- Describe the features of reflecting and reflecting telescopes (detailed ray diagrams not needed) - Demonstrate an understanding of why the world's largest telescopes are reflectors rather than refractors

WARNING: Never view the Sun with a telescope or binoculars

Telescopes let us see the finer details of the Moon and planets and other objects in the sky that we otherwise would not be able to see.

We each have our own set of 7 millimetre telescopes - our eyes. To see further we must rely on binoculars and telescopes.

Telescopes have a lens, mirror or both to enlarge images by collecting light.

There are two types of telescope that we will study: refractor and reflector.



A convex lens is used at the end of a tube to bring an image into focus at a point. Another convex lens called an eyepiece is used at the opposite end of the tube to magnify the image.

The lens is typically made of glass coated in a film to allow less light to be reflected and more to pass through the lens.


A reflector collects light at one end of a tube and reflects it off a concave mirror. It is brought to a focus by a secondary mirror further up the tube at a 45 degree angle which is then magnified using an eyepiece.

This type of reflector is called a Newtonian reflector.

Another form of reflector is the Cassegrain reflector. This reflects light from its concave lens to a secondary mirror. This mirror is facing the primary mirror and reflects the light back down towards the primary but focuses through a small hole behind the primary.

A reflector's mirror is usually made of glass with an aluminium coating.


Right Way up

Depending on the type of telescope you use, you will notice that images may be upside down or back to front.



Although neither type of telescope is perfect there are several reasons why reflectors have gained in popularity over refractors:

  • Higher resolution can be acheived with a smaller tube
  • Refractors suffer from chromatic aberration. This means that colour can sometimes be warped
  • Refractors suffer from lens sagging. The weight of the lens is supported by the edges and gravity distorts this. The maximum size refractor can be up to 1 metre wide.



Shaped so both sides bend outwards

Shaped so both sides point inwards

Objective or Primary lens
The main lens in a telescope that collects the light

A small lens used to enlarge the image.


Why are the world's largest telescopes reflectors rather than refractors?