When I started with visual astronomy, I read that when using an eyepiece that provides you an exit pupil greater than your pupil actually is, you lose "visual" information. Now, I understand the basic concept of exit pupil and how it relates to telescopes and eyepieces, but I have no idea how it looks like in practice.
So my question is: if your pupil is 5mm big and the eyepiece gives an exit pupil of 7mm, does it mean than what you see through the eyepiece is similar to an eyepiece that gives you an exit pupil of 5mm? For the sake of the example, let's assume that both views have the same FoV at the end.
So, we go back to your original question about, does it look similar?
Similar in what way?
Any two dissimilar eyepieces will yield different results and different views in some way.
Assuming your pupil opens to 5 mm, will an eyepiece that produces a 7 mm exit pupil produce a dimmer image than one that produces a 5 mm exit pupil? No.
As long as the eye, the pupil, is fully illuminated they will be equally bright, at least to the extent that you can likely notice.
However the eyepiece that produces the 7 mm EP will have a numerically higher focal length and thus a lower magnification. So, from a magnification point of view, it will look different, but not because of the exit pupil, because of the magnification.
As has been pointed out to me, we should address the difference between refractors and reflectors. With reflectors, Netwonian telescopes, you have a central obstruction. You do run into limits on how low you can go in magnification. At some point you begin to see a shadow of the central obstruction. I don't know if this is directly related to exit pupil, but I don't think so.
If you look at the specs for most Newtonian scopes (Dobs and non Dobs) you will usually see a "lowest useful" magnification. This is typically calculated based on a 7 mm exit pupil. I presume this also applies to SCTs and Maks, but I am not sure.
What happens if you put in an eyepiece that goes lower than that mag? Maybe nothing bad, or maybe you see the shadow of the central mirror.
That 38 mm/70 eyepiece that I use in my Apertura AD12 yields 40X. The published lowest useful mag in a scope of those specs would be 43X, yet I have no problems at all.
I have had 3 Newtonian scopes.
I have used that 38 mm/70 degree in the 8" (6.44 EP) and 12" (7.6 EP) with no noticeable shadow of the secondary mirror. I use this eyepiece often.
However, if I use a 25 mm (6.4 EP) in the 4" F4 I start to see the central shadow. A 32 mm can't be used in this scope. I typically don't go longer than 20 mm in this scope.
While this does not directly relate to your question, it is something to be aware of. If you buy a reflector, plan for no wider than a 7 mm EP you should be safe in most, but perhaps not all.
Edited by aeajr, 20 November 2019 - 05:16 PM.