A few quick eyepieces questions

[bombassa]

A few quick questions -

Is the exit pupil the same of equal focal lenght eyepieces regardless of the design? For instance is exit pupil of 6mm ortho=exit pupil of 6mm plossl=exit pupil of 6mm other multi lens design?

How do you calculate field of view and exit pupil of 2" eyepieces? Is it the same as for 1,25 eyepieces?

Thanks in advance

[brantuk]

This link will tell you everything you need to know about ep's and exit pupils - including how to calculate

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

[jamespels]

Hi bombassa,

not exactly an expert on this myself but I do know the exit pupil size is not (necessarily) the same as the eyepiece focal length - the human eye can only really dilate to about 5mm (max 7mm for younger folk) so EPs are designed to about this limit.

According to a popular online encyclopaedia (which is often but certainly not always correct!), the exit pupil size can be calculated as the focal length of the EP divided by the focal ratio of the telescope, so a 6mm EP on a f8 scope would have an exit pupil size of 6/8 of 0.75mm whereas a 24mm EP on the same scope would have an exit pupil size of 24/8 = 3 mm. Like I said, I am not an expert and cannot validate this but I have no reason to believe it's wrong...

[jamespels]

Thanks brantuk - I've managed to miss that primer for the last few weeks - will be very helpful choosing EPs!

At least it confirms that I'm not talking cobblers...

[bombassa]

Hi bombassa,

not exactly an expert on this myself but I do know the exit pupil size is not (necessarily) the same as the eyepiece focal length - the human eye can only really dilate to about 5mm (max 7mm for younger folk) so EPs are designed to about this limit.

According to a popular online encyclopaedia (which is often but certainly not always correct!), the exit pupil size can be calculated as the focal length of the EP divided by the focal ratio of the telescope, so a 6mm EP on a f8 scope would have an exit pupil size of 6/8 of 0.75mm whereas a 24mm EP on the same scope would have an exit pupil size of 24/8 = 3 mm. Like I said, I am not an expert and cannot validate this but I have no reason to believe it's wrong...

Forgive my bad english - I know the maths but was wondering if it's valid for all types of eyepieces of equal focal lenght _(of the eyepiece).