Once you’re familiar with the night sky as seen through binoculars, you’ll want to go on to a telescope. A telescope lets you see the rings of Saturn, surface features on Mars, storms in the atmosphere of Jupiter, and to divide close double stars.
But which should a beginner choose? There seems to be a confusing variety of telescopes on offer. Fortunately, there are just two main types: refractors, which use lenses, and reflectors, which use mirrors. There is also a hybrid type, which uses both mirrors and lenses, but we’ll deal with those under reflectors.
Refracting telescopes are like larger versions of the spyglasses used by seafarers of old: they have a long tube with a main lens at one end to collect and focus light, and an eyepiece at the other to magnify the resulting image. The only real difference is that an astronomical telescope has interchangeable eyepieces which offer different magnifications. A refractor with an aperture of about 60 mm (2.4 inches) is a common starter telescope, particularly for young people. (The aperture is the diameter of a telescope’s light-gathering lens or mirror.)
Incidentally, the first thing you may notice when you look through an astronomical telescope is that the image is upside down. This is deliberate. They could turn the image the right way up, but it would mean a more complex eyepiece, which is unnecessary for astronomy. After all, there is no “up” or “down” in space!
Most telescopes have a small ‘finder’ mounted on the tube, which does just what the name says – it’s a low-power refractor that helps you home in on the general area of interest.
Eyepieces and magnification
Beginners may be tempted to use the highest magnification, but this doesn't always give the best result. As magnification increases, the image gets fainter and more blurred. In practice, whether your telescope is a refractor or reflector, the maximum usable magnification for any telescope is twice the aperture in millimeters (or 50 times per inch).
Three eyepieces are usually enough, offering low, medium and high powers. The low and medium powers are best for looking at extended objects such as star clusters, nebulae and galaxies. The highest powers will be most used for seeing fine detail on the Moon and planets, and separating close double stars.
A reflecting telescope has a main mirror which collects incoming light. In the traditional Newtonian design (devised by Isaac Newton) the focused light is bounced back up the tube to a small secondary mirror which diverts it into an eyepiece near the top of the tube. Therefore the eyepiece is easier to reach than in a refractor, where you may have to stoop or crouch awkwardly if the telescope is pointing high up.
Mirrors are much easier to make than lenses so, size for size, reflectors are much cheaper than refractors. Reflectors are the usual choice of astronomers for apertures larger than 75 mm (three inches) or so. Remember that the wider the aperture of the telescope, the more you will see with it, so size is important.
These days, a commonly encountered design of telescope is what’s called a Schmidt–Cassegrain, which combines a thin lens at the front of the tube with a main mirror at the rear. In these telescopes, the light is reflected back to an eyepiece in the center of the main mirror. Schmidt-Cassegrains are popular because they have much shorter tubes than either traditional type, which compensates for their higher cost. Above all, remember that a telescope is a precision optical instrument, so expect to pay as much as for a good-quality camera.
Just as important as the telescope itself is the mounting. A telescope on a mounting which shakes in the wind, or is difficult to steer, is not of much use.
There are two main types of mounting. The simplest form is like the pan and tilt head used for a camera. Technically, this is known as an altazimuth mounting because it lets the telescope pivot up and down (in altitude) and swing from side to side (in azimuth). These must be adjusted all the time to keep an object in view as the Earth turns.
A more sophisticated type is the equatorial mounting. In this type the "pan" axis is parallel to the Earth’s axis of rotation. As a result, the object can be kept in view simply by turning this axis as the Earth spins. This is often done by a motor drive, leaving the observer’s hands free to draw or photograph the object of interest.
In recent years, traditional designs have been joined by computer-controlled GOTO mountings. These will automatically point at any object programmed into the telescope’s handset, and follow it as the Earth turns. You don’t even need to know where the object is! Needless to say, these are more expensive. Purists think they take some of the fun away from observing, although anyone who has struggled to find some elusive galaxy might not agree.
A good book for users of small to medium-sized telescopes: Stars and Planets Guide by Ian Ridpath & Wil Tirion, Collins, London (ISBN 978-0007251209) and Princeton University Press, Princeton (ISBN 978-0691135564).