: January 31, 2018, 01:27:37 PM »
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...ranging from 7-8mm in youth, to 4-5mm in old age.

It doesn't get to 8mm, even in small children and infants.
...generally, perhaps not.

jumphindnore

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Re: help understanding exit pupil
« Reply #22 on: February 02, 2018, 04:37:52 PM »
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While there are SCT owners who do use 55mm Plossls, were 68 degree and 82 degree eyepieces available and affordable with a 60 mm focal length, I am quite sure you would find them popular with the SCT owners, that give them more of an equal footing with then Newtonian.

Jon

Jon, they *are* available, from Russell optics. Or they were once upon a time. 68 degrees and 60mm focal length??! Can you spell v-i-g-n-e-t-t-i-n-g? The televue 55mm plossl has 50 degree afov, and for good reason. You don't get a dime's worth more fov over a 70ish 40mm ish eyepiece. You do get a very gray, dispiriting sky.

I would not use it in a boat, not in a moat, not with a goat, etc. Not in an SCT and not in an f/4.5 Newt and not in a 5" f/6.3 apo. They are not likable, Sam-I-am, and their rarity on Astromart is for good reason....well OK, 55 mm brings up 43 hits over 2 years, 40mm brings up 6 times that, and you can throw in another dozen or so for the under-appreciated and pricey 41 Pan.  I might buy a 55mm plossl (or one of those weird Russells) for a night at the Wilson observatory but aside from that....no.

And it's not the case that a 5mm exit pupil is just as good as a 3.5. It is arguably just as good if you like gray washed out views and if you like being informed of the least bit of astigmatism in your organic eyeball. It's certainly not something you would want to put into an SCT. Not if you own an alternative.  I think some of these opinions are in the category of let's-make-a-market for crummy eyepieces by dumping them on the SCT owners, and telling them that with long FRs it doesn't matter. Yeah right. 

(Now watch someone come up and say I use my c8 with an f/6.3 reducer and a 55 mm plossl all the time and it's the cat's meow....)

It might be the case that a 55mm is actually better in a fast Newt because the huge exit pupil will cut off much of that horrid view, but having tried it, I'd rather have cataracts and a 41 Pan.  I have too much respect for Newtonian aficianados to wish such a thing upon them.

Greg N

David Reynolds

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Re: help understanding exit pupil
« Reply #23 on: February 02, 2018, 05:50:06 PM »
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While there are SCT owners who do use 55mm Plossls, were 68 degree and 82 degree eyepieces available and affordable with a 60 mm focal length, I am quite sure you would find them popular with the SCT owners, that give them more of an equal footing with then Newtonian.

Jon

Jon, they *are* available, from Russell optics. Or they were once upon a time. 68 degrees and 60mm focal length??! Can you spell v-i-g-n-e-t-t-i-n-g? The televue 55mm plossl has 50 degree afov, and for good reason. You don't get a dime's worth more fov over a 70ish 40mm ish eyepiece. You do get a very gray, dispiriting sky.

I would not use it in a boat, not in a moat, not with a goat, etc. Not in an SCT and not in an f/4.5 Newt and not in a 5" f/6.3 apo. They are not likable, Sam-I-am, and their rarity on Astromart is for good reason....well OK, 55 mm brings up 43 hits over 2 years, 40mm brings up 6 times that, and you can throw in another dozen or so for the under-appreciated and pricey 41 Pan.  I might buy a 55mm plossl (or one of those weird Russells) for a night at the Wilson observatory but aside from that....no.

And it's not the case that a 5mm exit pupil is just as good as a 3.5. It is arguably just as good if you like gray washed out views and if you like being informed of the least bit of astigmatism in your organic eyeball. It's certainly not something you would want to put into an SCT. Not if you own an alternative.  I think some of these opinions are in the category of let's-make-a-market for crummy eyepieces by dumping them on the SCT owners, and telling them that with long FRs it doesn't matter. Yeah right. 

(Now watch someone come up and say I use my c8 with an f/6.3 reducer and a 55 mm plossl all the time and it's the cat's meow....)

It might be the case that a 55mm is actually better in a fast Newt because the huge exit pupil will cut off much of that horrid view, but having tried it, I'd rather have cataracts and a 41 Pan.  I have too much respect for Newtonian aficianados to wish such a thing upon them.

