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There are thee main types of telescope - Refractor, Reflector and Catadioptric
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 DSOs.
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 much larger.
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 Finally
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
Alt AZ Mount
Red Dot Finder
© Galloway Astronomy Centre 2016 All images are copyright – M Alexander unless otherwise stated