The correctors make it possible to increase significantly the useful field of view. In the Cassegrain and Ritchey-Chrétien designs, light travels to the focus through a central perforation in the primary mirror.Ĭorrector lenses (correctors) are often used in reflectors to correct residual aberrations. Ritchey-Chrétien systems with secondary mirrors that reduce beam convergence and are positioned in front of the prime focus have become most widely used by virtue of their design features. Spherical aberration and coma are corrected in all nonaberrational reflector systems. Nonaberrational reflector systems are possible the Schwarzschild and Ritchey-Chrétien systems use a primary mirror with a hyperboloid figure, and the primary mirror in Maksutov systems has the figure of an ellipsoid, sphere, or oblate spheroid ( see MAKSUTOV TELESCOPE and MENISCUS SYSTEMS). Astigmatism can be corrected only in systems where the secondary mirror increases beam convergence and is positioned in front of the prime focus such systems are inconvenient in operation.
The length of the comatic spot is equal to 3/16 A 2wf here, f is the focal length, A is the aperture ratio ( A = D/f, where D is the mirror diameter), and w is the angular distance of the star from the optical axis of the reflector (in radians). This coma is equal to that of an equivalent single parabolic mirror.
In all classical reflectors, spherical aberration is completely corrected, but appreciable coma restricts the field of view. In a Cassegrain reflector with a secondary mirror that reduces the beam convergence and is positioned in front of the prime focus (Figure 1, c), the secondary mirror is a convex hyperboloid in a Gregorian reflector with a secondary mirror that increases beam convergence and is located behind the prime focus (Figure 1, d), the secondary mirror is a concave ellipsoid and in the afocal Mersenne reflector system (Figure 1, e), the secondary mirror is a paraboloid.
In classical two-mirror reflectors, the primary mirror is a paraboloid and the secondary mirror has the form of some quadric surface of revolution. Each of these arrangements can reduce the convergence of a beam of light, thus increasing the focal length and image scale in comparison with the focal length and image scale at the prime focus, or it can increase beam convergence, thus reducing the focal length and image scale. The secondary mirror may be placed in front of the prime focus of the primary mirror or behind the prime focus. Optical diagrams of reflectors: (a) prime focus, (b) Newtonian, (c) Cassegrain, (d) Gregorian, (e) Mersenne, (f) NasmythĪ two-mirror reflector uses two mirrors-a primary and a secondary-with curved figures and an arbitrary number of plane mirrors that direct the light to a point convenient for observation.
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