Greg N

Greg:

A 60 degree eyepiece with a 68 degree AFoV requires a field stop that is approximately 71mm = 2.8 inches in diameter. Clearly that could not be a 2 inch eyepiece and probably not a 3 inch eyepiece. That is where the 55mm Plossl comes in, a bright exit pupil with the maximum possible TFoV in the 2 inch format.

As far as 3.5mm exit pupil being superior to a 5mm exit pupil, in sometimes yes, sometimes no. For dim, diffuse objects, the increased magnification and dimmer image can be a detriment and a 5mm or larger exit pupil will provide "better views" than a 3.5mm exit pupil. As noted in my previous post, SCTs are not well suited for viewing large, dim objects, in part because of the limitations of the 2 inch eyepiece format. As Dick Gentry pointed out in his recent post, he found a 63mm eyepiece much better than a 30mm eyepiece in a 16 inch F/10 SCT for viewing the Horsehead. Filters work best with bright images.

Jon

Matt Haines

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Re: help understanding exit pupil
« Reply #24 on: February 03, 2018, 07:52:53 AM »
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...ranging from 7-8mm in youth, to 4-5mm in old age.

It doesn't get to 8mm, even in small children and infants.
...generally, perhaps not.

 

Dark-adapted pupil diameter as a function of age measured with the NeurOptics pupillometer.

http://www.ncbi.nlm....pubmed/20506961

"RESULTS:
Two-hundred sixty-three individuals participated. For participants aged 18 to 19 years (n=6), the mean dark-adapted pupil diameter was 6.85 mm (range: 5.6 to 7.5 mm); 20 to 29 years (n=66), 7.33 mm (range: 5.7 to 8.8 mm); 30 to 39 years (n=50), 6.64 mm (range: 5.3 to 8.7 mm); 40 to 49 years (n=51), 6.15 mm (range: 4.5 to 8.2 mm); 50 to 59 years (n=50), 5.77 mm (range: 4.4 to 7.2 mm); 60 to 69 years (n=30), 5.58 mm (range: 3.5 to 7.5 mm); 70 to 79 years (n=6), 5.17 mm (range: 4.6 to 6.0 mm); and 80 years (n=4), 4.85 mm (range: 4.1 to 5.3 mm)."

In this study, dark adapted pupil diameter ranged from 3.5 mm to 8.8mm with the mean ranging from 7.33mm for people in their 20s to 4.85mm for people in their 80s. My guess is that the techniques we use to measure the diameter of our dark adapted pupils likely under estimates it.

The important result is that the variation as a function of age is less than the variation within the individual group. There were people in their 60's with dark adapted pupils greater than 7mm in diameter. The groups for people in their 70s and 80s were small, 6 people and 4 people respectively.. Maybe if 30-50 people had participated in each of those age groups, there would have been people with 7mm diameter dark adapted pupils.

What I see, it's worth measuring your own eyes. Clearly age related assumptions cannot be trusted.

Jon

Stephen Artman

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Re: help understanding exit pupil
« Reply #25 on: February 08, 2018, 05:24:26 PM »
One thing to note is that a lot of people 60+ take blood pressure drugs or cholesterol drugs, etc. Almost all of these
drugs shrink the pupils.

dsepinumer

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Re: help understanding exit pupil
« Reply #26 on: February 09, 2018, 01:01:08 AM »
I'm not an exit pupil decider...I use the ahhh/ewwww method...some nights I'm in the mood for the sit back and take in the big screen...and some nights I want to see the individual actors...its more about what I see, then what I think I should see. I fully admit I'm more into it for the fun then the science....clouds permitting.

Nathan Sorgaard

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Re: help understanding exit pupil
« Reply #27 on: February 09, 2018, 09:19:28 AM »
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I'm not an exit pupil decider...I use the ahhh/ewwww method...some nights I'm in the mood for the sit back and take in the big screen...and some nights I want to see the individual actors...its more about what I see, then what I think I should see. I fully admit I'm more into it for the fun then the science....clouds permitting.

Me too.. It's a hobby, recreation. I am out there because I enjoy looking at the heavens above.

Jon