Exit pupil help please

[LondonNeil]

Could someone tell me,  how much does exit pupil really matter?

My limited understanding so far is:

1. Your eye only dilates so far, and less as you age. So maybe 8mm for a child,  6 or maybe 5mm ish for someone like me about 50

2. If the exit pupil of light from the ep is smaller then the image isn't the brightest possible from the scope

3. If the exit pupil is larger than your eye pupil the image isn't brighter,  you can't get all the light in your eye

3. Equal pupils is 'just right'

4. Exit pupil size is EP FL/tube FR, so for example my f5 frac with a 10mm EP, the exit pupil is just 2mm. With a 5mm EP it's just 1mm.

Okaaaay, so I can see this helps you to understand the sweet spot for your scope and you.  If for example,  you know you will do alot of viewing of X target,  and want Y ish magnification,  you can chose the scope f ratio and EP to give you a good match on exit pupil and they sweetspot image should be good for you.   You can chose to go all in on the best scope a best EP for that one quality image role. 

But many of us are going to view a range of targets, using a range of magnifications, so it's a compromise.   How much of a compromise is it really?  

 

Hmm.....is this question really just 'why is a long FL tube/sloe scope, better for high magnification?'  It is isn't it?  It's as simple as that... maybe?

[Stu]

I think you can observe quite successfully without knowing anything about exit pupil, however understanding it can just help you know why certain combinations work better than others, and why you see certain effects.

For instance, observing Planets in a 100mm refractor at x200 is likely to show floaters if you have them. Understanding that they start to become prominent at an exit pupil of 1mm or less starts to explain why. In this instance it would be 0.5mm, so you options then become using binoviewers or, say, moving to a 200mm scope which would give you a 1mm exit pupil at the same mag.

At the other end of the spectrum, if, say, you were using a 55mm Plossl in an 8” dob, you would notice that the background sky is washed out if observing under any form of light polluted skies, plus it feels like looking down a straw. Knowing about exit pupils and apparent fields of view might take you down an expensive path to a 21mm Ethos which gives nearly as large a field of view but with a 3.5mm exit pupil rather than 8mm. Much darker sky background and more satisfying image.

If viewing through a narrowband filter, using too small an exit pupil makes the image much too dim. This help understand that you CAN use these filers in small scopes but you need to keep the magnification low to keep the exit pupil large. Similarly in unfiltered views, if you use too high a mag, you will end up with an image which is too dark; x200 on the Ring nebula gives a nice big image scale but if you exit pupil is 0.5mm you will hardly see the target. Options are again, drop the mag or increase your aperture. That’s why big dobs are used to hunt down small faint galaxies; you can use high powers whilst still maintaining sufficient image brightness to see the target.

So you see it does matter what exit pupil you are using, and knowing that helps you understand what your best options are in terms of kit combinations.

[Zermelo]

Stu has covered the essential points there. One lesser consideration is that if you have significant astigmatism in your observing eye, this tends to be more visible when using larger exit pupils. So, it can be preferable to obtain the same true field by using an eyepiece with shorter focal length but wider apparent field, the same tactic as for obtaining a darker background sky. As Stu says, it will be more expensive to get these wider fields that are well corrected to the edge. And it may force you into a 2" format.

[Mr Spock]

I only use low power, wide field eyepieces as finders. The sky is too bright here for serious dark background observing.

Most would say an exit pupil of 0.5mm is the lowest you should go due to floaters and the fov getting too dark. I stretch that to 0.45mm with my 100mm on the moon, and a more comfy 0.54mm on Jupiter; I reckon that's about as low as I want to go for each. 

[vlaiv]

Here are some interesting points regarding exit pupil:

- telescope can't make extended objects brighter, only dimmer. The brightest view is with our naked eye. Exit pupil matched to our "entrance" pupil will give the same brightness of extended object as when viewed with naked eye. Smaller exit pupils will dim the object brightness for extended objects.

- Sky is also extended object. Contrast ratio, or ratio of target brightness to sky brightness can't be changed by a telescope. It is always the same. Only way to change this is by use of special filters (for example UHC filter if target is emission nebula).

- Human vision is complex topic. Perceived contrast ratio depend on amount of light. It also depends on angular features of target - look at this image:

Frequency changes from left to right and contrast from on Y axis. It shows that not all frequency components are perceived the same although they have the same physical contrast (ratio of dark and light). This even changes with amount of light and our night vision adoption.

This is why we can "dial" in magnification for particular target and why larger scopes show faint fuzzies more easily - it is because of this combination of absolute levels of light + magnification. On physical level - small scope with exit pupil of 5mm delivers same amount of surface brightness as large scope with 5mm exit pupil.

[Don Pensack]

6 hours ago, LondonNeil said:

Could someone tell me,  how much does exit pupil really matter?

My limited understanding so far is:

1. Your eye only dilates so far, and less as you age. So maybe 8mm for a child,  6 or maybe 5mm ish for someone like me about 50

2. If the exit pupil of light from the ep is smaller then the image isn't the brightest possible from the scope

3. If the exit pupil is larger than your eye pupil the image isn't brighter,  you can't get all the light in your eye

3. Equal pupils is 'just right'

4. Exit pupil size is EP FL/tube FR, so for example my f5 frac with a 10mm EP, the exit pupil is just 2mm. With a 5mm EP it's just 1mm.

