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A reflector telescope is a type of telescope in which the objective lens used to collect light is a mirror. There are a number of variations on the basic reflector telescope design, ranging from hand-held telescopes designed to be used by amateur enthusiasts to huge observing telescopes located in laboratories all over the world. The quality of the telescope can vary considerably, depending on the size, the materials used in construction, and how well it is maintained.
This telescope design was developed in the 1700s, when designers of optics struggled with chromatic aberrations which made observations unreliable. The original reflector telescope consisted of a simple cylinder with a large parabolic mirror mounted in one end. The light hit the mirror, and the curved shape reflected the light back and focused it into a point which would meet the viewer's eye at the eyepiece. Later designs used a secondary mirror to reflect the focused light to an eyepiece. Magnifying lenses can also be installed to enlarge the image.
In some designs, the eyepiece is located at the side of the cylinder, with the secondary mirror being placed at an angle in the cylinder so that the light is bent to meet the viewer. The eyepiece can also be behind the parabolic mirror, in which case the secondary mirror is aligned so that the light passes through a small hole in the objective lens to meet the eye of the viewer. Cameras can also be used instead of mirrors or eyepieces to capture the image.
With a reflector telescope, the bigger the objective lens, the better the image quality. Big mirrors can pick up much more light, thereby allowing observers to see faint objects, and to pick up greater detail. For example, a planet at the same magnification can appear markedly different in two different telescopes, depending on the size of the objective lens.
Various optical defects can still occur with a reflector telescope. Any irregularity in the surface of the mirrors used can distort the image's shape or color. Dust in the cylinder can also cause abnormalities. Sometimes, specialized correction lenses will be used to correct errors so that the image is reliable, as was done in the Hubble Telescope after researchers realized that its images were not as crisp and focused as desired. In the case of the custom built massive telescopes used in research, correcting problems with additional lenses is usually easier that retrofitting the entire telescope.