The Cassegrain reflector uses an analogy to the Newtonian concave primary mirror that focuses beam Optical forward, just before the fire place there is a second convex mirror that multiplies and sends the fire back toward the primary mirror in the center of which is made a hole act to allow the passage of the optical beam ; the focuser is positioned posteriorly the tube, as in the case of refractors. In order to avoid the presence of spherical aberration is necessary that the curvature of the parabolic primary mirror has convex, while the secondary is - in the classical Cassegrain configuration - section hyperbolic. Compared to the Newton
Cassegrain reflector telescope allows for a very long focal (usually this type of instrument has an aperture of f/15 and beyond) while maintaining a compact appearance.
Like the Newton, the secondary mirror is maintained at the center of an optical ray structure called in slang "cruise" or "spider" (spider) which must be the least intrusive as not to cause diffuse light .
The classical Cassegrain is a suitable tool high resolution observation of the Moon and planets, and for the separation of double stars while - because of the long focal length - is deficit in the comments (and especially the photo) works deep sky . The coma may reach the edge annoying levels.
However, there are several variants such as the Dall-Kirkham (hyperbolic primary mirror and spherical secondary mirror) is used to obtain a tool for opening relations with f/11...13 (more suitable for universal use) or the Ritchey- Chretien (both working hyperbolic mirrors) which features a very wide field proper, which makes it very suitable for photography at the prime focus.
Other variants are based on Nasmyth Cassegrain , which uses third flat mirror which deflects the optical beam at the side of the tube like the Newton, and Coud which is basically a Nasmyth with an additional plane mirror located in the axis polar mount German or English that allows the viewer to observe always from the same point regardless of the position of the telescope.
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