So confused with exit pupil!

[vlaiv]

I think I managed to terribly confuse myself and I can't seem to get out of this crazy "rational" loop.

A bit of background - I was contemplating (as one does) - best possible M31 scope. Idea was - start with eyepiece and take it's field stop. Calculate needed focal length to project 3.5° degrees on it (nicely framed M31) and then put as much aperture on this as possible. I concluded several things:

- max focal length for 46mm field stop is 750mm

- max focal length for 27mm field stop is 440mm

- obviously one does not need more aperture than is needed for 7mm exit pupil as this light will be wasted

Then it hit me - best scope, one that delivers the most light for given magnification is one that produces the largest exit pupil.

There is also advice that you need 2-4mm exit pupil for best views of faint DSO targets. This will depend on level of light pollution and rationale is that by increasing magnification you dim both the target and background LP thus creating more contrast (eye non linearity).

Now I wonder - is this true? Because, if it is true, what about following example:

You have your very nice 42mm eyepiece with you and your F/6 scope. You are in light polluted skies and want to observe a target. You don't have any other eyepieces with you. This eyepiece gives you 7mm exit pupil, but we all "know" that 3.5mm exit pupil will be better suited in your conditions as it will darken the sky. What do you do? Ah, solution is simple - put aperture mask on your scope, make it F/12.

You half the aperture of your scope and suddenly - you should be able to see better your target in light pollution. Does this seem reasonable to you? I simply can't wrap my head around this.

On the other hand, from my reasoning on best M31 scope - you want 7mm exit pupil for magnification of your choice.

Which one is correct? If aperture rules, and we all know it does - otherwise, everyone would use 2.5" super APO scope and be happy with it's DSO capabilities, why is there notion that 2-4mm exit pupil gives best results?

As a side note, let's compare 32mm plossl (max field stop in 1.25" format) to 55mm plossl (max field stop in 2" format). I'll assume both have 50° FOV.

32mm for 7mm exit pupil will give us ~ 4.57143 F/ratio. 440 / 4.57143 = ~96.25mm

55mm for 7mm exit pupil will give us ~ 7.857143 F/ratio. 750 / 7.857143 = ~95.45mm

What? 96mm scope is the best M31 scope? Regardless if we use 1.25" or 2" eyepiece? Or to put it in another way - 4" scope is the best M31 scope as long as you want to frame M31 nicely. Depending on F/ratio of the scope, you'll choose eyepiece that has max field stop and max exit pupil of 7mm. This holds as long as my ratio with max aperture at magnification holds of course and 2-4mm exit pupil is nonsense.

[Stu]

@vlaiv, I can’t answer all your questions, but my experience confirms that a widefield scope of around 700mm focal length does indeed work very well on M31 under a dark sky. I’ve used various scopes such as an Astrotech 106mm f6.5 triplet, 690mm focal length, a Televue Genesis 100mm f5 (500mm) and Tak FC100DC f7.4 (740mm) with a 31mm nagler with very good results. The Genesis is probably most suited to this as it gives a 6.2mm exit pupil and a wider field of view which frames M31 beautifully and allows you to tease out the fainter outer reaches.

This is relevant if you want to see the whole object, but if you want to see detail then a big dob at higher power is the better tool. That’s when the 2 to 4mm exit pupils kick in I guess; at x200 in a 500mm dob you can be looking for the globs which are visible around M31.

I guess it’s similar with the Veil; you can fit the whole object in a widefield scope at large exit pupils in a 100mm refractor, but a large dob gives you much better detail and contrast on small sections of it.

Not sure if you are familiar with Mel Bartels but he has written a lot about this so I think you might find his information and calculators interesting.

http://www.bbastrodesigns.com/tm.html#myArticles

http://www.bbastrodesigns.com/Calculators.html

[vlaiv]

9 minutes ago, Stu said:

This is relevant if you want to see the whole object, but if you want to see detail then a big dob at higher power is the better tool. That’s when the 2 to 4mm exit pupils kick in I guess; at x200 in a 500mm dob you can be looking for the globs which are visible around M31.

This is my confusion exactly. Say you want to go x200 and observe globs around M31. Say it is in 16" dob with exit pupil of 2mm. Wouldn't 20" dob at x200 work better and show you more at the same magnification? How about 1.4 meter mirror? You could still use that one for x200 and it will gather much more light than starting 16" telescope.

12 minutes ago, Stu said:

Not sure if you are familiar with Mel Bartels but he has written a lot about this so I think you might find his information and calculators interesting.

http://www.bbastrodesigns.com/tm.html#myArticles

http://www.bbastrodesigns.com/Calculators.html

Yes, I read this before. Will need to revisit it though. Thanks for the links.

[PeterW]

Aperture gives you image scale on fuzzies and deeper magnitude for point source stars. As Stu points out it depends on your skies, if you can get to dark ones then a big exit pupil will give you more..... there are number of posts about binocular observers picking up all sorts of stuff that you’d assume were in big dob territory. 
 

peter

[vlaiv]

That has very interesting consequence.

