Eye relief - can someone give a concise explanation?

[Gem]

Hi

I think I get it, but could someone please give a brief explanation of eye relief in an eyepiece? What is ideal?
I am long sighted but don't wear glasses when observing (except occasionally to read a star atlas by red torch light!).

Thanks! :)

[Merlin66]

When you look into the rear of an eyepiece fiited to the scope you'll see a bright circle. The size of this circle is equal to the diameter of the objective divided by the magnification ie a 100mm f10 (focal length = 1000mm) telescope with a 20mm eyepiece will give a circle 100/(1000/20) = 2mm. (The size is independent of the size of the eye lens in the eyepiece.)
This is the exit pupil. And represents the beam of light you want to focus with your eye.
Now move your eye forward/ back and side to side. You see that the exit pupil changes size/ shape....there's only one point behind the eyepiece where the beam from the edge of the field and the beam from the centre of the field cross and form a perfect exit pupil.
The distance behind the eyepiece where this occurs is called the eye relief.
This distance can vary from a millimetre or so, up to 20mm, depending on the type and design of the eyepiece.
If you normally wear glasses, you need extra eye relief to give that extra space between the eyepiece your glasses and your eye.
You will see some eyepieces marked LER = Long Eye Relief.
http://en.wikipedia.org/wiki/Eye_relief
HTH

[Gem]

Thanks! Good explanation!

[rmcconachy]

May I make one small addition to Merlin66's explanation above? If you normally wear spectacles because you are long or short sighted then you can do without your spectacles at the scope if you wish. Your best focus point will be different to that of somebody who is not long or short sighted but the image you see will be just as good. However, those of us who wear spectacles to correct astigmatism cannot focus away that aberration (although it becomes less bad with decreasing exit pupil size).

[Gem]

Thanks. I am long sighted but hardly ever wear my glasses when observing. The only times I do is when I am too lazy to remove them from reading the atlas and just want a glance in the eyepiece! I definitely need glasses to read small print using a dim, red torch!!