To: ALL
Date: 02/27/97

From: CVNV05A JEFFREY CHRISTLIEB Time: 9:23 PM

The following was something I found on the World Wide Web. Just thought
I'd share it with those who may be in the market for a new scope. --Jeff

Science and Engineering Research Council
Royal Greenwich Observatory

Information Leaflet No. 11: `Telescopes'.
 

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.
 
 

The Refractor.

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 at the Old Observatory in
Greenwich.
 
 

The Reflector.

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 onto it,
in a vacuum, a thin film of aluminum.
 
 

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.
(See the pamphlet on Detectors.) 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.
 
 

Radio Telescopes.

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.
 
 

Telescope Mountings.

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 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
analyze 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 analyzed. (See the pamphlet on
Spectroscopy.) 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
radiation. There are other wavelengths, however that are absorbed by the
atmosphere and do not reach the ground. These include Xrays, 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 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. 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, Xrays and 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.
 
 

Produced by the Information Services Department of the Royal Greenwich
Observatory. PJA Thu Nov 25 15:37:00 GMT 1993

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