Okaaaay, so I can see this helps you to understand the sweet spot for your scope and you.  If for example,  you know you will do alot of viewing of X target,  and want Y ish magnification,  you can chose the scope f ratio and EP to give you a good match on exit pupil and they sweetspot image should be good for you.   You can chose to go all in on the best scope a best EP for that one quality image role. 

But many of us are going to view a range of targets, using a range of magnifications, so it's a compromise.   How much of a compromise is it really?  

 

Hmm.....is this question really just 'why is a long FL tube/sloe scope, better for high magnification?'  It is isn't it?  It's as simple as that... maybe?

An exit pupil matching the pupil size of your eye is the brightest image you can get in the scope.  Lower magnifications will be equally bright, but you will lose resolution due to the smaller effective aperture.

And on a reflector scope, that can cause a problem with the secondary shadow becoming noticeable.  The largest exit pupil will also reveal the astigmatism in your own eye better than smaller exit pupils.

Exit pupils around 2-3mm usually have the highest visual acuity for more observers.  It's no accident this is also a common size for the pupil in the daytime.

Exit pupils of ~1mm is where the telescope achieves its maximum resolution because the Airy disc becomes visible.  Higher magnifications yield a larger image, but not an improved resolution.

Exit pupils smaller than 1mm starts getting into where out eyes have problems--floaters in the eye and corneal issues and developing cataracts can yield poor image quality.  Not to mention that small exit pupils mean high powers,

and the atmosphere Seeing starts interfering with image quality.

 

As I see it, Exit pupil choice is the realm between large pupil astigmatism and small pupil eye problems.

For me, that is 1-4mm exit pupils, though I have a range of eyepieces that yield 0.65-5.2mm exit pupils just to cover the objects that are best at those magnifications (example: Pleiades at one end, Neptune at the other).

 

In practice, I pay no attention whatsoever to exit pupil--I merely try to frame the object well and use the right magnification for the object to see it best.

Whatever exit pupil that results in is the exit pupil it happens to be.  I would never look at exit pupil to decide what eyepiece to use.  I wonder if anyone does.

[Naughty Neal]

Like most I select an EP for the seeing often starting at lowest power then increasing to a higher power (sometimes) until the seeing worsens,  when the seeing starts to worsen then I back off the power and may select other similar FL's.   Typically though I know my Exit Pupil will be within a certain range , for my two refractors that is approx. 0.45mm - 2.91mm.

As Don say's , I doubt few bother or even worry about Exit Pupil as it is the power/ ep we  select for the object.

Edited March 14 by Naughty Neal

[LondonNeil]

Thanks,  I didn't shan't fret then

[Stu]

On 14/03/2024 at 15:46, Don Pensack said:

Whatever exit pupil that results in is the exit pupil it happens to be.  I would never look at exit pupil to decide what eyepiece to use.  I wonder if anyone does.

I think the only time I do is when using narrowband filters in smaller scopes. It just helps make sure you don't end up with a dim view whilst wondering why.

Other than that it’s normally about picking the right high mag for planets or doubles etc, or at the mid/low end choosing an eyepiece which frames the object. I find exit pupil useful to explain what effects you see, but I agree it’s not something I generally use in choosing the eyepiece to use.

[Louis D]

The other day I was trying to look at the moon while the sky was still bright in my 6" f/5 GSO Newt using a 40mm eyepiece to center it.  Man, was I fighting the secondary obstruction.  A quick calculation in my head of 40/5 = 8mm and my likely 2mm entrance pupil explained to myself why.  After dark, even with the 40mm, I wasn't noticing the issue partly because of the dark background making it easy to shift the shadow to the side of the moon's image by not keeping it centered.

[AstralFields]

If you want to dig in real deep and understand the physics/biology behind the whole '2mm exit pupil is best' , have a look here:

The human eye (telescope-optics.net)

If you are happy with some general rules of thumb, I would go with:

1) If you want to enjoy a nice, sharp, maximum resolution image - choose a 2mm eye pupil.

2) For dimmer objects, the bigger the exit pupil, the better as long as the object doesn't get too small. For galaxies it is worth having a Zoom eyepiece as you can dial in the optimum magnification/brightness = exit pupil for a certain object, under certain conditions.

3) Exit pupils above 6mm , or even above 8mm and even up to 10mm can be useful. Say you have a 16" F/4.5 telescope and you want to use a 2" 40mm as a finder. Even though you will be losing some potential light, you are getting a nice field of view instead. The secondary obstruction and shadow is usually not an issue unless you are looking at the Moon, which normally you wouldn't on such a telescope with a 40mm.

4) Exit pupils below 1mm start to run into a lot of problems due to the design of our eyes.. floaters, blurry image etc. I very rarely go below 1mm but on some very rare exceptions I have found Saturn in an exit pupil of only 0.3mm quite pleasant to look at. I was quite surprised as the whole reason why I put in a 3x Barlow with a 7mm was to show my friend that such magnification is useless... I was wrong. It was 3 AM and the seeing was exceptional. Amazing view.