I have 100cm F.L objective and 10cm F.L eyepiece 10cm in diameter and double convex both. what are the planetary objects I can see with telescope made with these lenses.

I also have a double convex and double concave lens of 20cm F.L both having 5cm diameter. will any of the above said lens help the telescope I'm going to make if included somewhere in the design?


2 Answers 2


If F is the focal length of the objective, and f the focal length of the ocular, then the magnification of the instrument is:

M = F / f

So you'll get 10x with the first combination. This is enough to see craters, mountains and valleys on the Moon. You will probably not see planetary disks or the rings of Saturn.

The large diameter of the ocular might be awkward, but it should work. Make sure both lenses are installed on a rigid tube - plastic tubes from water pipes should work. Make sure the lenses are perpendicular to the axis of the tube - small errors there reduce overall performance.

You will need a way to finely adjust the distance between objective and ocular - the ideal distance is F + f but don't just glue them in place; instead, see if you can make two concentric sliding tubes, you know, like the spyglasses from pirate movies. Then you'll be able to look through the instrument and adjust the distance until the image is sharp.

If there's any way you could find a used ocular from an old microscope or binoculars, use that as the ocular in your instrument. It will provide higher magnification (smaller focal length) and its performance should be much better than a simple biconvex lens - fewer aberrations, since true oculars have internal corrections (well, most of them). With the higher magnification you will probably see the rings of Saturn, the moons of Jupiter, or even some equatorial belts on Jupiter.

Take it slowly and do it right. This is totally doable with simple materials. This is the design of the first telescope I've built many years ago. You'll be very proud when something you made with your own hands actually works.

EDIT: Yes, you can stack lenses for extra power in the ocular. If you stack two convergent lenses, so close that they touch each other, having the same focal length, the resulting system has about half the focal length of one lens. If there's some distance in between them, things are a bit more complex.

You will notice that power is not everything, and sometimes more power is beneficial while other times it creates problems. It depends on many factors, which you can discover through experimentation.

Also read about the various eyepiece designs, which are made for specific purposes - not just magnifying power, but also lack of aberrations and a high quality image. But those require special components to make.


By all means go ahead and try various combinations yourself. There's a lot to discover. It's a lot of fun.


Telescopes for astronomy cannot be used hand-held. You must use some kind of mount. You could make one, doesn't matter how simple, as long as it's stable and it can turn in both directions (up/down and left/right). E.g. you could make a mount out of a cardboard box:


enter image description here

Google "diy refractor mount" or "galileoscope mount" for more ideas.


When the eyepiece is convergent, the image is reversed. This is normal for usage in astronomy, where the orientation of the image doesn't matter.

There are special devices called prism diagonals that can be inserted between objective and ocular, that fix the reverse image problem. Star diagonals show the image correct-side-up, but left and right are reversed. Correct-image diagonals show the image correct-side-up and also left and right are correct. They achieve this by doing one or multiple reflections in prisms.

  • $\begingroup$ lovely answer. thanks. any idea including another convex in between or somethere improves magnification? $\endgroup$
    – Khaleel
    May 4, 2018 at 5:28
  • $\begingroup$ @Khaleel I made an edit to my answer related to your new question. $\endgroup$ May 5, 2018 at 8:28
  • $\begingroup$ yeah Gotcha, I built one. it is around 100cm in length. problem is when hand held it is very tough to get things inline.! also is it possible to make the inverted image upright again using some other convex lens? $\endgroup$
    – Khaleel
    May 11, 2018 at 9:30
  • $\begingroup$ @Khaleel I made two more edits answering your new questions. $\endgroup$ May 11, 2018 at 21:57

If you use the 100cm FL convex lens as an objective and the 20cm FL concave lens as an eyepiece, the telescope will have only 5x magnification but otherwise be similar to the one Galileo used to discover the four largest moons of Jupiter. Here is documentation of such a telescope built by students. Beyond 10x or so, you need a steady mount for the telescope, and simple lenses may show chromatic aberration.

  • 1
    $\begingroup$ A few random thoughts: The concave lens eyepiece will definitely work, and it will show a non-inverted image (like binoculars do), but it does have one drawback: the field of view is tiny. It's like looking through a keyhole. This is a major reason why simple divergent lenses didn't catch on as oculars. Agreed on the aberrations of simple lenses: chromatic, spherical, etc. Most of these can be controlled by reducing the diameter of the lens via a simple piece of cardboard with a hole in it (but that reduces the resolving power also). $\endgroup$ May 3, 2018 at 23:42

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