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Andy Resnick
Feb14-13, 08:24 AM
Sci Advisor
P: 5,515
Eyepiece for refractor telescope

Quote Quote by PhysicoRaj View Post
Hi, Iam thinking of making my own telescope and I have a magnifying glass lens, biconvex, about 10 cm in diameter and 27 cm focal length. I think it would be better as the objective. For the eyepiece, what type of lens, of what focal length and aperture diameter should I use? (I hope to identify the rings of Saturn.... I may not be able to .. but at least the craters of the moon are okay.)
The ratio of the objective and eyepiece focal lengths provides the (angular) magnification, so the shorter the focal length of the eyepiece, the greater the magnification. Using a negative (concave) lens for the eyepiece is worse than a positive lens when you are trying to look through the telescope (see Galilean vs. Keplerian telescopes).

The f/# of your objective is 2.7, so the f-number of your eyepiece does not need to be much larger than 2.7 as well.

This simple analysis completely ignores aberrations, which are likely to be large.