The prime focus of the reflector is inside the main tube of the telescope and thus the image cannot be observed there without blocking part of the incoming light. A variety of schemes are employed to divert the image to a more convenient location. The simplest of these, constituting the Newtonian reflector, is the placement of a flat secondary mirror in the path of the converging light just before the prime focus. The small secondary mirror, which blocks a negligible portion of the primary mirror, is tilted at an angle of 45° in order to reflect the convergent light at right angles and bring it to a focus outside the telescope tube. In the Cassegrain system, the secondary mirror is convex and reflects the convergent light directly back along the axis of the telescope through a hole in the center of the primary mirror. By causing light to traverse a longer path, the effective focal length is increased and a larger image is formed. The Gregorian system is similar to the Cassegrain, except that the secondary mirror is concave. The Coudé system uses both a convex secondary mirror and one or more diagonal flat mirrors to produce a focus outside the tube. The secondaries are arranged so that the position of the focus remains stationary as the telescope rotates, allowing the use of image-recording and analyzing devices that would be too heavy to mount directly on a moving telescope.
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