Ritchey-Chretien Research Articles

Zero-Coma Condition For Decentered And Tilted Secondary Mirror In Cassegrain/Nasmyth Configuration

The tolerance on decentering and tilt of the secondary mirror of a Cassegrain/Nasmyth telescope is considerably broadened when the zero-coma condition is applied, the limit being set by the allowable residual coma and tilt of the focal surface. The dependency of field tilt on decollimation is defined. A simple set of steps for effective collimation of a telescope is outlined, along with practical engineering recommendations concerning the Ritchey-Chretien (R-C) astigmatism corrector and focal surface subsystem. It is shown that when the proper engineering degrees of freedom are provided, essentially ideal R-C imagery can be obtained over the original optical design field, even in the presence of relatively large angular errors in the optical axes of the primary and secondary mirrors. The specific example given is the case of the proposed University of Texas 7.6 m telescope.

Better but bigger prime focus corrector lenses for Ritchey-Chretien telescopes

La resolution sur un large champ au foyer principal des telescopes de Ritchey-Chretien peut etre amelioree par des correcteurs a lentilles triples. Des telescopes tels que celui de Kitt Peak (f/2,8-4 m) pourraient etre ameliores, principalement pour une spectroscopie sans fente d'objets multiples

Refractive vs Reflective Correctors

SummaryImproved designs of refractive correctors produce excellent images with fast telescopes such as those with an F/1.5 prime focus and F/3.5 secondary focus. The fields are flat and there is compensation for the chromatic effect caused by windows. Disadvantages of such correctors are that stray light is produced at the optical surfaces, the elements must be supported at their edges, prerequisite high quality glass is available in only limited sizes, and all wavelengths are not transmitted.Reflective correctors, on the other hand, can produce diffraction limited images at all wavelengths and the mirrors can be supported across their backs as well as at their edges. Disadvantages are that the images are degraded by any substantial window (such as a detector faceplate), there is more central obstruction, and the correctors are sometimes very large and heavy.Except, perhaps, for a specialized telescope, such as one devoted to multi-object slit spectroscopy using fibres, the refractive corrector is preferable at fast foci.A good combination is a Ritchey-Chretien (R-C) telescope with refractive correctors at the fast prime and secondary foci, and a reflective corrector-magnifier for the slow infrared focus.

Spectroscopic equipment for the Space Telescope

The Space Telescope will provide views of the Universe of remarkable clarity. Central to its capabilities is an f/24 Ritchey-Chretien telescope with a 40000 cm 2 unobscured collecting area, which will provide point-source images less than 0.1" in radius at wavelengths below 633 nm. It will operate over the range 110 nm to 1 mm. The initial flight instrumentation includes two spectrographs, the Faint Object Spectrograph (F.O.S.) and the High-Resolution Spectrograph (H.R.S.). The F.O.S. is sensitive from 115 to 800 nm. It will provide data at resolving powers 10 3 -10 2 on extremely faint sources. The H.R.S. operates at the wavelengths 107-320 nm. It will achieve high photometric accuracy at resolving powers 10 5 , in the echelle mode, or 10 4 -10 3 in first order, on brighter targets. The two cameras on board the S.T. will provide ancillary spectroscopic capabilities.

Optical characteristics of the UNAM 2-m Ritchey-Chretien telescope

Measurements made on the components and on two assembled systems of a 2-m Ritchey-Chretien telescope have been used to determine the actual characteristics of the telescope. Sufficient precision has been obtained so that other focal ratio systems can be designed.

Hypersensitization of Kodak infrared plates: Results of some chemical methods and pre-exposure

Tests have been carried out on Kodak Spectroscopic Plates Type I-N, in order to find a treatment which could be easily applied to the infrared plates to be used with the Loiano 152 cm,f/8, Ritchey-Chretien telescope. Different techniques have been tested, not only with the aim of achieving the higher speed gain, but also to obtain the better uniformity and reproducibility. Our results suggest that the advantages of preexposure techniques are underestimated in this respect. Furthermore, we find that blue and infrared preexposures (at 0.8 μs) give different results, suggesting a different degree of dispersity in the latent image produced by pre-exposure in the two cases.

Field correction of a Ritchey-Chretien telescope at several focal ratios

The two mirrors of a Ritchey–Chrétien telescope have shapes chosen to give aplanatic imagery, generally at a focal ratio around ƒ/8. If the secondary is replaced by one giving a materially different focal ratio, the spherical aberration of the combined system can be corrected by an appropriate mirror form, but the coma cannot also be corrected, and it may be large. It is shown that both coma and astigmatism can be corrected by the addition of an afocal thin doublet lens between the secondary mirror and the focus, together with a modification to the secondary. The analysis leads to a discriminant that determines the range of two-mirror systems for which such correction is possible. Two numerical examples are given, for conversion of the 2.5-m du Pont ƒ/7.5 telescope to work at ƒ/15 and ƒ/30.

Method for Obtaining a Flat Field with Rosin-Amon Tilted Plate Correctors

The properties of two-conic reflecting aplanats are analyzed and discussed on the basis of third order aberration theory. Techniques for designing infinite conjugate two mirror aplanats and computing their image properties are developed. The secondary mirror alignment characteristics of Ritchey-Chretien and aplanatic Gregorian telescopes are examined and neutral point locations defined. Design configurations corrected for a third Seidel aberration (astigmatism, image curvature, or distortion) are identified and their properties discussed. The properties of Ritchey-Chretien and aplanatic Gregorian telescopes are compared.

National Mapping’s Astro-Geodetic Complex

The Division of National Mapping has received, on long term loan from NASA and in co-operation with the Smithsonian Astrophysical Observatory, a Lunar Laser Ranging instrument consisting of a gigawatt pulsed ruby laser, a 150 cm aspheric Ritchey-Chretien telescope, and associated electronic equipment. The instrument was formerly operated by the Air Force Cambridge Research Laboratories at Mount Lemmon in Arizona. Its principal use by National Mapping will be direct determination of the distance between the telescope and any of the three retro-reflector arrays placed on the Moon at Hadley’s Rille, Fra Mauro and Mare Tranquillitatis by Apollo astronauts. Full scale operation in conjunction with similar instruments well separated in latitude and longitude, in particular at Mount McDonald in Texas and Mount Haleakala in Hawaii, will permit determination of Earth rotation and polar motion, lunar ephemeris and libration, and tectonic plate movement or continental drift, which justifies its use in a geodetic environment.

Extending the Stellar Field of View of Ritchey-Chretien Telescopes

The preferred modern type of large telescope today is the Ritchey-Chretien form. The field size is aberrationally limited by undercorrected astigmatism and field curvature. A pair of equally and oppositely oriented flat transmittng plates can neutralize either or both of these aberrations locally in the field to restore the image. These plates, relatively inexpensive and positionally noncritical, may be used to extend the available image area by an order of magnitude.

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