Making a telescope

Class practical

Using two convex lenses to make a simple astronomical telescope.

Apparatus and Materials

For each student or group of students

• Telescope mount or metre rule with Plasticine or Blu-Tack
• Retort stand and bosses, tall
• Convex lens (+14 D), plano-convex if available
• Convex lens (+ 2.5 D), plano convex if available
• Greaseproof paper or frosted screen
• 200-watt carbon filament lamp (one per class)
• Mounted lamp holder (one per class)

Health & Safety and Technical Notes

The mains lampholder must be fitted with a suitably-fused 13 A plug. It is best if the batten holder is one of the safety pattern where inserting the bulb operates a switch.

Read our standard health & safety guidance

Suitable plano-convex lenses are available from the supplier: ASCOL.

It helps if the room is three-quarters blacked out.

Procedure

1. Put the weak + 2.5 D lens in a holder at the far end of the mount or fix it firmly to the metre rule with Plasticine or Blu-Tack. If it is plano-convex, the convex face should be towards the object.

2. Raise the telescope mount to shoulder height. Turn and tilt the mount until it is aimed at the distant lamp. Catch the real image of the filament on a scrap of tissue or greaseproof paper - like the back of the pinhole camera.
3. Install the eyepiece, + 14D, as a magnifying glass to view the scrap of tissue with one eye. Again, if a plano-convex lens is used, the convex face should be towards the tissue. (The system is not symmetrical.)
4. Slide the magnifying glass nearer and then farther until you see the scrap clearly magnified and in focus. A partner could hold the greaseproof paper to enable the observer to slide the eyepiece. Encouragement to watch the place where the paper is also helps.
5. Take away the tissue, so that you are looking through a telescope at the lamp, with one eye.

6. Open the other eye and use your two eyes to get the telescope properly focused. For comfortable use, the final image should be as far out as the object.
7. The eye at the eyepiece looks through the telescope at an image of the distant lamp; while the other eye, the naked eye, looks straight at the distant lamp.
8. Does the telescope picture of the lamp look larger than the picture seen by the naked eye?
9. If they are not both in focus at the same time, move the eyepiece forwards or backwards a little until you do see both clearly at the same time. You could say to students:
10. Raise your eyebrows and keep your eyes open with "wide eyed surprise".
11. Direct the telescope at familiar objects and look out through an open window. DO NOT LOOK AT THE SUN - take care that this is not possible. This could damage students' eyes badly.

Teaching Notes

• Looking at the printing on spines of books across the room can persuade students that their telescopes do really magnify. They may be surprised to find that the telescope image is inverted; this can make it hard to scan a scene.
• A look at the Moon is not only a special delight; it can persuade pupils who have found focusing their telescope difficult, to succeed at last. (A half moon shows the mountains particularly near the edge: a full moon glares.)

This experiment was safety-tested in February 2007