A refractor telescope, is one that and uses a glass lens to gather light instead of mirrors, which are used in reflector telescopes. This lens, also known as an objective, is arguably the most important part of the telescope since its quality will determine the quality of the overall magnification, and of the resulting viewable image. Most people who have a telescope stored in the attic will find that it is a refractor telescope.
Refractor telescopes are smaller than reflectors, because they are able to focus more light onto a smaller area. This size advantage makes them easier to store and transport than the much larger reflector. This makes them popular to sell in department stores and the like.
A refractor telescope works by bending, i.e. refracting, light. A curved glass objective is found in one end of the telescope. Light from a viewed object passes into the objective, and is bent into the tube of the telescope. The light is ultimately reflected upwards by a mirror at the other end of the tube. The mirror shines the image into an eyepiece for viewing.
There are some characteristics of refractor telescope that makes them more appealing than reflectors for those who are new to star gazing. The optics are less likely to shift than the mirrors on a reflector, an occurrence that causes misalignment and blurring of objects. Reflectors need to be "collimated," or realigned, with just about every use, and sometimes even during use as temperatures change. Refractors require no calibration by the user at all. A reflecting telescope is open at one end, meaning that the objective can get dirty and must be cleaned regularly, a process that can be dangerous to the expensive mirrors if not performed properly. A refractor telescope has a closed tube, which means it can not accumulate dirt and dust on the inside, and therefore does not require internal cleaning.
Refractors do have disadvantages, particularly for those with a more advanced knowledge of astronomy. They are subject to a number of light anomalies, the most problematic of which is chromatic aberration, a distortion that causes a rainbow of light around viewed objects. A refractor telescope has difficulty taking in certain kinds of light, particularly ultraviolet light, which is unable to pass through the objective. The larger the telescope, the more obvious these problems become. This makes the refractor telescope impractical to use for serious research. In general, refractors most suited to planetary and moon viewing by amateur astronomers.
