I am making a telescope, but I had meniscus lens of power +1 (so focal length 1 Metre). I brought it in a optic store since I cannot find the lens of my need in any shop in my city.

When I put it in sunlight it does not focus the light at a dot, instead it focus as a small circle of diameter approximately 1 inch.

Can I use it as telescope's objective. If not, how to use it as a normal lens (or how to make it to focus light at point)


1 Answer 1


When I put it in sunlight it does not focus the light at a dot, . . . . . which is to be expected.

Given that the Sun has an angular diameter of about $0.5^\circ \approx 0.009^{\rm c}$ when using a $+1\rm D$ lens one would expect the image of the Sun in the foical plane of the lens to have a diameter of about $0.9 \,\rm cm \approx 1/3 \,inch$.
The theoretical diameter exceed the one you measured but it might be worth checking.

Instead of sunlight try and focus a distant object, eg house, tree, etc, onto a white screen/paper which is about $1\rm \, m$ from the lens to see if you can obtain a sharp image of the distant object on the screen.
You might have to "shade" the screen to observe the image.
In particular see if the image is in focus over its whole extent.

If the image is distorted then / out of focus in parts, then the lens will probably not be suitable as the objective lens of a telescope.
Also if the lens is of very small diameter, say less that a couple of centimetres, then its light gathering capacity will be small and the final image that is formed may not be very bright.

  • $\begingroup$ Have you checked how "good" your lens is? What diameter is it? $\endgroup$
    – Farcher
    Sep 22, 2021 at 11:26
  • $\begingroup$ Diameter of my lens in 65mm $\endgroup$ Sep 22, 2021 at 12:15
  • $\begingroup$ Can I use normal convex lens at the focal point of meniscus lens to bring light to focus so than I can use it in telescope. $\endgroup$ Sep 22, 2021 at 12:17

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