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When we look through a telescope with our eyes we can only see the light that is arriving at that moment. For some objects that are very far away there is not enough light for us to be able to see them properly. Detectors work by collecting the light from an object over a much longer time so that it builds up to make a much better and brighter picture. This lets us look at things that are much further away. There are two types of detector that are used a lot today. These are called charge-coupled devices and photographic plates.

Charge-coupled device (CCD)

CCDs are grids of tiny squares called pixels. When light hits a pixel it charges it up. The brighter the light that hits it, the more charged the pixel gets. The whole grid of squares can be read one by one and a picture can be seen almost straight away on a computer. The main problem with CCDs is that they must be much smaller than photographic plates.

Photographic Plates

Photographic plates are plates with a special coating that is changed by light. The more light that hits the plate, the darker it will be when it is developed. Photographic plates can look at a large area, but they are not very good if the light is very faint. The plates also take a long time to develop, so the results are not as quick.


Spectrographs spread light out into different colours. This is called a spectrum. You can see a spectrum if you shine light through a prism, or if you look at a rainbow. White light is split into all of the colours of the rainbow: red, orange, yellow, green, blue, indigo and violet. The really interesting thing about a spectrum, though, is that each one looks slightly different depending on the type of material that the light started from. Each spectrum from a different type of material is unique, just like your fingerprints are unique. This means that we can look at the spectrum of light coming from a star and use it to tell the type of material that it started from. Spectrographs can therefore be used by astronomers to tell what gases the stars are made of.

The spectrum of white light
The spectrum of hydrogen gas
The spectrum of carbon

Click on the links below to find out more about telescopes

Telescopes Introduction

Types of Telescope

Data Reduction

Faulkes Telescope

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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001