Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Using the telescope like an astronomer

Class practical

Placing the final image at reading distance from the eye, as an astronomer would.

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

Mounted lamp holder (one per class)

200-watt carbon filament lamp (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' types, where inserting a bulb operates a switch.



Ariel view of apparatus set-up

a Hold a page of print beside the telescope, about 25 cm from your eye. Look at the remote lamp through the telescope with one eye, while you read the print with your other eye. 

b Concentrate hard on the naked eye, while you move the telescope eyepiece to bring the lamp into focus.

Teaching notes

1 This is the same procedure as in the experiment Making a telescope, but try to place the final virtual image only 25 cm away - as an astronomer who wished to make sketches in a notebook would place it. The astronomer would want to move quickly from telescope to notebook without having to re-focus their eyes. 

2 If a student finds this too difficult, yet wants to succeed, do the focusing for them. Let them have a good look, with both eyes open; then move the eyepiece and let them try for themselves. This works well for a student who is held back by not knowing what to look for. 

This experiment was safety-checked in March 2007


Related guidance

About telescope lenses

Using a model telescope


Related experiments

Making a telescope

Model of a telescope with ray streaks