Ritchey–Chrétien telescope

The Ritchey -Chrétien Cassegrain telescope (also called RC telescope) is a further development of the Cassegrain telescope, in which the two mirrors are hyperbolic in shape. Thus, a coma-free imaging is achieved without a corrector as Schmidt plate, meniscus or another lens. However, since the field is not level, is set in the photographic Ritchey -Chrétien use of large - Cassegrain telescopes close in front of the focus lens system that corrects this problem.

The Ritchey -Chrétien system was developed in the early 20th century by George Willis Ritchey and Henri Chrétien.

Known Ritchey -Chrétien telescopes, the Hubble Space Telescope and the Very Large Telescope ( VLT ) of the Paranal Observatory in Chile. Variants of this type are now being offered by various providers of amateur telescopes.

Comparison with the classical Cassegrain system

The size of the usable field of view is an important parameter for the performance of a telescope. It is therefore useful to assess, from which image field diameter aberrations a value of 1 "to 2" exceed. This limit was chosen because it can be resolved with an average seeing 2 ".

The following is a Cassegrain telescope and a Ritchey -Chrétien Cassegrain telescope are compared with the same technical data:

• Diameter of the opening: 500 mm
• Focal length of the primary mirror: 1500 mm
• Focal length of the entire system: 4000 mm

The radius of the field curvature in the two systems is more than 500 mm.

Effective image field diameter:

As the table shows, the size of the field at the Cassegrain telescope is limited by the coma aberration, the RC telescope, however only by the astigmatism. Thus, the RC image telescope with a maximum error of 1 " is about 3.3 times larger useable field of view than the classical Cassegrain, with a maximum error of image 2 ", the useful field of view of telescope RC is still 2.3 times as large as the Cassegrain.

By coma freedom the RC telescope can be built with larger aperture ratios in a particularly compact design.

A disadvantage of the RC telescope compared with the Cassegrain is the need of corrector lenses, if you want to work with removed secondary mirror with the primary focus. The parabolic mirror of the Cassegrain shows then that is on the optical axis of a perfect image, while the RC hyperbola reflects very badly.

Design characteristics of the Ritchey -Chrétien Cassegrain telescope

The radii of curvature of the two mirror surfaces of the optical axis - described by the following two equations - as with all Cassegrain telescopes:

It includes:

• R1 and R2 are values ​​of the radii of curvature of the primary and secondary mirror
• Effective focal length F of the overall system
• B distance from the secondary mirror to the focal point
• D spacing between the two mirrors

Note: In the literature, these radii are often specified as negative numbers. Order is brought formally expressed that the light on its way through the instrument first passes through the center points of the radii of curvature and then is incident on the surfaces.

Because of the two hyperbolic mirror surfaces difficult to inspect the manufacture of small Ritchey -Chrétien Cassegrain telescopes is hardly useful. The grinding of the mirror is generally beyond the reach of an amateur astronomer. The alignment of the two mirrors must be carried out with high accuracy. Thus, a very rigid telescope is required.

Large Ritchey -Chrétien Cassegrain telescopes

• The two 10 -m telescopes of the Keck Observatory in Hawaii
• The four 8.2-m telescopes at the Paranal Observatory in Chile
• The 4-meter Mayall telescope and the WIYN 3.5 m telescope of the Kitt Peak National Observatory
• The Hubble Space Telescope with a 2.4 m primary mirror diameter