Any scope that is above F/8 is really not optimal for deep sky observing unless it can't adopt some sort of reducer.

This is of course if we assume 7mm exit pupil and 56mm eyepiece as being max (at least affordable maximum). It also limits usability of telescopes with 1.25" focuser to about F/5.7 (40mm plossl).

That puts whole new twist on things, doesn't it?

[Ags]

My best view of M31 was dark skies and 16x50 binoculars. I could see dust lanes and all! 

The only thing a bigger scope can offer is more scale for the same exit pupil. That doesn't help with M31 which is too huge already. So best views of M31 are in bins and small scopes.

Edited December 26, 2020 by Ags

[Kyle Allen]

From home (Bortle class 5) I find above a 4mm exit pupil gives too washed out an image. So unless you’re at a very dark site, I agree with the above that a smaller scope would be better for M31.

[Stu]

2 hours ago, vlaiv said:

This is my confusion exactly. Say you want to go x200 and observe globs around M31. Say it is in 16" dob with exit pupil of 2mm. Wouldn't 20" dob at x200 work better and show you more at the same magnification? How about 1.4 meter mirror? You could still use that one for x200 and it will gather much more light than starting 16" telescope.

So yes, I would say a 20” would show you more at x200 due to the better resolution. I guess a lot depends on the skies. Dark skies allow you to use larger exit pupils without the sky washing out so the contrast is maintained.

[vlaiv]

7 minutes ago, Ags said:

My best view of M31 was dark skies and 16x50 binoculars. I could see dust lanes and all! 

The only thing a bigger scope can offer is more scale for the same exit pupil. That doesn't help with M31 which is too huge already. So best views of M31 are in bins and small scopes.

If you happened to have 16x100 binoculars on that particular night do you think image would be even better? Would you be able to see more / or at least would it be easier to see what you saw?

16x50 binoculars give 3.125mm exit pupil. 16x100 binoculars give exit pupil of 6.25mm

I think that main issue is that people look at things from - fixed aperture point of view. If you fix another parameter - like magnification, then it becomes obvious that best view will be provided by largest aperture on that fixed magnification.

What I do have issue with is the fact that people prefer 2-4mm exit pupil in some cases and I can't figure out why is that. Possible reasons:

- Contrast thing due to sky brightness. If this is the case then something highly counter intuitive would happen. Imagine you have f/5 telescope and 35mm eyepiece. This combination gives you 7mm exit pupil. If magnification is proper for object that you are viewing - it follows that using aperture mask on telescope - thus stopping it down to create narrower exit pupil - would make image better.

This is the main problem that ties my head in the knot - reducing light makes image better somehow.

- Angular size / contrast thing. It is known that human vision system does not equally perceive contrast on different frequencies.

This would mean that magnification is responsible for better image rather than exit pupil. But then again, changing magnification would create better image on same aperture, but then, adding more aperture would again improve things??

I simply can't understand why best images are at 2-4mm exit pupil and not 7mm. Maybe that is just a myth?

 

[vlaiv]

1 minute ago, Stu said:

So yes, I would say a 20” would show you more at x200 due to the better resolution. I guess a lot depends on the skies. Dark skies allow you to use larger exit pupils without the sky washing out so the contrast is maintained.

Not sure why resolution is discussed here - atmosphere limits resolution so there will be not much difference between 16" and 20" at x200.

I think that contrast is maintained across exit pupils, isn't it?

Say you have brightness A for target and brightness B for sky at exit pupil of 6mm. Getting exit pupil down to 3mm will make both target and sky reduce apparent brightness for same amount - thus their ratio is maintained, isn't it? Why would then contrast increase?

Only explanation is that it has something to do with "nonlinarity" of human perception. Maybe two times brighter does not look like same contrast if absolute amount of light changes. But I never heard something like that for stellar sources. Two stars having magnitude of difference in brightness between them - have the same "contrast" regardless if they are mag5 and mag6 or mag11 and mag12, right?

Or to put it in another words - contrast does not change with distance. Distance determines absolute amount of light hitting our eye - further away from sources we are - less light is reaching us, it changes with square of distance. Standing next to billboard and looking at it from half a mile away - makes no difference to contrast on the image (except for atmosphere, but on a very transparent day).

[Ags]

11 minutes ago, vlaiv said:

If you happened to have 16x100 binoculars on that particular night do you think image would be even better? Would you be able to see more / or at least would it be easier to see what you saw?

I think it is a delicate balance between image scale and contrast. A bigger exit pupil might have made the sky background too bright, while less magnification might have made the structures smaller and harder to see.

[andrew s]

On the eye, it has the least aberation at 2 to 3mm entrance pupil. Also if you want to have the best eye resolution you need to excite the cones. So you need to present the eye with an exit pupil of 2 to 3mm with an intensity high enough for colour vision to get the maximum from the image or at least that's how I understand it. If the intensity is high enough the eye is limited by the "fast" seeing not the long term that limits imaging.

