Exit pupil?

[Nova2000]

Hi. 

I don't know much about eyepiece. Some said cause I wear specs I need big exit pupil number. Can someone explain this to me

 

[kilix]

no, with eyeglasses you are looking for bigger "eye relief" number.

How big, that depends on personal preferences, but most of the time, eye relief smaller than 10mm is not recommended for eyeglasses users.

Also, bigger eye relief tends to be easier to use also for non-eyeglass users.

[kilix]

one picture instead of 1000 words:
 

[rockystar]

as said above, it's Eye Relief, which is how far away you need to put your eye from the eyepiece - wearing glasses will naturally push your eye further away.

Exit Pupil is the size of the light cone that the eyepiece produces - and it will be far better explained by others.

[laudropb]

If you have to wear glasses to observe then I would suggest eye relief of between 15 and 20 mm is required.

The exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio of the telescope e.g. for my 24 mm eyepiece in my 200 Dob 24/5.9=4.06 mm. Normally the human eye fully dilated is about 7 mm in diameter. Unfortunately with age this decreases. If the exit pupil is larger than your dilated pupil you will not see the whole image.