Page 37 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHA
WHAT TO USET TO USE

Finderscope
WHAT TO for a useful fi nder is
The minimum size
LOOK FOR IN 5x24: that means
5x magnifi cation
with a 24mm lens.
A REFRACTOR

Mount
The mount should
hold the scope and
tripod fi rmly so that
everything is sturdy.

can be magnifi ed so we can see incredible details, such
as craters on the Moon. Eyepieces
Eyepieces should have
Whatever you do, never buy a telescope that sells 1.25-inch diameter
itself on how much it magnifi es – there are many Focuser
Rack-and-pinion barrels. Good focal
more important things to consider. Most of the sky’s lengths to look for are
focusers are usually
fi ne sights can be seen with quite low magnifi cations, moved by turning a 25mm and 12mm.
by which I mean less than 75x. Telescopes that have dial. The movement
statements on the box like “Enter space with this 300x should be smooth
magnifi cation!” should be avoided. but fi rm.
DECIDE ON A DIAMETER
Another critical consideration when you’re deciding
what to buy is the telescope’s diameter. More correctly,
this is called the aperture size – of the lens in the case
of a refractor telescope, and of the mirror in the case
of refl ectors. In either case, the larger the lens or the
mirror, the more light the instrument will be able to
capture, and the more you can magnify.
Not surprisingly, the cost goes up as the various bits
get larger. So go back to the dedicated suppliers who
will consider your needs, your ability and your budget
before giving you expert advice on what to buy.

JARGON BUSTER

• APERTURE This is
the diameter of the main
objective (or primary) lens,
which determines how much
Focal length P light a scope can capture.
Prime focusrime focus
• FOCAL LENGTH The
Secondary econdary
S distance from lens to focus
mirrorrror in a refractor, or from prime
mi
focus to primary mirror in a
refl ector (see diagram, left).
• MAGNIFICATION To work
Sta out magnifi cation, divide a
Starlightrlight
scope’s focal length by the
Ey focal length of the eyepiece.
Eyepieceepiece
REFLECTOR For example, a scope with
This second type of telescope 900mm focal length and a
captures light with a curved mirror instead of a lens. It is 12.5mm eyepiece will give
the mirror that does the focusing towards the secondary you 72x magnifi cation.
UPSIDE-DOWN IMAGE
mirror, which defl ects the light into the eyepiece.
skyatnightmagazine.com 2012 37