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In every-day 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 that we use determines how much detail we can see with it and how bright the image looks. Astronomical telescopes are used basically to do these two things. They are big so that they can collect a lot of light from a faint star or galaxy and so that their resolution, the ability to see small detail, is 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.
The problems inherent in supporting the lens in a refractor and the light losses due to the light passing through thick pieces of glass are overcome in the reflecting telescope by using a mirror instead of a lens to collect the light. The mirror of a reflector is at the bottom end of the telescope tube. It consists of 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. This mirror can be supported, not only around its edge, but also all over its back surface. The top surface is made highly reflecting by evaporating a thin film of aluminium on to it in a vacuum.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 takes a paraboloid shape. 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. This used to be done using photographic plates but these have largely been replaced by more efficient digital detectors, called Charge Coupled Devices (CCDs).
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
The classical mounting for an astronomical telescope is to have an axis parallel to the Earth's north-south axis. 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. This is called an equatorial mounting. 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?
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, however, that are absorbed by the atmosphere and do not reach the ground. These include X-rays, the ultraviolet and the 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, not as pin-points, but as fuzzy blobs. 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, Xrays 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.
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