CCDs and Telescopes

Astronomers use telescopes (such as the Liverpool Telescope in Figure 1) to observe objects such as asteroids, comets, planets, stars and galaxies. They are able to collect more light than the eye can so that we can observe faint objects. Telescopes were originally designed with eyepieces to let astronomers look through them, but nowadays professional telescopes have special instrumentation which allows the light to be detected more efficiently. A key property of a telescope is its size, which usually describes the diameter of its primary mirror. This primary mirror is the area that the telescope uses to collect incoming photons. A typical amateur telescope might be 30 - 50 cm, whereas the Liverpool Telescope has a 2-metre primary mirror. The largest optical telescopes in the World are in the 8 - 10 metre range allowing them to collect between 16 and 25 times as much light as the Liverpool Telescope.

Figure 1: The Liverpool Telescope on La Palma.
Credit: Liverpool John Moores University/Astrophysics Research Institute
CCD cameras are very sensitive digital cameras, built around a Charge-Coupled Device (or CCD; see Figure 2) which is able to detect photons falling on its surface allowing measurements to be made. CCD cameras only measure the brightness of an object, so filters are used in conjunction with them to measure an object's colour.

Figure 2: A typical CCD chip.
Credit: National Schools' Observatory

How much more light can a 2-metre telescope collect than a 1-metre?

Twice as much
No, think about the area of the mirror (rather than its radius)
Four times as much
Yes, this is correct - the area is four times greater
Eight times as much
No, think about the area of the mirror
1.41 times as much (the square root of 2)
No, think about the area of the mirror