So far we have considered the bare minimum requirements in order to get started. But maybe you can afford more and want a bigger scope? Fine, but before we look at those, let's consider some of the accessories that you will probably find that you really need, whatever size of scope you buy - assuming that it is more than a casual interest.
Eyepieces: The 25mm eyepiece that comes with the scope plus the 10mm and 2x Barlow, if you have them, will not get you far. Eyepieces come in many sizes, shapes and weights - not to mention their price. A single Televue Ethos 21mm can cost over £700 and weighs 2.25lb!!! It will give you a fantastic wide 100 degree apparent field of view and it is, so it is said, a bit like going on a spacewalk. Fortunately reasonably good quality eyepieces can be had for a 'mere' £25 - £100, depending upon what you want. But apart from price let us look at
Field of View: The higher the magnification the smaller the field of view will be for any given eyepiece and telescope. The Moon as seen in the sky has a diameter of 1/2 deg. Use a typical 25mm eyepiece with a 130mm Telescope with a tube length of 900mm and you will get a True Field of View of about 1.5 deg. Change that to a 10mm eyepiece and the Field of View is around .6 deg, and with a 254mm Telescope with a 1200mm tube length that comes down to around .46 deg. So you can still see almost all of the Moon.
Wider fields of view. Deep sky objects like the Andromeda Nebula at around 3 deg are much larger than the Moon, hence the desire, usually, to have eyepieces with wide fields of view. They also have the advantage of making it easier to find objects in the first place - with GOTO as well as manual scopes. But they cost more. And if you are to match the eyepiece to the viewing conditions then you need a range of eyepieces - which can start to become quite expensive. So a relatively cheap solution to get started with is to get a
Zoom Lens: These are composite eyepieces which give you a range of magnifications, typically in the range about 7- 21mm or 8 - 24mm.. Prices from around £40 for a cheap but perfectly adequate 7 -21mm. With this you effectively get 5 eyepieces for the price of one - 21, 15, 11, 9 and 7mm - suitable for most viewing conditions. All you do is turn the knob and refocus if necessary. No need to fiddle around in the dark and change eyepieces. The only slight disadvantage is that the field of view is not as good at the lower 21 range as with single eyepieces. Of course if you pay more you will get better quality zoom lens, probably slightly wider fields of view and slicker operation. And still on the subject of Field of View.....
Red dot Finders: These are zero magnification devices which sit on top of the main scope. You switch it on, peer through it and will see a red dot projected on to the sky. If it is correctly aligned with the main scope then any object you point it at will be centered in the eyepiece of the main scope. But this only works for objects you can see with the naked eye. This can be limiting, even with GOTO scopes where, in theory, you are only concerned with the initial alignment using bright stars. However, GOTO scopes are seldom 100% accurate and the object you are seeking may be on the edge of the field of view, or even just outside. There are more sophisticated red dot finders, in particular the Telrad and Rigel ones. Some astronomers use a combination of red dot finders and
Finderscopes: These are small refractor telescopes which have a fixed magnification in the range of about 6 - 9, and apertures of from around 20 -50mm. They enable you to see the fainter objects and have a relatively wide field of view. This is useful even with GOTO scopes where the object you are seeking isn't precisely centered. Indeed, you may not see it at all in the main scope, but looking through the finderscope with its wide field of view you will be able to see which way you have to move the scope in order to centre the object. And with manual scopes this can mean the difference between finding the fainter objects or not.
Reticule Eyepieces: These are eyepieces which have a single or more usually a double cross etched into them. They enable you to accurately centre a star or other object in the eyepiece by placing it at the centre of the cross and can greatly improve the accuracy of GOTO scopes. They usually have a narrow field of view, but this is not a disadvantage as you are only looking at the central section. There are both illuminated and unilluminated ones and even with the former it is possible to turn off the light and still see the crosses. Indeed, the light even when adjusted to a minimum can almost swamp the star you are looking at.
As well as the different sizes of finderscopes you also have differences in the way they present their views. Astronomers traditionally do not worry about which way is up, so unlike terrestial telescopes and binoculars they present images which are inverted up and down, or side to side, or even upside down and side to side. This creates problems for a beginner when, for instance, you move the scope up and the object in the eyepiece goes down, or you move it left and the object goes right. You will get used to it, but when using a finderscope it can be nice to have one that presents the image as you see it. And for yet more comfortable searching you can get them complete with 90 degree diagonals thus making overhead searches much easier. Prices can range from about £6 for the smallest straight through finderscope, up to around £70 for a 9 x 50 right angle erect image one depending on make and supplier.
Filters: If there were such a thing as perfect light pollution filter then much of this site would hardly be relevant. Unfortunately the glare from suburbia covers such a wide wave-length spectrum that no one filter can cut it all out without also cutting out the star-light. Light pollution filters try to cut it down and different filters will be suitable for different objects. Prices range from around £20 for the simplest which are best described as contrast boosting filters, up to maybe £180 for the specialist items. As the latter are often suitable for only a very small range of objects they are not really something for the beginner to consider.
However, when viewing the Moon, particularly when at or near full, a neutral Moon filter can be very useful as it reduces the glare. There are also various colour filters for the planets which can help to enhance markings such as Jupiter's red spot and the bands on Saturn. These typically cost around £6 or so, but with the smaller scopes it is important to make sure that their light transmission levels are high enough (70%+) otherwise you see nothing.
Clothing: This may seem a strange item to include, but if you are to observe in relative comfort for any length of time, warm clothing is absolutely essential. I wear a woolly hat, at least 2 jumpers and a thick shirt, 2 pairs of socks, Dunlop Purofort Wellington boots (insulated, comfortable and keeps my lower legs warm), and my over trousers over my trousers - much better than longjohns.
All the items mentioned above are additional costs which you will almost certainly incur in one way or another, that is if you become more than just the casual observer content to use the bare minimum that comes with your first purchase.
Please note that all prices quoted are approximate, subject to change
and for guidance only