Measuring effective aperture of telescope

[vlaiv]

I just had a discussion on my local forum and learned something interesting that I would like to discuss.

We often hear of some scopes working at stopped down aperture in comparison to what has been declared and there are tests to measure actual aperture involving lasers / torches, etc ...

I've learned about rather interesting way to test if aperture is stopped or not in telescope. It is not convenient test to measure how much stopped down aperture is - it is just a test if it is or not (one can then proceed to repeat the test with ever smaller apertures until one matching true aperture is found).

Test consists of creating square cardboard cutout / sort of square aperture mask with diagonal being aperture that is being tested. Say I want to test if my Maksutov 102 has 102mm of aperture and it is not stopped down by baffle or something. I would make square aperture mask with diagonal of 102mm

I would then take eyepiece - focused to infinity in the scope and put aperture mask in front of the scope. I would then observe exit pupil at large distance (or record it by camera). If I see perfect square with sharp corners - aperture is not stopped down, but if I see corners being clipped - then aperture is stopped down.

I had a go with finder and its cap (not square - but I just wanted to test the effect) - and indeed, from far enough, FOV gets tiny - but shape of field stop becomes the shape of aperture - if I place cap half way on the aperture - I see semi circle blocking the exit pupil.

If I look from close thru the eyepiece - I see nothing at all (which is to be expected).

[neil phillips]

7 minutes ago, vlaiv said:

I just had a discussion on my local forum and learned something interesting that I would like to discuss.

We often hear of some scopes working at stopped down aperture in comparison to what has been declared and there are tests to measure actual aperture involving lasers / torches, etc ...

I've learned about rather interesting way to test if aperture is stopped or not in telescope. It is not convenient test to measure how much stopped down aperture is - it is just a test if it is or not (one can then proceed to repeat the test with ever smaller apertures until one matching true aperture is found).

Test consists of creating square cardboard cutout / sort of square aperture mask with diagonal being aperture that is being tested. Say I want to test if my Maksutov 102 has 102mm of aperture and it is not stopped down by baffle or something. I would make square aperture mask with diagonal of 102mm

I would then take eyepiece - focused to infinity in the scope and put aperture mask in front of the scope. I would then observe exit pupil at large distance (or record it by camera). If I see perfect square with sharp corners - aperture is not stopped down, but if I see corners being clipped - then aperture is stopped down.

I had a go with finder and its cap (not square - but I just wanted to test the effect) - and indeed, from far enough, FOV gets tiny - but shape of field stop becomes the shape of aperture - if I place cap half way on the aperture - I see semi circle blocking the exit pupil.

If I look from close thru the eyepiece - I see nothing at all (which is to be expected).

In a ideal world couldnt we just ask the manufacturer ? There is the problem. Not being forthcoming to begin with. But still a answer shouldnt be that difficult from them really should it. 

Edited November 27, 2021 by neil phillips