# Telescope mount or metre rule with Plasticine or Blu-Tack

Students need to assemble and adjust a simple telescope and other instruments comfortably, and quickly. For this, we recommend the following:

The mount
This should be a simple bar to carry sliding lens-holders. It should be free from distracting centimetre and millimetre graduations. It need not be as heavy or as long as an optical bench - 75 cm will suffice.

Squatting or bending over to look through a telescope at bench level puts the observer at a considerable disadvantage. He must roll his eyes upward in a way that easily produces headache. Therefore, the telescope bar should be held at head height on a tall retort stand, so that the lenses are at eye level. The clamp holding the bar should allow it to tilt so that students can direct their telescopes at an object higher up.

Lens-holders
These should accept lenses easily and hold them firmly with their axes parallel to the bar. They should be carried on simple saddles or bulldog clips so that students can move lenses easily in a half-dark room.

The lamp as an object
A large carbon-filament lamp run at normal voltage serves very well, in a room that is half or three-quarters dark.

Students hunting for an image - which is an unfamiliar thing - find it more easily if the object is bright.

However, some teachers prefer a less bright filament. For that, a rheostat should be inserted in the lamp circuit.

With a small class, all the students set up their telescopes at one end of the room and view a lamp placed at head-height at the other end. With a large class, arrange half the class at one end of the room and half at the other end. Then two lamps are needed, one at each end of the room. They should be placed high up, well above head-height, so that students at one end of the room can view the lamp over the heads of the students at the other end.

Lenses for the telescope
The objective is a weak spectacle lens, preferably plano-convex or meniscus, of power about 2.5 D (f = 40 cm). It is placed with the convex face towards the object, away from the observer.

The eyepiece is preferably plano-convex or meniscus (though a biconvex one will do) of power about + 14 D (f = 7 cm). It is placed with the convex face away from the observer.

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