Where are you going to use your Telescope? A question often asked is, 'can I use it indoors looking out of my bedroom/conservatory, etc?'. The simple answer to that is NO. There will be reflections from the glass and air currents which will badly distort and diminish the views. So you either use it outdoors in your back garden or travel somewhere. Which brings us to the issue of
Portability - and a frequent comment, even from non-beginners when the scope first arrives is 'I didn't realise how big/heavy it would be'.
Even if you do most of your observing from your back garden the weight and size of a scope can be an issue, especially if you have to negotiate steps or any sort of slope. And there are times when you may want to take your scope elsewhere - to a local astronomy society Star Party for instance. There is also the not so small matter of setting it up every time you want to use it, especially if you are old or suffer some kind of infirmity which can make it difficult to carry heavy, bulky loads. Once you get up to the 8ins size and beyond most scopes can be quite heavy and bulky.
In this repect Refractors and Reflectors are far longer and bulkier than Catadioptrics of the same aperture. Which is one reason why the latter have become so popular. Fortunately most small to medium size scopes up to about 6 ins of the type we are considering are usually reasonably manageable. And if you can store the mount without having to completely dismantle it then you don't have to go through the business of setting it up and making sure it is level every time you use it.
Mount: A good mount is very important. It must be stable enough not to tremble at the slightest breeze - which is where buying from a reputable supplier rather than a department store is highly recommended - see Suppliers.
Basically there are 2 types of Mount, the Alt-Az ones which you move in two directions, up and down and side to side, and the Equatorials which you align with the sky according to your latitude so that you only have to move it in one direction.
But Equatorial mounts are not as easy to use as the Alt-Az mounts. However they do have the advantage that once you have found your object you can track it by just turning one knob. And if you have a motor attached that will do it for you.
Of course if you have a GOTO telescope it doesn't really matter which type of mount you have unless you are seriously into AstroPhotography, as the computer will take care of all of the tracking once you have done the initial alignment. But for serious astrophotography the Equatorial mounts are much preferred - the motors only have to move in one direction not two and will give much better tracking.
Viewing position: This concerns Newtonian types of telescope in particular. Moving the scope around to a different part of the sky can mean that you end up with an eyepiece in an impossibly difficult position for viewing. The solution is to loosen the clamps holding the tube and rotate it so that the eyepiece is in a better position. Not a very nice thing to have to do in the dark on a cold night as I discovered when I nearly dropped my tube - it had ice forming on it! Some specialist makers have special tube rings which allow for easy rotation and it should surely be a standard feature for all of these scopes! By contrast, Refractors and Catadioptrics usually have a diagonal which you can easily rotate to a convenient viewing position. And the compactness of Catadioptrics also score highly in regard to..
Eyepiece height: With a Refractor or Reflector the variation in the height of the eyepiece between looking at something near the horizon and something overhead can be considerable. One moment you are almost on tip-toes, the next minute crouching down or even flat on your back. And the longer the tube, the greater this difference becomes. With scopes larger than those we are considering you may need steps.
For really comfortable viewing you need a height-adjustable chair or, especially if you use binoculars some sort of reclining deck-chair, hammock or the equivalent - there are many 'solutions'.
Currently Skywatcher sell a height-adjustable chair for around £99 but a cheaper alternative is a gas lift bar stool - particularly good for the Catadioptrics where the difference between viewing something near the horizon and overhead is small enough so that you can be seated most of the time. But for cheapness a popular solution is an ironing chair from the likes of Aldi. Comfortable viewing not only makes observing a much better experience but it is said that it 'adds' the equivalent of 1/2 to 1" to the aperture.
Collimation: This refers to the alignment of the mirrors and the lens. For instance, if the main mirror and the secondary mirror/lens are misaligned then you will never get a sharp image in the eyepiece. This is a potential problem with all types except Refractors, unless they aren't correctly aligned to start with, which may be the case with the very cheap toy ones. And the shorter the focal ratio the more precise the alignment has to be. So moving your scope, particularly when transporting it by car, etc., can cause problems - but if you are careful you should be ok.
Collimation is something you will often find mentioned on the Forums as one of the first things to do. For beginners this is not terribly helpful. Certainly if you get a fast scope - one with a focal ratio of 5 or less - it can, if you are unlucky or very finicky become an issue - eventually. But for most beginners worrying about this can be very much like trying to run before you can walk, especially as all you really want to see is just anything.
Please note that all prices quoted are approximate, subject to change
and for guidance only