Published on Feb 15, 2016

### Abstract

The objective:

You must have heard about telescopes. If you are lucky you might have even seen one. A telescope is an instrument to view distant celestial objects, like the Moon, planets etc. Common telescope are called optical telescopes. They come in two varieties: refractors and reflectors. In refracting telescope (refractors), the optical system consists of lenses. Both the Objective (the optical system at the front end of the telescope) and the eyepiece (the optical system used to look through the telescope) are lenses. In the reflecting telescopes the objective is a parabolic mirror and the eyepiece is made of lenses.

The telescopes used by astronomers are often huge and cannot be constructed very easily. Often the most difficult job when a young enthusiast decides to make a telescope is to find lenses or mirrors. But we have found a design that is suitable for most young students in India. Parabolic mirrors for a reflecting telescope must be ground, which is rather arduous and time consuming.

In this activity we shall make a reflecting telescope. Let us be first of all be familiar with its constraints. The magnification power of a reflecting telescope is equal to the ratio of the focal length of the objective lens to that of the eye piece. For example if the focal length of the objective lens is equal to 100 cm and that of the eye pierce equal to 1 cm then we shall be able to see a distant object almost a hundred times bigger. The next constraint is the length of the telescope. A reflecting telescope will be at least as long as the focal length of its objective lens. Thus, the length of the telescope in the above example will be about a meter. We have chosen a design for this activity that would be practical to make under typical Indian condition.

You can acquire the lens for the objective of your telescope from an opticians shop (a shop from where one gets one's spectacles made). When you go to buy one ask him for a + .5 diopter lense with a diameter of about 5 cm. Should you fail to get a +0.5 diopter lense, take a +1 diopter lens. A magnifying glass ( a double convex lens) with focal length of 2 cm. or more can serve as the eye piece. You can take the help of your science teacher to get it.

A lense of power 1 diopter has a focal length of 1 meter and a lense of power 0.5 diopter has a focal length 2 meters. Thus if we have a lens of power 1 diopter for the objective we shall need a tube at least 1 meter long. One can use PVC tubes normally available at a hardware store (a shop that sells sanitary ware, paints etc.) The tube that we can use for our telescope must have a diameter slighly larger then that of the lens.
Since for a telescope we need to adjust the distance between the two lenses, we need two tubes. One longer for the objective lens and the other shorter for the eyepiece. The diameter of these tubes must be such that they can slide one over the another.

In case, it is difficult to find such tubes, you can make your own tubes with the help of some thick paper and some adhesive tape or glue. The two lenses have to be secured firmly at the end of these tubes very firmly. You may use some bangles for this purpose. Once you have made up the telescope, you will find that it is very difficult to focus on any distant object, while holding it in you hand. So you will also need to improvise a method of clamping it to a firm object. Think, and improvise according to the conditions around you.