All About Space Astronomer Book - 2014 UK

Choosing and using a telescope

Second-Hand Options

There are a number of options available if you are happy to purchase a

second-hand telescope. There are numerous online second-hand sites

where you may be able to pick up a bargain. eBay is another potential

source. As usual when buying anything privately, be cautious. Ask whether

they are willing for you to see the scope. Even if you don’t inspect beforehand,

this will help to verify that there is actually a telescope at the other end!

Mounts

So far we’ve made little mention of telescope mounts despite these being

a vital component to ensure that you get the best out of your purchase.

Telescope mounts are analogous to speakers in a hi-fi system. After

spending your money on a new amplifier, there can be a reluctance to spend

a fortune on high-quality speakers. Buying cheap here may result in a sound

quality which doesn’t reflect the quality of the amplifier at all well.

Similarly, if you put a decent telescope on a wobbly, cheap mount,

you’ll degrade the quality of the view you get and your experience may be

disappointing and rather frustrating. Some telescopes are sold with mounts

included while others require you to buy the mount separately. Some scopes

are also sold integrally connected to the mount. There are many different

makes of mount available but, fortunately, the number of different types is

use of optical amplifiers will mean that you can partially adapt a long-focal-
length scope for use as a deep-sky instrument and vice versa. However, the
results achieved will not be as good as a dedicated scope.
To assist you further, below is a general indication of the best type of
scope to buy for each budget group. It’s by no means meant to be a definitive
list but hopefully it will point you in the right direction.

Up to £100 This is probably too low a budget for a new telescope so the
recommendation here would be to purchase a good pair of, say,
10 x 50 binoculars.
£100–200 There are numerous small refractors available in this price range with
apertures 60–90mm. Avoid computerized control and concentrate on
optical quality and a good mount.
£200–500 A 10-inch Dobsonian or equatorially mounted 8-inch reflector make great
deep-sky telescopes. A 4-inch refractor is another option for lunar and
planetary observing.
£500–1000 At this level look for good-quality optics on a robust equatorially
driven mounting.
£1000 plus Optics and sturdy mount again come fi rst. Colour-corrected refractors or
catadioptric instruments for high-magnifi cation views of the Moon and
planets are recommended.

Try Before You Buy
If you want to try out a particular telescope before you buy it, have a word
with your local astronomical society to see whether anyone in the group has
the same or a similar model that they are willing to let you look through and
examine. If you have no luck here, have a word with your telescope stockist
to see whether they know of anyone locally who may be able to offer the
same service.
An internet search on a particular type of telescope, stating the
manufacturer, type and size, is a good way to see whether there has been
any user feedback about the instrument online. Here you can often pick up
existing problems or even learn that the model you have chosen is regarded
as a superb bargain.

[7] Optical amplifiers such as Barlow lenses can be used to alter the effective focal

length of a telescope.

[8] Different telescope designs shown

schematically. Yellow lines represent parallel

light rays entering the scopes and being

brought ino focus at f, the focal plane, which is

where the eyepiece is located. l is the lens in

the first image, of a refracting telescope. The

rest are reflecting telescopes, and the labels

P, H, E, S indicate the shape of the mirror

bringing the light into focus, explained in the

diagram. F stands for flat – a flat mirror.

Paraboloid

Hyperboloid

Ellipsoid

Spherical

Lens

Flat

Focal plane

Altitude axis

Declination axis

Right ascension

Refractor Prime focus

Catadioptic

prime focus Newtonian Nasmyth

Schmidt camera

(or Maksatov)

Schmidt

Cassegrain

Cassegrain

Ritchey

Chrétien

Folded

Cassegrain Coudé

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