© Galloway Astronomy Centre 2016
All images are copyright – M Alexander unless otherwise stated
There are thee main types of telescope - Refractor, Reflector and
Refractor - telescope has two or more lens at the front.
Minimum lens diameter recommended is 80mm. This
type of telescope is good for viewing the Moon and
planets, but limited to brighter Deep Sky Objects
(DSOs) ie Galaxies and nebulae, except for larger
aperture ones, but these are expensive and heavy.
Short focal lengths (400 or 500mm) give a wide field of
view so the planets will look quite small.
For a decent view of planets a focal length of 900mm or
more is recommended.
Reflector - telescope uses mirrors. Minimum mirror
diameter recommended is 130mm. The smaller
aperture telescope is good for viewing the Moon and
planets, but limited to brighter DSOs. A 150mm
telescope will give much brighter images allowing
fainter objects to be seen more easily. Larger apertures
of 200mm or more are more expensive and can be
cumbersome, but will give unrivalled views of fainter
Short focal lengths (700mm or less) give a wide field of
view so again objects will look small. Longer focal
lengths of over 1000mm allow objects to be made
Catadioptric - telescope uses both a glass “corrector”
plate and mirror. Even small ones are expensive
compared to the other two types due to the complexity
of design. For apertures of 105mm or more they offer
good views of the Moon, Planets and brighter DSOs
equal to high quality refractors for a similar aperture,
but at a lower price.
All telescopes need a mount to allow it to be
pointed at the sky accurately and firmly.
An Equatorial mount is complex and, therefore,
expensive. It allows the telescope to be moved in
only one axis to track the motion of the stars, etc.
The mount can be fitted with motor drives to
automatically follow the sky. This type of mount is
used for all three types of telescope.
A Dobsonian mount is simple, sturdy Alt Az
(Altitude and Azimuth) mount constructed like a
wooden box which makes it relatively cheap. It is
only sold with reflector telescopes. The telescope
can be moved left and right, up or down easily.
Their advantage is that most of your money goes
into the optics. The only drawback is that the
telescope needs to be moved a little bit on two
axis when viewing an object (an easily mastered
technique) otherwise it will move out of the field
of view due to the Earth's rotation. Motorised
versions are available but add to the price.
Alt Az (Altitude and Azimuth) mounts has recently
seen a revolution with the introduction of the
single arm motorised mounts. This light weight,
strong, stable mount is designed for small
refractors and reflectors. Also see GOTO Mounts in
Note: Large catadioptric telescopes are often sold
on sophisticated heavy weight single arm and
forked Alt Az mounts.
Typically, smaller telescope come with a red dot
finder (RDF), while suitable for locating bright
objects like stars and planets they can be difficult
to use when finding DSOs.
Larger telescopes are fitted with a magnifying
finder (effectively a mini telescope), typical sizes
are 6x30 (6 times magnification with 30mm
aperture) and 9x50.
Note – cheap, poor quality telescopes are often
fitted with a tiny 20mm finderscope which is
virtually useless - avoid telescopes supplied with
these or at the least replace it.
Most telescopes come with one or more "Plossl"
eyepieces (EP). The focal length (FL) of the EP
appears as a number on the barrel. By dividing the
FL of the telescope by the FL of the EP the result is
the magnification with that EP.
E.g. 1000 / 25 = 40x magnification.
You can reverse the calculation to determine the eyepiece FL to
achieve a specific magnification.
Typically only 3 EPs are needed to give a range of magnifications
between 30x to 100x and if a 2x Barlow is used as well this will
double these magnifications. There is little point exceeding 160x
with telescopes under 150mm as the image becomes too dim.
You can work out a rough maximum magnification for any
telescope by multiplying its aperture (in inches) by 35.
Many telescopes are fitted with a focuser capable of taking both
1.25" and 2" EPs (although Skywatcher only include 1.25" EPs).
This size is the diameter of the EP barrel. 2" EPs are physically
larger, more expensive and offer wide-angle views.
One note of caution – Ultra Wide-angle EPs, both 1.25" and 2",
have become very common. While they work well with the long
FL Refractors and Catadioptric telescopes, these EPs should be
avoided for the shorter FL refractors and reflectors.
There is a huge range of eyepieces on the market, again we can
offer advice on what best suits each type of telescope.
Next - And Finally
Red Dot Finder
Alt AZ Mount
Tel: 01988 500594