Until you actually do any astronomy it is very difficult to know exactly what will capture your interest, or indeed if your initial enthusiasm will even disappear altogether, or not. The realities of actually getting that experience, whether visual or photographic may completely change your ideas as to what you really want to do, as it did with me. Hence the advice to take things slowly and, as mentioned throughout, seek out help, particularly at the Star Parties that the local astronomical societies organise. Nevertheless many of you will no doubt be like me and, once you have the idea in your head, you will want to go out and buy a scope. So let us begin....
How much? You can spend many thousands of pounds on a telescope, not to mention all the accessories, but to begin with the consideration is for a budget in the region of £300 - £400 max.
How big? We all know that the bigger the telescope the more you can see? Well, yes and no.
When my son first saw my 130mm reflector he commented on how big it was - he was thinking that a big tube - 900mm long - meant a big scope. The tube of my second, larger 150mm scope is only 41mm long. It is the size (aperture) of the main mirror or lens that determines light gathering capacity - how 'big' a telescope is. My 150mm (6 in) scope gathers 33% more light than the 130mm (5.1 in).
Magnification: The other question people often ask is, what is the magnification? In strict terms the answer is that the maximum useable magnification of any scope is around 50 times the diameter in inches. Thus 250 for a 5 inch, 300 for a 6 inch. However, it is very, very rarely possible to achieve anywhere near the maximum because of
Viewing conditions: Here used to describe the overall effect of Seeing Conditions and Transparency. Even if the sky seems to be perfectly clear, Seeing conditions may be such that the atmosphere is swirling around in a most unsteady manner, or the Transparency is poor because it contains lot of water vapour or dust particles. Indeed sometimes a smaller scope will give a better image than a larger one.
The magnification for every telescope will depend on both the eyepiece used and the
Focal length: This is the distance between the main mirror or lens and the eyepiece. With Refractors and Reflectors this will normally be the length of the tube. With the Catadioptrics the beam of light entering the scope is bent back and forth between the various mirrors and lens giving a short tube and a long focal length.
Focal Ratio: This is the aperture divided by focal length A telescope with an aperture of 100 mm and a focal length of 800 mm has a focal ratio f/8.
For any one eyepiece and for scopes of the same aperature a long focal length telescope
will give greater magnification than one with a short focal length. Good for the Sun, Moon and planets but not for deep space objects.
The 80mm Refractor shown here has a tube length of 400mm which gives it a short focal ratio 5 (400/80).
The 130mm Reflector has a tube and focal length of 900, which gives a medium f/r of 6.92 (900/130).
The 90mm Catadioptric has a tube length of about 400mm, the same as the 80mm Refractos but a focal length of 1250mm which gives a long f/r of 14 (1250/90).
For AstroPhotography telescopes with short or fast focal lengths ( f/3 to f/6) are reckoned to be best for Deep Sky Objects - Nebulae, Galaxies, etc - whilst those of moderate (f/7 to f/11) and long or slow focal lengths (f/12 and above), are better suited for the Moon and planets.
Most small to medium sized telescopes come with a 25 or 26mm eyepiece. This will give a magnification of about 16 (400/25) with the above-mentioned Refractor, 36 (900/25) with the Reflector and 50 (1250/25) with the Catadioptric.
But the greater the magnification the smaller the field of view,
which can make it more difficult to find anything - which brings us to
Finderscopes: These are the small 'telescopes' that sit on top of the main scope. Without these it would be difficult to find anything, and this is true even with most computerised GOTO scopes. There are basically 2 kinds of finderscopes, the zero magnification red dot finders and the small refractor telescopes.
Red Dot Finders project a small dot into the sky and if you previously aligned it with the main scope you will, if you aim it at any object, be able to see that same object in the centre of the eyepiece of your telescope.
The other finderscopes are small refractor telescopes of up to 50 mm in diameter and with limited, usually fixed magnifications of 6 to 9 and give you a fairly wide view of the sky. Again, if you centre the object in the finderscope it should be visible in the eyepiece of your main scope.
All finderscopes have to be carefully aligned with the main scope so that an object viewed through the finderscope will also be visible in the main scope, preferably in the centre! This is best done during the day or maybe at night using a bright star or the Moon. Unfortunately, if light pollution is bad and/or you don't know your way around the sky, it can be difficult to find anything other than the brightest objects especially with the red dot finders where the red dot obscures the star you are looking at.
GOTO Telescopes: These are computerised scopes which, once aligned with the sky can, in theory, find any object that is in their databases, and many others if you know their coordinates. The latest versions have wi-fi andCCD cameras which scan the sky and will self-align, so you don't need to know anything about the night sky - in theory at least.
As well as finding objects for you these scopes will give you information about them. You can also take tours of constellations and other categories such as named stars, variable stars, etc., - all extremely informative and useful. However, even the smallest of these cost around £550, which is a lot to spend if you aren't sure how far you will want to take this hobby.
Skywatcher 80mm Refractor
Skywatcher 130mm Newtonian Reflector
Skywatcher Heritage 130P FlexTube Dobsonian