The magnified images from telescopes can be seen through an eye-piece, however, our eyes can only see the light arriving at the moment we are looking. This means that faint and far-off objects may still not be visible. Detectors work by collecting the light from an object over a period of time so that it builds up to produce a far clearer and brighter image. This allows us to study much more distant objects. The two most common detectors used today are charged coupled devices and photographic plates.
CCDs are silicon chips made up of an array of light-sensitive diodes. Each of these pixels becomes charged when light falls on it, the amount of charge depends on the amount of light. The pixels are then read column by column to construct a signal of the image on the chip. This can then be digitised and viewed on a computer.
CCDs are more sensitive to light than photographic plates and their resolution does not deteriorate in low light levels. They also have the advantage that the image can be displayed almost immediately and improved using computers. The detector area, however, is much smaller than that of a photographic plate or film.
Photographic plates are plates covered with a special emulsion that reacts with light as it falls upon it. The greater the number of photons that hit the plate, the darker the image when it is developed. Photographic plates can image a large field of view and are therefore still used in large-scale sky surveys. At low light levels, however, they are no longer linear, which means that the image does not darken by the same amount for each photon, making the images taken difficult to analyse. The plates also take a long time to develop and digitise and canít provide the almost instantaneous results of CCDís.
Spectrographs are used to disperse light into a spectrum. This means that the intensity of different wavelengths of light can be measured separately. A spectrograph consists of a slit to select an object, a collimator that turns the light into a parallel beam, a prism or grating to separate the light and a detector such as a CCD to record the spectrum.
Spectra can be used to study the composition and movement of distant objects because different gases give out different wavelengths of light. Each gas will produce a unique spectrum which can be used to identify it. The emission and absorption lines can also be used to determine the motion of the source by looking at the Doppler shift of the lines.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001