The reflector telescope uses a parabolic-shaped mirror to
gather and focus light. All modern research telescopes and large
amateur ones are of the reflector type (click
here to know more about telescopes)...
The simplest design is prime focus. In such a design, the detector lies in front of the mirror. The advantage of a prime focus system is that it requires a minumum number of reflections; on the other hand, some of the incoming light rays are blocked. If the mirror is large enough, the fact that the detector lies in front and therefore blocks some of the incoming rays doesn't matter that much in terms of losing light, but some distortion may occur because of diffraction or scattering off the blocking structure.
Most reflector telescopes will use a smaller secondary mirror in front of of the large primary mirror to reflect the light to a more convenient viewing spot.
Isaac Newton used a flat secondary mirror at a 45° angle to reflect the light to an eyepiece at the side of the telescope tube near the top. Such an arrangement, called a newtonian design is used by many amateur telescopes.
Many reflector telescope use another light path design called the cassegrain design to reflect the light back through a hole in the primary mirror, so that detectors or the eyepiece can be conveniently placed behind the telescope, in a convenient location for making adjustments. Most of the large telescopes used for research, including the Hubble Space Telescope, are of this design.
There are many types of hybrid telescopes, such
as the Schmidt design and the Schmidt-Cassegrain...