Philips Atlas of the Universe

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Choosing a Telescope

A

stronomical telescopes are of two basic types. First

there is the refractor, which collects its light by means

of a glass lens (or combination of lenses) known as an

objective or object-glass. Secondly there is the reflector, in

which light is collected by a curved mirror. The aperture of

the telescope is determined by the diameter of the object-

glass (for a refractor) or the main mirror (for a reflector). In

each case the actual magnification is done by a smaller lens

known as an eyepiece or ocular. Obviously, the larger the

aperture of the telescope, the more light can be collected,

and the higher the magnification which can be used.

Each type of instrument has its own advantages, and

also its own drawbacks. Aperture for aperture, the refrac-

tor is the more effective, and it also needs comparatively

little maintenance; but it is much more expensive than a

reflector of equivalent light-grasp, and it is less portable.

There are various forms of reflectors, of which the

most common is the Newtonian; here the main mirror is

parabolic, and the secondary mirror is flat. The main prob-

lem is that the mirrors need periodical re-coating with

some reflective substance, usually aluminium, and they

are always liable to go out of adjustment. Compound tele-

scopes such as Schmidt-Cassegrains are becoming very

popular, and have the advantage of being more portable

than Newtonians, but unfortunately they are very costly.

In choosing a telescope, much depends upon the main

interests of the observer; for example, anyone who intends

to concentrate on the Sun will be wise to select a refractor,

while the deep-sky enthusiast will prefer a reflector.

Moreover, there is always the temptation to begin with a

very small telescope – say a 5-centimetre (2-inch) refrac-

tor, or a 7.5-centimetre (3-inch) Newtonian – which will

cost a relatively small sum. This is emphatically not to be

recommended. A telescope of this kind may look nice, but

the mounting will probably be unsteady, and the field of

view will be small. Moreover, there is the question of mag-

nification. In general, it is true to say that the maximum

useful power for a telescope of good optical quality is


20 per centimetre of aperture (
50 per inch) – so that,

for example, a 7.5-centimetre (3-inch) reflector will bear no

more than a power of 150. If you use too high a power, the

image will be so faint that it will be completely useless. If

you see a telescope which is advertised by its magnifying

power only, avoid it; it is the aperture which matters, at

least 7.5 centimetres (3 inches) for a refractor and 15 cen-

timetres (6 inches) for a reflector for serious work.

Take care with the choice of eyepieces. At least three

will be necessary: one giving low power (wide views), one

moderate power (general views) and one high power (for

more detailed views, particularly of the Moon).

Refractor. This 10-cm

(4-inch) refractor with a

high-quality equatorial

mount is more than twice

the cost of a reflector of the

same aperture, but offers

the ability to do detailed

lunar and planetary work

thanks to the high-contrast

image which results from

an unobstructed light path.

Scale indicating declination,

the telescope’s ‘east to west’

movement

Adjustable counterweight

Slow motion control on flexible

cable for fine directional

adjustments

Metal straps attach telescope tube

to equatorial mounting

Scale indicating right ascension, the

telescope’s ‘up and down’ movement

Low-power sighting telescope with

a wide-angle view, used to locate

the astronomical targets

Eyepiece, or ocular, magnifies the

image after it has been brought to

focus by the objective

Sturdy metal tripod carrying the

equatorial mounting

Objective lens inside telescope tube

brings the rays of light to a focus

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