In everyday life we use a telescope or a pair of binoculars when we want to see greater detail on a distant object. The size of the telescope determines how much detail we can see and the brightness of the image.
Astronomical telescopes perform these two tasks. They are big so that they can collect a lot of light from a faint star or galaxy, making their resolution – the ability to see small detail – as good as possible.
Most every-day telescopes and binoculars use lenses to gather the light which we see through an eyepiece. Astronomical telescopes that use lenses in this way are called refracting telescopes because the objective lens (at the end furthest from the eye) refracts the light to a focus which is magnified by the eyepiece.
Astronomers do not use refractors very much nowadays because if we wished to collect a lot of light from a faint object we would need a very large objective lens. The only way to support a large lens is around its edge. The force of gravity would bend the lens away from its design shape when we moved the telescope around the sky.
The biggest refractor in the world is the 40-inch Yerkes refractor near Chicago in the USA. The largest in Britain is the 28-inch here at the Observatory in Greenwich, known as the Great Equatorial Telescope (see the image above). Find out more about the Observatory's Great Equatorial Telescope.
Over 150 telescopes from the National Maritime Museum's collection are available to view on the Collections Online website.
A reflecting telescope uses a mirror to collect light instead of a lens. This overcomes the problems inherent in supporting the lens in a refractor and the light losses due to the light passing through thick pieces of glass. The mirror of a reflector is at the bottom end of the telescope tube.
The mirror is a fairly thick, rigid disk of glass whose top surface has been accurately ground and polished so as to reflect all the light falling on it to a focus near the top end of the telescope tube. The top surface is made highly reflecting by evaporating a thin film of aluminium on to it in a vacuum. The mirror can be supported around its edge and back surface.
Small reflectors are often in a Newtonian configuration (shown above). They have a paraboloid primary mirror which brings the light of any object in the field of the telescope to a focus near the top end of the tube, called the prime focus. A flat mirror is placed at 45° to the axis of the tube and reflects the light out to an eyepiece at the secondary focus.
The classical Cassegrain
In the classical Cassegrain telescope, the primary mirror is paraboloid shaped. This brings the light of any object in the field of the telescope to a focus near the top end of the tube, called the prime focus. This is used on big telescopes to take pictures of small areas of the sky, using digital detectors called Charge Coupled Devices (CCDs). CCDs replaced photographic plates as they are more efficient.
As any instrument at the prime focus will obstruct the light on its way towards the primary mirror, we can not put large instruments there. Instead we place a smaller curved mirror, called the secondary, just inside the telescope focus where it reflects the light down the telescope tube and through a hole in the primary mirror to a focus just behind it, called the Cassegrain focus. Large instruments, such as a spectrograph, can be placed there.
Unfortunately, the field of a classical Cassegrain telescope is rather small. This problem can be tackled by putting a complex lens system, called a corrector, into the light beam or by changing the classical design by altering the curvature of the primary mirror.
The Schmidt Telescope
For photography of large areas of the sky the primary mirror is made with spherical curvature and an aspheric 'corrector plate' is placed at the top end of the telescope tube. There are three large Schmidt telescopes in the world with fields about 6° across (the Moon's apparent diameter in the sky is half a degree). The oldest of these is the Palomar Schmidt (not to be confused with the Palomar 200-inch) and the other two are the ESO Schmidt in Chile and the United Kingdom Schmidt in Australia. These have been used to produce photographic charts of the whole sky.
Most radio telescopes work in the same way as an optical reflecting telescope except that the mirror is made of metal, which reflects the radio waves up to a detector at the prime focus.
Some radio telescopes are single, large, steerable dishes and others are used as arrays whose signals can be linked together to act as a single very large telescope with very high resolution. There are large radio telescopes at Jodrell Bank in Cheshire, the heart of the MERLIN array - a series of six radio telescopes linked across the UK.
The classical mounting for an astronomical telescope is to have an axis parallel to the Earth's north-south axis, called an equatorial mounting. As the Earth rotates once a day about its axis the telescope is rotated, in the opposite direction, at the same rate. This results in the telescope remaining pointing at a star in the sky as long as it is above the horizon.
The making of a drive to work at a constant speed about one axis with small corrections when necessary is a simple problem but the mechanical design of the mounting, with no vertical axis, is neither simple nor cheap. Many different forms of equatorial mounting have been devised; the Northumberland telescope in Cambridge, the Isaac Newton and the Jacobus Kapteyn in the Canary Islands all have different types of equatorial mountings.
Now that computer controlled drive systems can be made which allow constantly varying drive rates to be used on two axes, we can use the much simpler Altazimuth mounting, which has a vertical and a horizontal axis. The William Herschel telescope in the Canary Islands has such a mounting.
Why do astronomers always want bigger telescopes?
The size of the primary mirror of a telescope determines the amount of light that is received from a distant, faint object. In order to begin to solve some of important astronomical problems, when, how and why the galaxies were formed, astronomers need to be able to analyse the light coming from the furthest and the faintest objects in the sky. For such objects, we need very large telescopes.
Observing from space
We have mentioned radio telescopes, which can be used from the ground because the atmosphere is transparent to radio waves, just as it is to visible light. There are other wavelengths that are absorbed by the atmosphere and do not reach the ground, including X-rays, ultraviolet and far infrared.
The atmosphere also stops us from seeing very sharp detail in images. When you look at the stars at night you can see them twinkle. This is the effect of layers of air at different temperatures, in the atmosphere, bending light towards and away from your eyes. The same bending affects optical telescopes and results in stars appearing as fuzzy blobs, not as pinpoints. Astronomers go to great lengths to put their telescopes where the atmosphere is most stable, but to get the best results we must go outside the atmosphere.
The Hubble Space Telescope was designed to give us this excellent resolution and to be able to work in the ultraviolet. It is giving us pictures better than any seen before and has changed our ideas about many things. Other satellites measure in the ultraviolet, the infrared, X-rays and gamma rays. They have revealed objects that we did not know existed and have resulted in an even greater demand for large ground-based optical telescopes to study these interesting objects.