The first decision must be: which type of telescope – reflector or refractor? The advantages and disadvantages can be summarized briefly as follows:
Newtonian reflectors produce images that are free of chromatic aberration (false colour). They are relatively cheap, and it is fairly easy to re-align the optical system. Among the disadvantages are the need to re-surface the mirror with an aluminium coating at regular intervals. In addition they are not so portable or convenient as the smallest aperture (60mm) refractors. There is little point in considering a reflecting telescope for astronomical use that is smaller than 150mm diameter, unless the optical components are of the highest quality. If you can afford a medium-priced reflector, then a Maksutov telescope or Cassegrain telescope could be considered. These may be harder to maintain if the best optical performance is to be obtained. With a Maksutov structure a compact instrument produces a wide, distortion-free field. Cassegrain telescopes have a short tube length but a long focal length. This permits larger magnifications without needing very short focal-length eye-pieces.
Refracting telescopes should have an aperture of at least 60 mm for attractive views, but 75mm is the minimum you should consider if you want to do serious observational work. Good refractors often cost rather more than reflectors, because more optical sur¬faces have to be worked, but the 75-mm refractor is really an ideal beginner’s instrument in terms of ease of use. Only simple adjustments to optical components are required, and the eyepieces are not elaborate. We have to offset these advantages against the fact that the tube is longer in the larger-aperture instruments, so heavy mountings are needed. In most practical cases, it is probably cost that will determine choice. However, there are 60-mm instruments which are somewhat cheaper than the cheapest 150-mm reflectors, and are capable of revealing Saturn’s rings, the markings on Jupiter, and the brighter galaxies: they are well suited to casual observing.
Whatever telescope is purchased, it is essential to provide a sturdy mount for it. Ideally a permanent mount to which the tele¬scope is clamped or bolted is required. Tripods, preferably of a heavy construction, may be suitable for the smaller instruments, but not if any heavy equipment, such as an electric drive is to be used. The heavier the mount the better, especially when there are slight winds about or inquisitive friends trying to view the wonders of the heavens! Quite frequently, it is better to make a permanent pillar or base to mount one’s telescope than to use the tripod sup¬plied by the manufacturer.
Once the telescope is attached to the mount, it should be free to move on two axes that intersect at right angles. The fitting that secures the telescope to its pier or tripod and permits this movement is called a HEAD. Two sorts of head are in widespread use: the altazimuth and the equatorial. An altazimuth mounting is the simpler, it has a long tradition of use for the mounting of terrestrial telescopes: the altitude axis is parallel to the horizontal plane, and the azimuth axis is vertical. It is not easy to undertake serious observing with this arrangement unless you have a special programme in mind, such as comet seeking, because the telescope must be moved along both axes simultaneously in order to compensate for Earth rotation and follow stars across the sky. The equatorial head is much preferable: one axis is parallel to the Earth’s rotation axis, so that movement on this axis alone cancels the stellar motion. This type of head makes the telescope slightly more difficult to set up initially, but the advantages outweigh this minor detail. Furthermore, an equatorial head can incorporate engraved scales, known as setting circles, which permit one to set the instrument to a specific hour angle and declination, a considerable help when searching unfamiliar star fields. If celestial photography is intended an equatorial head is essential.
Your choice of telescope and its mounting should also be influenced by a realistic prediction of how much you think your interest will grow. For example, if astronomical photography is of potential interest, a drive, clockwork or electric, will be required at some stage. Therefore it will be essential to acquire an instrument to which a drive can be fitted later. If you are determined to observe faint variable stars or faint galaxies, a larger aperture (upwards of 250mm) will probably be required.
On the larger refractors and all reflectors, a finder telescope is needed. This gives a wide-angle view of the field; the object of interest is then centred on cross-wires and will, in a correctly adjusted telescope, be in the field of view of the more powerful telescope.
If the telescope does not have a mechanical or electric drive, check to see that it has manual slow motion controls to compensate for the motion of sky. Without these an instrument is absolutely useless for more than a glance at celestial bodies.
When purchasing a telescope, you can seek the advice of reputable dealers, obtain the brochures of manufacturers, and consult your local astronomy club. All these sources of information should help you to narrow the choice.
Finally, it is worth mentioning that you can buy all the materials to grind your own mirrors and construct a home-made telescope. This can be a disheartening procedure for one who is not reasonably skilled at mechanical tasks, or prepared to spend some tens of hours on the grinding and polishing of the mirror. The mirror is made by grinding two circular glass blanks against each other until the correct concave curve is obtained. If you do have a work bench and like constructing things, then this project is worthy of consideration, but no beginner should tackle an instrument with a mirror larger than 200mm (ideally try 150mm first), and in any case specialist books on telescope making will have to be consulted. Really skilled, amateur telescope-makers will tackle apertures of up to 400mm, but this is out of the question for a complete novice.