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Ritchey Chretien Telescopes

A specialized Cassegrain telescope, the Ritchey-Chretien telescope (RCT) is found in most observatories around the world, ideally designed for wide-view observing and research. For example, NASA's Hubble Telescope is a Ritchey-Chretien.

Its name is a tribute to its inventors: American astronomer George Willis Ritchey and French optician Henri Chretien. Designed in the early 1910s, the RCT took the basic design of the Cassegrain and made a few modifications to help counter typical aberrations associated with reflector telescopes, namely coma and astigmatism. Through analyses and trial-and-error, Ritchey hit upon the right construction to minimize these aberrations. The key? Hyperbolic mirrors for both the primary and secondary mirrors. This change corrected coma and spherical aberration, while there still remains the presence of astigmatism and also field curvature.

Specifically, a slightly hyperbolic mirror is used for the primary and a more strongly hyperbolic mirror is employed for the secondary. The model does not insert a correcting lens at this point, which ends up creating a wider field free of coma. Due to the resultant curvature however, the correcting lens is placed closer to the focal plane (the surface on which the device focuses an image). This solves the curvature problem and as a result, the RCT is able to snapshot a wide useable field at high resolution. Stars are imaged as circles.

The first successful RCT was built in 1927, with a diameter of 0.5 meters. The second had a one-meter diameter and was constructed specifically for the United States Naval Observatory. What makes the Ritchey-Chretien so popular with observatories is 1) its Cassegrain short-tube length, 2) reduced aberration, and 3) sharp images across a wide field view.

Though the RCTs have been historically limited to professional instruments, manufacturers such as Meade have begun to experiment with the design for amateur astronomers as well, utilizing the same concept, on a smaller scale. The roadblock had previously been the expense of the hyperbolic mirrors, but as is the case with all invention, necessity may be leading to breakthroughs in this area, with less expensive devices possessing a similar capability, just with a few tweaks here and there. The objective however for wide, sharp imaging, remains the same.

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