Stargazing Eyepiece for refractor telescope

  1. PhysicoRaj

    PhysicoRaj 430
    Gold Member

    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.)
  2. jcsd
  3. russ_watters

    Staff: Mentor

    The rings of Saturn are a piece of cake, but with that short focal length, you'll need a short focal length eyepiece and maybe a Barlow.
  4. PhysicoRaj

    PhysicoRaj 430
    Gold Member

    Then what type of a configuration you suggest(focal length combination and size)? May be I will try to get both new, the eyepiece and the objective.
  5. Andy Resnick

    Andy Resnick 6,087
    Science Advisor
    Education Advisor

    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.
  6. A common simple refractor design is a 100 mm doublett lens with a focal length of 1000 mm. For maximum FoV you'd need a 50 mm eyepice. Together with a 2xbarlow lens and a second eyepiece around 12 mm. This way you'd get magnifications of 20, 40, 83 and 167.

    I'd recommend that you start by getting your hands on a book on building telescopes and checking if there is an amateur astronomer club nearby. There are a lot of gotchas if you haven't built a telescope before.
  7. PhysicoRaj

    PhysicoRaj 430
    Gold Member

    Thanks a lot...
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