Today's amateur astronomers have a wide and varied selection of equipment from which to choose. To the novice stargazer, it all appears very enticing but very complicated. Here is your first lesson. Many different kinds of telescopes have been developed over the years. Even though their variations in design are great, all fall into one of three broad categories according to how they gather and focus light.
Aperture Let's begin with the basics.
Regardless of type, when we refer to the size of a telescope, we speak of its aperture. In the case of a refractor, the diameter of the objective lens is cited, while in reflectors and catadioptric instruments, the diameters of their primary mirrors are specified. The focal ratio is simply the focal length divided by the aperture. Magnification Many people, especially those new to telescopes, are under the false impression that the higher the magnification, the better the telescope. It's true that as the power of a telescope increases, the apparent size of whatever is in view grows larger, but what most people fail to realize is that at the same time, the images become fainter and fuzzier.