The astronomical telescope and the telescope

Observing distant objects

Unlike a magnifying glass, a telescope is used to observe very distant objects, such as stars or planets: it does not magnify their size but rather the angle at which we view them. It consists of two converging lenses: an objective lens with a long focal length f’1 and an eyepiece with a short focal length f’2.

The afocal setting

A telescope is said to be afocal – the standard setting for an eye at rest – when the image focus of the objective lens coincides with the object focus of the eyepiece. A beam of parallel incoming rays then emerges parallel: the final image is formed at infinity, which is comfortable for a normal eye that does not need to accommodate.

Magnification

For a telescope set to the afocal configuration:

G = f'1 / f'2

The greater the focal length of the objective lens and the shorter the focal length of the eyepiece, the greater the magnification.

Example: with f'1 = 100 cm and f'2 = 2 cm, we obtain G = 50.

Refractor versus telescope

Instrument	Objective	Main advantage
Astronomical refractor	Converging lens	Simple construction
Telescope	Concave mirror	No chromatic aberration, large diameter possible

Large observatories use telescopes because a mirror several metres in diameter is easier to manufacture and support mechanically than a giant lens, which would warp under its own weight.

Common misconception

The magnification of a refracting telescope does not depend on the diameter of the objective lens. A large diameter mainly improves light-gathering power and resolution – that is, the fineness of visible detail – two qualities distinct from magnification.