Regards Andrew 

[vlaiv]

3 minutes ago, Ags said:

I think it is a delicate balance between image scale and contrast. A bigger exit pupil might have made the sky background too bright, while less magnification might have made the structures smaller and harder to see.

Magnification would stay the same, won't it?

Larger aperture just means more light collected - both LP light and target light.

That is the part that confuses me. Larger aperture collects more of both LP and target light and their relative intensities remain the same.

We know that larger telescopes simply work better, and for fixed magnification - that means larger exit pupil.

That line of reasoning says that large exit pupil should make it easier to see. On the other hand people say they see easier with smaller exit pupil. So confusing.

[Ags]

What I heant is 16x50 worked on the night, possibly better than 7x50 or 16x100 would have.

[vlaiv]

7 minutes ago, andrew s said:

On the eye, it has the least aberation at 2 to 3mm entrance pupil. Also if you want to have the best eye resolution you need to excite the cones. So you need to present the eye with an exit pupil of 2 to 3mm with an intensity high enough for colour vision to get the maximum from the image or at least that's how I understand it. If the intensity is high enough the eye is limited by the "fast" seeing not the long term that limits imaging.

Regards Andrew 

I understand what you are saying, I'm just not sure how resolution impacts all of this.

Maybe it would be best to limit magnification range to x15-x80 for example. Here aperture size and resolution won't have much impact. We are looking at extended objects of relatively uniform surface brightness.

Say we have nice elliptical galaxy and we want determine which scope makes it possible to see furthest extents of it at same magnification.

Take example I gave with a telescope and eyepiece and aperture mask. Same scope, same eyepiece that natively gives 7mm exit pupil - 4 different cases:

- dark sky no aperture mask

- dark sky 50% aperture mask (3mm exit pupil)

- LP sky no aperture mask

- LP sky 50% aperture mask (again 3mm exit pupil)

Can we make definitive ordering on furthest extents of nebulosity seen and explain why is that?

Edited December 26, 2020 by vlaiv
need to be careful of what I'm typing :D

[vlaiv]

3 minutes ago, Ags said:

What I heant is 16x50 worked on the night, possibly better than 7x50 or 16x100 would have.

That is the part that I don't understand.

Why do you think that 16x50 would work better than 16x100 on any particular night?

[Stu]

27 minutes ago, vlaiv said:

Not sure why resolution is discussed here - atmosphere limits resolution so there will be not much difference between 16" and 20" at x200.

I think that contrast is maintained across exit pupils, isn't it?

You referred to globular where resolution is very important for what you see.

re the last point, I was just saying that you tend to avoid larger exit pupils under light polluted skies because the sky background washes out. Under darker skies and/or with filtered views, larger exit pupils are much more useful.

[andrew s]

@vlaiv I don't think you can be definitive as eyesight varies. However, in telescope-optics.net section 6.6.1 is a diagram with plots including bright, dim and high low contrast thresholds introduced by Rutten and Venrooij. Maybe this will help.

Regards Andrew 

[faulksy]

32 minutes ago, vlaiv said:

Magnification would stay the same, won't it?

Larger aperture just means more light collected - both LP light and target light.

That is the part that confuses me. Larger aperture collects more of both LP and target light and their relative intensities remain the same.

We know that larger telescopes simply work better, and for fixed magnification - that means larger exit pupil.

That line of reasoning says that large exit pupil should make it easier to see. On the other hand people say they see easier with smaller exit pupil. So confusing.

image scale is the important thing here. as you say it gathers more lp and target light. but from a dark sky aperture is king . come and look through my 20" just keep hold of your jaw 🤣

[Stu]

1 hour ago, vlaiv said:

That is the part that confuses me. Larger aperture collects more of both LP and target light and their relative intensities remain the same.

Don’t forget the impact of image scale on perceived contrast. Large dobs work because they can maintain surface brightness at larger image scales, and the eye perceives larger objects more easily. For small, faint galaxies for instance this works very well, so a 20” dobs will show you objects that a small scope won’t, because they will be much larger at the same exit pupil.

[vlaiv]

5 minutes ago, Stu said:

Don’t forget the impact of image scale on perceived contrast. Large dobs work because they can maintain surface brightness at larger image scales, and the eye perceives larger objects more easily. For small, faint galaxies for instance this works very well, so a 20” dobs will show you objects that a small scope won’t, because they will be much larger at the same exit pupil.

I'll need to do some testing for this.

I remember one occasion where we put 4" vs 8" scope on galaxies in Markarian's Chain.

8" F/6 scope used 28mm 68° eyepiece giving ~ x43 and exit pupil of 4.72mm

I can't remember what eyepiece F/5 4" scope used - probably 17mm plossl? But it was no contest at all

Dob showed all of the galaxies and ST102 showed only 3 main ones.

[Ags]

3 hours ago, vlaiv said:

Why do you think that 16x50 would work better than 16x100 on any particular night?

Trial and error mostly.