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Hi readers, I am planning to build my telescope. Does this very simple 2 convex lens arrangement even possible to observe the moon clearly with high magnification? I am planning to buy 5 cm diameter convex lens with 10cm focal length. What further modification should I make to enhance its quality?


2 Answers 2


If you buy that lens as an objective you will learn a lot very quickly!

  1. Spherical aberration is a monster!
  2. Chromatic aberration is a monster!

But you will be very very disappointed when you try to see something.

You must check the catalog and only order "achromatic doublets"

Companies like Thorlabs and Edmund Optics will have these but they are a bit expensive for hobby applications. You may be able to find discount outlets.

If you want to keep costs low, look for "hobby" lenses, but make sure they are achromatic lenses and the focal length is 8 or 10 times longer than the diameter.

For example D = 60 mm, f = 500 mm would do.

Then for 20x magnification for example, use an eyepiece with a focal length 20 times shorter, or 25mm.

My first refractor telescope was with simple glass lenses like this (but with the long focal length and smaller diameter like I'm suggesting) and I had big problems with achromatic aberration.

My second one was made from the objective and eyepiece from my grandmother's bird watching binoculars (she didn't need them anymore) Basically they were ideal lenses but I put them in a cardboard tube with a cardboard and tape sliding focusing holder so I could have the feel of making a telescope.

After that I realized that cheap lenses are only good to learn about aberrations, not to look at the Moon.

Look for used optics parts, somebody selling an old telescope or pair of binoculars and just get some low cost achromatic lenses to experiment with.

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    $\begingroup$ Nice answer.... $\endgroup$ Dec 27, 2021 at 15:35
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    $\begingroup$ Broken/junk telephoto lenses might sometimes be available at fleamarkets or camera fairs.... some of them are based on an achromatic doublet or pair in the front... $\endgroup$ Dec 28, 2021 at 19:51

As an investigation into how telescopes work, this will be instructive. You should choose a lens with a longer focal length for the objective (big) lens. and a lens with a short focal length for the eyepiece. So 10cm focal length is too short for the main objective lens (and too big and long for the eyepiece)

The magnification is simply the ratio of focal lengths, so if the objective lens has a focal length of 1000mm and the eyepiece has a focal length of 10mm, your telescope will have a magnification of 100× (but a pretty small field of view)

If you just get two lenses of the same focal length, you will get a magnification of 1 (ie no magnification at all!)

You'll also need some way to mount and move these lenses.

However, these are probably "magnifying glass" quality lenses, they will suffer from all the problems of spherical and chromatic aberration. You will get colour fringes from the lens acting as a prism, and you'll get distortion from the edges of the lens not focussing to exactly the same plane as the centre. As you might expect, a "real" telescope costing $1000 will be "better" than one with lenses costing RM5.5.

However, at that price point, and if you want to learn how telescopes work, then go for it! Remember one big, weak, long-focal-length lens. One strong small short-focal-length lens, and some way to mount and focus them. You should certainly be able to see craters on the moon with this sort of set-up. (But probably not details on the planets)

  • $\begingroup$ Hi, thanks for your reply. Do these telescope that use convex lens to magnify only better than those which uses concave mirror to magnify? Do you think that concave mirror is able to gather more light than convex lens which makes image sharp and clear? Merry Christmas :) $\endgroup$
    – Testing Su
    Dec 27, 2021 at 13:12
  • $\begingroup$ The problem is the use of lenses. You can make quality telescopes with quality lenses. But these cost RM550 or more, and not RM5.5 But most amateurs who make serious astronomical telescope use mirrors. You'll still pay hundreds for a high quality mirror, but it will be larger and won't suffer from chromatic aberration. However, to learn how telescopes work, these lenses are fine. You can make a Galilean telescope with a concave lens. The field of view tends to be very small. Be realistic on how much money you are willing to spend! $\endgroup$
    – James K
    Dec 27, 2021 at 13:22
  • $\begingroup$ It is always the convex objective lens which gathers the light, not the eyepiece lens. In a galilean telescope only the eyepiece is concave. $\endgroup$
    – James K
    Dec 27, 2021 at 13:49

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