Buying A Telescope

1. Try to buy from a shop/store that specialises in astronomical equipment (OK they're thinly scattered around the UK). They can advise on the best equipment to purchase on a given budget. You may also find that, with more choice, a little more money buys a much better telescope. Search the internet or look in astronomy magazines for a suitable shop.

2. Telescopes are about gathering light from faint objects. Many objects in the sky are not small just dim. So, it's about aperture (the diameter of the main lens/mirror), the bigger the better, and not magnification. Claims of staggering magnifications in telescope adverts are unreliable because over-magnified images are faint and fuzzy. In practice, magnifications up to 50x, maybe 100x, are ample. Astronomical telescopes come with interchangeable eyepieces; low magnification ones are much easier to use, giving brighter crisper images. To determine magnification you divide the focal length of the eyepiece (written on it) into the focal length of the telescope (indicated on the lens collar or, for reflectors, on a plaque on the barrel). For example, a 40mm eyepiece used with a 2000mm (2m) telescope gives a magnification of 2000/40 = 50x.

3. The mount (tripod and steering mechanism) is as important as the telescope (indeed astronomers can spend more on the mount than the scope itself). A wobbly mount and a telescope that cannot be firmly locked into position can ruin observing. So, give it a prod before buying.

There are two basic kinds of mount; altazimuth and equatorial. Altazimuth mounts rotate horizontally (left-right) and vertically (up-down) and are logical to use. Equatorial mounts are tilted so the 'left-right' rotation is about an axis parallel with the Earth's rotation axis (N-S). This enables the telescope to easily follow objects across the sky for photographic purposes. However, equatorial mounts have to be aligned before use and are not intuitive to use. If you just want to point the telescope at something and look, buy an altazimuth mounted telescope.

4. For more money one can buy a telescope which can aim itself at objects in the sky and some can now take images automatically too.  Such telescopes are said to have 'GoTo' capability. GoTos have to be set up first but you can put the automated scopes outside and leave them to get on with it. There is some fun in exploring the night sky for yourself though. Arguably the most important criterion for a telescope is 'can I be bothered to get it out and use it'. If the telescope takes ages to set up (and is too heavy and bulky) the user may soon get tired of it.

5. Many shops sell long thin refractor telescopes, typically with apertures under 60mm, simply because they look like what everyone thinks a telescope should (i.e. something Admiral Nelson might have looked through!). However, they can be quite difficult to use, requiring precision aiming, and return dim fuzzy images with high magnification. With filters they can be used for solar observation though.

A quick search of the internet will show that modern large observatory telescopes are wide and short, with a curved mirror at the back, i.e. nothing like the scopes you see in department stores...astronomy has moved on. You can buy amateur-sized versions of the same thing; Newtonians, Cassegrains etc. 'Short fat' scopes (said to have low f numbers) can give brighter images and have the additional advantage of being less prone to wobble.

6. Finally, this isn't the whole story. Your local astronomy society (of which MARS is just one) has members with experience of many telescopes and who can offer good advice. If you get to one before Christmas, it may guide your buying choice but even after Christmas they can give you assistance in making the best use of the scope you've got.

More detailed advice is provided by the Society for Popular Astronomy, at:

http://www.popastro.com/main_spa1/help-advice/choosing-a-telescope/