Ritchey–Chrétien telescope
A Ritchey–Chrétien telescope (RCT or simply RC) is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors (coma).
The RCT has a wider field of view free of optical errors compared to a more traditional reflecting telescope configuration.
As with the other Cassegrain-configuration reflectors, the Ritchey–Chrétien telescope (RCT) has a very short optical tube assembly and compact design for a given focal length.
The RCT offers good off-axis optical performance, but its mirrors require sophisticated techniques to manufacture and test.
If the mirror is made parabolic, to correct the spherical aberration, then it still suffers from coma and astigmatism, since there are no additional design parameters one can vary to eliminate them.
The remaining aberrations of the two-element basic design may be improved with the addition of smaller optical elements near the focal plane.
Alternatively, a RCT may use one or several low-power lenses in front of the focal plane as a field-corrector to correct astigmatism and flatten the focal surface, as for example the SDSS telescope and the VISTA telescope; this can allow a field-of-view up to around 3° diameter.
However, the Schmidt requires a full-aperture corrector plate, which restricts it to apertures below 1.2 meters, while a Ritchey–Chrétien can be much larger.
In a Ritchey–Chrétien design, as in most Cassegrain systems, the secondary mirror blocks a central portion of the aperture.
This ring-shaped entrance aperture significantly reduces a portion of the modulation transfer function (MTF) over a range of low spatial frequencies, compared to a full-aperture design such as a refractor.
The radii of curvature of the primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are: and where If, instead of
of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is: and Note that
However, professional optics fabricators and large research groups test their mirrors with interferometers.
A Ritchey–Chrétien then requires minimal additional equipment, typically a small optical device called a null corrector that makes the hyperbolic primary look spherical for the interferometric test.
On the Hubble Space Telescope, this device was built incorrectly (a reflection from an un-intended surface leading to an incorrect measurement of lens position) leading to the error in the Hubble primary mirror.
[7] Incorrect null correctors have led to other mirror fabrication errors as well, such as in the New Technology Telescope.
In practice, each of these designs may also include any number of flat fold mirrors, used to bend the optical path into more convenient configurations.
This article only discusses the mirrors required for forming an image, not those for placing it in a convenient location.
