Exit Pupil

[Rob63]

I am looking at EP's at the moment, and I hear a lot about making sure it has the correct Exit Pupil.

Can someone tell me how to calculate exit pupil size please ?

[John]

The exit pupil is the bundle of light that exits the eyepiece and enters your eye. It's diameter depends on the specs of the scope and eyepiece - the formula I use is exit pupil = focal length of eyepiece divided by focal ratio of scope. So a 30mm eyepiece in an F/10 scope gives a 3mm exit pupil and the same eyepiece in an F/5 scope gives a 6mm exit pupil, and so on.

The issues seem to come at the extreme ends of the exit pupil scale. A small exit pupil of .5 mm can show the floaters in your eye for example. A very large exit pupil of, say, 8mm probably means that not all the light being bought to focus by the scope & eyepiece will get into your eye as the dilated pupil is normally only around 7mm and this gets smaller as you get older.

I'm just over 50 and I try and keep my max exit pupil to around 6mm with my scopes and around .5mm as the smallest.

Hope that helps a bit

[Moonshane]

what John said! I'd add that larger exit pupils also mean that in skies where you have any light pollution the views are very grey and washed out. it matters a little less at darker sites though. many observers use large exit pupils and love them, personally I like to keep around 6mm max

[nicks90]

on a related note then - what is kidney beaning? Is that when the exit pupil is too large?

[umadog]

Kidney beaning is spherical aberration of the exit pupil (explained below). It's distinct from blackout, with which it is sometimes confused. A diagram showing kidney beaning is here: http://www.telescope...berration_2.htm So basically, kidney beaning means that rays from different angles have different eye reliefs. So your eye has to be at different distances so see the full field and so the full field is never visible at once.

EDIT:

the exit pupil is the number that determines the image brightness. Smaller is dimmer (and higher power). People typically don't stray much below 0.5 to 0.25 mm. Larger is brighter (and lower power). It ceases to get brighter once the exit pupil matches your eyes dilated pupil. If exit pupil does get larger than the eye's dilated pupil, the image doesn't become dimmer. So the world does end it just means that at very low powers you're not using the whole of the objective.

[Rob63]

Thanks for the information, I am pushing 50 myself so I will be looking at EP's in the range suggested (0,5mm - 6mm).