I am currently trying to make a Keplerian telescope from "scratch" with plano-convex lenses bought online.

enter image description here

I have found my objective and eyepiece lenses. I'm just not sure if they will suffice to observe Mars with detail, I'm especially doubting my eyepiece choice because of the lens' diameter. Here are my lenses:

-Objective: enter image description here

-Eyepiece: enter image description here

So if there's any other lenses combo that you could recommend that aren't too pricey (~300$CAD) that could make observing Mars possible, that would be wonderful!!

  • $\begingroup$ For C$300 you could buy a telescope that would give much better viewing than your home made Keplerian telescope. One of your problems with the objective is its long focal length, which will make the scope quite unwieldy. $\endgroup$
    – Dr Chuck
    Commented Jun 9, 2019 at 21:33

1 Answer 1


There are a lot of potential problems here. First, double-convex lenses are not necessarily the best so far as chromatic distortion, or, more important, astigmatism & coma go. Most likely plano-convex will work better.
As you suggest, in general the ocular (eyepiece) lens can be much smaller in diameter and still cover the stop-limited field of view of the objective.

I recommend you do some searching for "make your own Keplerian" to see what sort of lenses are recommended vs. telescope size, power, cost.

I also strongly recommend you get or borrow a copy of Smith's "Modern Optical Engineering," which will give you a great introduction to simple optical systems and the difference between axial ray-trace and full-aperture performance. Once you're comfortable with the concepts of Field Stop and Aperture Stop, and lower-order aberrations, you may be better set to design your own 'scope.


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