Make Your Own Telescope


The holy month of Ramadan will start in few days, bringing its blessings along. To determine the holy month's start date and the beginning of fasting, telescopes are used to observe the crescent moon of the Islamic calender month. To be able to observe Ramdan's crescent moon by yourself, we offer you the following simple steps to craft your own telescope.

Different Types of Telescopes

Telescopes are used to observe celestial bodies by harnessing the light they reflect and condensing it to generate a closeup view. Italian scientist Galileo Galilei was the first to use a telescope to explore the skies, and he was able to observe Jupitor's moons.

There are different types of telescopes according to the type of object lense(1) that is used to gather light. If the object lense is made of glass and positioned at the fore part of the telescope, then it is a refracting telescope. In refracting telescopes, the light is gathered at the focal level, whereas the ocular lens is positioned behind it so that the observer can see the celestial bodies. Sometimes, there is a need to add further lenses to reduce chromatic aberration(2) and the dispersion resulting from the different wavelengthd of light.

If the object lense is a glass mirror positioned at the lower part of the telescope, it reflects light; here, we are talking about the reflecting telescope, aka Newton's telescope. In reflecting telescopes, the object lense is usually concave; since it reflects light, the image is inverted at the focal level.

Another type of telescopes is the small-sized Schmidt–Cassegrain telescope. It depends on a series of mirrors to bend the path of light, and a spherical primary mirror to condense the incoming parallel bundle of rays on a secondary convex mirror. This light is then reflected once more to the lense through an opening in the primary mirror.

Now, let us see how to make your own telescope. Before we start, it is essential to understand the following terms:

  • Lense focal point: The point where the light from a parallel bundle of rays comes to focus on the lense.

  • Focal length: The distance between the center and the focal point of the lense. There is a positive correlation between the object lense's focal point and the telescope's capacity, whereas there is an inverse correlation between the focal length of the ocular lens and the telescope's capacity.

  • The telescope capacity: the quotient of the focal length of the object lense divided by the focal length of the ocular lens. In other words, the telescope capacity increases as the focal length of the object lense increases, and that of the ocular lense decreases.

  • Dioptre: a unit of measurement of the optical power of lenses or mirrors. It is the quotient of 1000 divided by the focal length of the object lense.

Required Tools

  • An object lense and an ocular lense, which you can buy from an optics store or a lenses workshop. The object lense should be convex, with a focal length equal to 1000 (+1 dioptre). Ask the manufacturer to adjust its size to the big cylinder's diameter. As for the ocular lens, you can use one magnifying lense, or two 4-cm-diameter lenses.
  • Two plastic or carton cylinders with different diameters; one should hardly slide into the other. As for the first, the diameter should be 5.5 cm, and the length 90 cm. As for the second, the diameter should be 4 cm, and the length 25 cm.


  • Clean the lenses.

  • Fix the object lense at the front-end of the long cylinder.

  • Join the two ocular lenses together using an adhesive tape, and fix them at the back-end of the small cylinder.

  • Adjust how the small cylinder slides into the big one. If there is extra space between the two cylinders, you can wrap the small one with carton paper to facilitate sliding.

  • Now, your telescope is ready. You can paint and decorate it, and use a tripod to make it more user-friendly.

  • Direct your telescope towards the sky and look through the ocular lens. Leave some space between your eye and the lense to widen the field of view.

  • Keep moving the sliding telescope forwards and backwards until you find the best vision.


(1) The object lense is the one at the end near the observed object, as opposed to the ocular lens, which is close to the observer's eye.

(2) Chromatic aberration refers to image distortion caused due to the varying refractive index of the lens and the different wavelengths of light.


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SCIplanet is a bilingual edutainment science magazine published by the Bibliotheca Alexandrina Planetarium Science Center and developed by the Cultural Outreach Publications Unit ...
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