refracting telescopes (refractors)

A refracting telescope uses a lens to focus light. A simple magnifying glass works in the same way – you’ve probably used on the project an image of a window onto a wall. You could then use a more powerful magnifying glass to look at this image in detail, but you get a much brighter view by looking at the image from behind rather than projecting it onto a screen. This makes a simple refracting telescope, with the more powerful magnifying glass being known as the eyepiece.

The distance between the lens and its image is the focal length. The magnification of a telescope is given by the focal length of the main lens divided by that of the eyepiece.

In practice, simple lenses give very poor images, notably as a result of chromatic aberration or false colour, which is caused by the outer parts of the lens acting like prisms and splitting up light into its component rainbow of colours. The steeper the curvature of the surface of the lens, the shorter its focal length but the worse the false colour. So in practice all but the simplest toy telescopes have lenses made of at least two components, each using glass with different refracting properties, so the false colour is largely cancelled out by the other. These are known as achromatic lenses. Better compensation for chromatic aberration is given by apochromatic lenses, which these days have a component made from either fluorite or ED (extra dispersion) glass. Eyepieces also have multiple components.

The main lens of a refracting telescope is known as the objective, sometimes called an objective glass or OG. The objective end of the telescope is usually extended with a dew shield, whose purpose is to restrict the amount of sky seen by the objective so as to slow down the formation of dew.

Refractor diagram
A refracting telescope with an achromatic lens. The purpose of the star diagonal is to make the viewing position more comfortable, but it laterally reverses the image. The total length of the red dotted line indicates the focal length of the objective