Make a telescope

This is how to make a simple telescope using two lenses and a cardboard tube

2 convex lenses of a differing focal length
a card board tube whose length is AT LEAST that of the two focal lengths of the lenses when added up.
Sticky tape
colour felt tip pen


Take your cardboard tube and cut it down to the length of the two focal lengths of the lenses

e.g. if I have 10cm and 20cm lenses I would need a 30cm tube.


Now fix the lenses to either end of the tube using the sticky tape, but try not to obscure the view through the tube.


Using your felt tip pen colour the end of the tube with the shorter focal length lens so you know which way round you telescope is.


Now look though the the end that you’ve marked and your looking through a telescope!

The first thing you'll notice is that the image you see is upside down. Real telescopes have many lenses in them so the image always appears the right way round.

If the focal lengths of you lenses were fairly close you might not think there's much of a difference in image size. If you look through the telescope with one eye, however, and look straight at the object with the other you should be able to see both the magnified and the unmagnified image and compare the two.

Have a guess at how many times larger the image is than the object. In theory if you divide the larger focal length by the smaller one you should get magnification of the telescope. If you take the case above where we had a 10cm focal length lens and a 20cm focal length lens then you divide 20 by 10 and get 2. Therefore that telescope would have a magnification of x2.

 Did you guess the right magnification for your telescope?

Telescopes use lenses to bend incoming light in such a way that the image appears larger. you can see in the picture below how this is achieved.

The magnification of a telescope depends on the arrangement and the focal length of the lenses within it. The quality of the picture depends upon the size (diameter) of the lenses. The bigger the better. You cant make good quality lenses over 1 foot in diameter so you have to use curved mirrors as lenses instead (see 'make yourself ugly' for more info) some of the largest telescopes have mirrors over 8 meters in diameter. you can see below the quality of some the pictures they produce.

 Click on the pictures to see larger versions.