Royal Greenwich Observatory
Information Leaflet No. 11: 'Telescopes'.
In every-day life we use a telescope or a pair of binoculars when we want to see something in greater detail that is far away. 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 that 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, where it 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 at the Old Observatory in Greenwich.
The Reflector: [Classical Newtonian]
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 [parabolic] 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 onto it, in a vacuum, a thin film of aluminium.
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 are rapidly being replaced by more efficient television type detectors called 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 prime focus, where it reflects the light down the telescope tube and through a hole in the center of 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, a "corrector plate", into the light beam, or by changing the classical design by altering the curvature of the primary mirror [Schmidt-Cassegrain telescope].
[The Maksutov telescope is similar to the Schmidt-Cassegrain, but with spherical mirror and corrector plates]
[The Ritchey-Chrétien telescope is similar to the Schmidt-Cassegrain, but with a hyperbolic primary mirror]
The Schmidt Telescope:
For photography of large areas of the sky, the primary mirror is made with a 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, like the Jodrell Bank telescope, 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, and at Cambridge.
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 axis vertical, 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 Alt-Az 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 [or lens] of a telescope determines the amount of light that is received from a distant, faint object. Some of the most important astronomical problems are, today, in cosmology. Astronomers want to know how the galaxies, of which the Milky Way is our galaxy, were formed, when, how and why. In order to try to solve problems like these, we need to be able to analyse the light coming from the furthest and the faintest objects in the sky. The light from these objects must be fed into instruments attached to the telescopes so that the light can be analysed. For such objects we need very big telescopes.
Observing from space:
We have mentioned radio telescopes; these, like optical telescopes, can be
used from the ground because the atmosphere transmits these sorts of
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. When you look at the stars at night you see them twinkle. This is the effect of layers in the atmosphere of different temperature bending the light towards and away from your eyes. The same bending affects optical telescopes, and results in stars appearing, not as pinpoints, 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. Unfortunately a mistake was made
with its primary mirror and it does not perform as well as it should.
Despite this, 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.
[ARVAL's Note: See The Hubble Space Telescope (HST) in ARVAL's Gallery]
Produced by the Information Services Department of the Royal Greenwich Observatory.
PJA Tue Apr 16 15:37:00 GMT 1996
Updated: September 30 '97, June 26 '14
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