When talking about telescopes, it is important to understand that telescopes used for night time viewing are primarily light gathering devices. Particularly at night, our eyes do not very effectively capture enough light to make distant or dim objects to make those objects visible. Telescopes help make up for this, by collecting more light and concentrating it, making it possible to see much fainter objects. This is also the reason that the better Earth based telescopes are those with the largest objective lens. The larger the lens, the more light that is gathered, as a general rule, so a 100 inch telescope will usually be better than one that is only 10 inches.
There are basically 3 kinds of optical telescopes; refractors, reflectors, and compound telescopes. These can be augmented in a number of ways, for instance by attaching a camera to the eyepiece, which allows for long exposure pictures to be taken. The longer the shutter of a camera is left open, the more light that is gathered, to expose the film.
In a refractor, incoming light is gathered by the large objective lens. Due to the convex shape of this glass lens, the light is then focused to a point inside the telescope. The eyepiece magnifies the focused light so that distant objects can be seen. Refractors are subject to a few problems. For one, part of the incoming light is lost. For another, there is a limit to the size that the objective lens can be. If it is too large, it can bend or break under its own weight.
A reflector, invented by Isaac Newton and later refined by John Hadley, tends to be more accurate than a refractor, that is, it can usually gather more light. It does this by using a concave shaped mirror rather than a glass lens. Due to the shape, the incoming light is focused and sent to a much smaller mirror, which reflects it to the eyepiece, often located on the side of the telescope, usually somewhat beyond half way up the side. Reflectors don’t have the problems that refractors have, and they are capable of capturing a much wider view. The larger the primary mirror, however, the more precisely it must be ground in order to prevent distortions. The grinding and polishing of the mirror can take years, and transporting a large mirror can present the engineers with some rather interesting challenges.
The third kind of telescope, usually called a Compound Telescope, contains elements from both refractors and reflectors. The first compound telescope was made in 1930, called the Schmidt telescope after its creator, and was used mostly for taking pictures. The Hubble Space Telescope is a compound telescope, and the images it captures are fed electronically rather than using photographic plates.
New styles and designs are being developed. However, regardless of the type, telescopes have greatly broadened our knowledge of space, and they have greatly enhanced the abilities of hobbyists and amateur astronomers. It will be quite interesting to watch the advancements made to telescopes over the coming years.