Exit Pupil Question

[chich0]

Hello,

Been reading about exit pupil and eyepeice selection.

Just curious does the exit pupil need to stay in a certain range?
(I have read between 5-7mm)

Is the way to calculate it F/number / eyepeice mm?

What happens if it goes over or under?

[ngcles]

Hi Chris,

The exit pupil of a telescope that is used visually is the diameter (in milliimetres) of the beam of light coming out of the eyepiece when used in the telescope.

It will vary according to the magnification you are using. It can be easily and fairly accurately approximated by dividing the magnification produced by a particular eyepiece with your 'scope, into the diameter of the telescope aperture expressed in millimetres. Alternately, you can divide the focal length of the eyepiece in millimetres by the f/ ratio of your telescope (same answer).

Example 1000mm focal length telescope of 200mm aperture makes the telescope an f/5 (1000/200=5)

Eyepiece used is 10mm fl.

Magnification is 1000/10= x100. Exit pupil is 200/100=2mm.

As to the consequences of exit pupil, please see the thread here and my comments:

http://www.iceinspace.com.au/forum/showthread.php?t=28240

Hope it is of some help!


Best,

Les D

[Merlin66]

The pupil of the eye is usually around 6mm diameter, so if the exit pupil from the eyepiece is larger than that, the light is lost. The lowest usable magnification is that which gives an exit pupil of around 6mm

[chich0]

Wonderful.

Thanks !!! :)