SAFIR is a 10-meter, 4 K space telescope optimized for wavelengths between 20 microns and 1 mm. The combination of aperture diameter and telescope temperature will provide a raw sensitivity improvement of more than a factor of 1000 over presently-planned missions. The sensitivity will be comparable to that of the JWST and ALMA, but at the critical far infrared wavelengths, where much of the universe's radiative energy has emerged since the origin of stars and galaxies. We examine several of the critical technologies for SAFIR which enable the large cold aperture, and present results of studies examining the spacecraft thermal architecture. Both the method by which the aperture is filled, and the overall optical design for the telescope can impact the potential scientific return of SAFIR. Thermal architecture that goes far beyond the sunshades developed for the James Webb Space Telescope will be necessary to achieve the desired sensitivity of SAFIR. By optimizing a combination of active and passive cooling at critical points within the observatory, a significant reduction of the required level of active cooling can be obtained.
NASA's Space Infrared Telescope Facility (SIRTF) is a 1- meter class cryogenically-cooled space observatory. The constituent sub-assemblies are currently in their assembly and verification phase. To facilitate the assembly and verification of the telescope, the Space Telescope Test Facility (STTF) has been built at the Jet Propulsion Laboratory. The STTF allows for the assembly, alignment, and optical characterization of individual components, as well as the telescope assembly with its cryogenic mechanism, at temperatures from 300 to 5 K in a chamber with interior diameter of 1.4 m, and a height of 2.3 m. The chamber is surrounded by a class 10,000 or better clean room. This paper reports on the functional and operational capabilities of this facility.
An optical test Dewar has been constructed with the unique capability to test mirrors of diameter <EQ 1 m, f <EQ 6, at temperatures from 300 to 5 K with a ZYGO Mark IV interferometer. The facility possesses extensive thermometry throughout for characterization of the test chamber thermal environment and Dewar performance. Optical accesss is controlled with cryogenically cooled shutters. The entire Dewar is vibration isolated by 40 dB where the fundamental resonances of the Dewar structure are highest. The facility has been brought on line for its first user, the Infrared Telescope Technology Testbed for the Space Infrared Telescope Facility at JPL. The design requirements for this facility and the resultant design and implementation experiences and challenges will be presented.
A two axis optical gimbal mechanism for aligning 1 meter diameter telescope primaries and test flat mirrors at temperatures from 300 to 4.2 K was constructed for use in the SIRTF Telescope Test Facility (STTF). This mechanism consists of an aluminum frame, pivoting on a monoball bearing, and driven in tip and tilt by tungsten di-sulfide lubricated lead screws with external drive motors. Flexures decouple the optical support frame from stresses generated by differential rates of cooling. A second set of flexures decouples the mirror mechanically and thermally from distortion in the gimbal mechanism. The mechanism provides sub arc-second resolution in either axis, while limiting the heat leak to less than 100 mW at 4.2 K. Linear variable differential tranformers are used at temperatures from 300 to 4.2 K to measure a home position. The STTF and gimbal are presently operational, and have been used in two separate interferometric measurements of a 0.5 meter, f 4.0 beryllium spherical mirror at 6 K. The gimbal will be used in the interferometric testing of a beryllium telescope primary mirror from the Infrared Technology Testbed, for the Space Infrared Telescope Facility at JPL.