Aperture Finder

Telescopes offer two advantages to the amateur astronomer. The most important is the ability to intercept more light from an object. The second is resolving power, the ability to separate the fine detail of objects that are close together. This is very important when observing planetary detail or double stars.

Aperture is the diameter of the principal light collector of a telescope, which can be a lens or a mirror. Aperture is basically responsible for determining the two advantages mentioned above.

Although most people think of telescopes, it may be in their best interests to consider binoculars first. They are portable, convenient and relatively inexpensive. Cheap binoculars are better than a cheap telescope. Consider something like 7 x 35 binoculars. The 7 is the magnification produced by the eyepiece, the other number is the aperture in millimeters.

If you are considering a telescope there are essentially three types available to the amateur astronomer.

Refractors are the most readily recognized type and consist of lenses at both ends of a tube. Whatever you do avoid the cheap models sold by department, nature, science and toy stores. They quote large magnifications, but their small apertures make them useless for astronomy. Remember a telescopes most important job is to collect light, not magnify an image. A normal terrestrial telescope has an extra lens to ensure the image is the right way up. Lenses can create false rainbow tints around very bright objects like planets. This is called chromatic aberration and can be ignored or corrected by a filter. Refractors generally cost more per inch of aperture than other types of telescope, and those of more than 4-inch (100mm) aperture are rather long and cumbersome.
The second type of telescope is the reflector. Light travels down a tube before reflecting off a couple of mirrors and through an eyepiece on the side of the tube. Reflectors need to be larger than a refractor to be equally useful. They do not suffer chromatic aberration, but the main mirror may occasionally need re-polishing or realigning (collimating). Reflectors are often the most comfortable telescopes to use because of the eyepiece position. You dont have to kneel and possess an elastic neck to look straight upwards as you would with a reflector.

The third type of telescope is the Schmidt-Cassegrain. This uses lenses and mirrors to fold the light path back on itself within a compact tube. They are generally cheaper than refractors, but dearer than reflectors. They are more portable and easier to handle than the other two.

Although you should buy the biggest aperture you can afford, dont get a large telescope if you will have to carry it a long way. A smaller telescope would be easier to set up and therefore more likely to be used. The general rule for calculating the maximum practical magnification a telescope can achieve is to double the aperture in millimeters, e.g. a 100mm aperture telescope should have a maximum magnification of 200x.

What can you expect to see with a beginners telescope? A 3-inch (75mm) refractor or 6-inch (150mm) reflector will allow you to see many galaxies and nebulae, Saturns rings, Jupiters largest moons and hundreds of craters on the moon.

Aperture Finder