Use of thermal sieve to allow optical testing of cryogenic optical systems.
Full aperture testing of large cryogenic optical systems has been impractical due to the difficulty of operating a large collimator at cryogenic temperatures. The Thermal Sieve solves this problem by acting as a thermal barrier between an ambient temperature collimator and the cryogenic system under test. The Thermal Sieve uses a set of thermally controlled baffles with array of holes that are lined up to pass the light from the collimator without degrading the wavefront, while attenuating the thermal background by nearly 4 orders of magnitude. This paper provides the theory behind the Thermal Sieve system, evaluates the optimization for its optical and thermal performance, and presents the design and analysis for a specific system.
Status of the James Webb Space Telescope (JWST)
The James Webb Space Telescope (JWST) is a 6.5-meter, space telescope designed for infrared imaging and spectroscopy. Its planned launch in 2013, aboard an Ariane 5, will place it in n L2 orbit. The JWST program is a cooperative program with the Goddard Space Flight Center (GSFC) managing the project for NASA. The prime contractor for JWST is Northrop Grumman Space Technology (NGST). JWST's international partners are the European Space Agency (ESA) and the Canadian Space Agency (CSA). JWST will address four major science themes: First light and re-ionization; the assembly of galaxies, the birth of stars and protoplanetary systems; and the formation of planetary systems and the origins of life. We discuss the design of the observatory as it is currently base-lined, and review recent progress with the observatory.
CONCERNING THE ALIGNMENT OF MODERN TELESCOPES: THEORY, PRACTICE, AND TOLERANCES ILLUSTRATED BY THE ESO NTT
A recent paper by Bhatia (1995) suggest from calculations based on the optical geometry of the NTT that the zero-coma condition is a necessary but insufficient condition for the alignment of modern telescopes and that analysis of separate decenter effects is necessary to cope with decentering astigmatism aberrations near the field c enter. We show here that, while Bhatia's calculations are quite correct, their basis shows a misunderstanding of how the NTT was actually aligned and that this "passive" alignment was sufficiently good that the decentering astigmatism effects he indicates were negligible. More dangerous are the wide-field effects of decentering astigmatism discussed by McLeod (1996). However, we show that these effects, linear with field, are also negligible with the NTT alignment quality achieved, provided the correct effective shift of the astigmatic field center is allowed for int he active correction software. This is of great importance in the NTT because the image analyser operates at the edge of the field and the correct field astigmatism must be subtracted to give the necessary active correction at the mechanical field center. This approach is valid for the technical use of the edge field of an aplanatic (RC) telescope limited by field astigmatism. Decentering astigmatism will be much more critical at the field edge for anastigmatic telescopes is a wide field is also used for astronomical observation.