ABSTRACT
The function of a telescope is quite similar to that of a microscope, but for a small difference. They both magnify the angular extent of an object that subtends a small angle at the human eye. When a telescope with a circular aperture is diffraction-limited, the image of a point source is not a point, but rather a bright disk surrounded by a series of bright rings of much smaller and decreasing amplitude. In addition to the diffraction effects, any optical instrument, including the telescope, also suffers from performance degradation due to aberrations, fabrication imperfections, and alignment errors. An object is seen or detected by the telescope detector only if enough energy is collected to produce the requisite signal-to-noise ratio in the focal plane. Many nominally mirror telescopes additionally incorporate transmissive components for specific aberration correction most often field correction; they are known as catadioptric field correctors. The simplest one-mirror system is a parabolic telescope.
