Having sampled what the sky has to offer to the
unaided eye, or experimented with capturing images
of the Moon, planets or stars with a digital camera,
the next step is to purchase a telescope. This is not
a decision to be taken lightly, and this chapter is
designed to provide the essential information that
you will need to choose a suitable instrument.CHOOSING AND USING A TELESCOPE
Astronomer Book
[1] Patrick Moore's first telescope. The
classic 3-inch brass refractor which he
has owned since boyhood.
[2] A modern computer-controlled
80mm (3.15-inch) starter scope.[3] A long-focal-length telescope is
best for getting close-up views of Solar
System objects such as the crater
Clavius shown here.Astronomical telescopes are of two main kinds, refracting telescopes,
or refractors, and reflecting telescopes, or reflectors. A refractor collects
the light from its target object and passes it through a glass lens (also
called an object glass or objective) where the rays of light are collected
and brought into focus. The image is enlarged by a second lens known as
the eyepiece or ocular. It is obvious that the larger the main object glass,
the greater the amount of light that can be collected. This in turn means
a higher maximum magnification can be achieved. Different eyepieces
can give various magnifying powers. An astronomical refractor gives an
upside-down image which, if required, can be turned the right way up with
the addition of a correcting lens. Since this slightly reduces the amount
of light reaching your eye, correcting lenses are rarely ever used for
astronomical purposes.
One disadvantage of a basic refractor is that the different parts of the
spectrum of light are brought to focus in different planes, so a bright image
tends to be surrounded by gaudy coloured rings which may look beautiful but
are a nuisance to the astronomer. This difficulty is avoided with a reflecting
telescope, where the light collection is done by means of a mirror. A popular
reflector is the Newtonian, first designed by the great scientist Isaac Newton
way back in 1651. The light falls on a mirror which is specially shaped andreflects back on to a smaller flat mirror at an angle of 45 degrees. This mirror
diverts the light to the side of the tube, where it is enlarged by an eyepiece as
before. In a Newtonian reflector you look into the side of the tube instead of
up it.
From the amateur’s point of view, these two types have their own
advantages and drawbacks. The refractor needs very little maintenance and
will last a lifetime if reasonably well treated. On the other hand, a reflector
has to have its mirrors coated periodically with some reflecting substance,
generally a thin layer of aluminium. This has to be done regularly. In addition,
a reflector is prone to going out of adjustment. Against this, a reflector is12