The pointing calibration and reference sensor for the space infrared telescope facility

The Space Infrared Telescope Facility (SIRTF) requires a visible light sensor at its focal plane to 1) calibrate the alignment between the externally mounted star trackers and the telescope boresight; 2) to establish the correspondence between the telescope coordinate system and the absolute J2000 reference frame; and 3) to provide starting attitudes for high accuracy absolute of fset maneuvers. The Pointing Calibration and Reference Sensor (PCRS) functions as the primary absolute attitude reference for the SIRTF telescope. It measures the J2000 position of Tycho Catalogue stars to an accuracy of 0. 1 4 arcsec 1-s per axis. To accurately measure Tycho objects, we have selected a silicon PIN photodiode operating in the Johnson V band, which we use with a cryogenic readout developed for the MIPS instrument on SIRTF. The PCRS employs a 4x4 Si:PIN detector array, using the outer rings for acquisition and the inner four pixels for precise measurements. Operation in the SIRTF focal plane presents us with several unique problems. Since the detector thermally links directly to the cryostat helium bath, it must operate at a temperature of 1.4 K. Additionally, the power dissipation must be less than 0. 1 mW to minimize the impact on helium lifetime. We describe low temperature characterization of Si:PIN detectors and readouts to verify their operability in the PCRS environment. Since the beryllium optics of the SIRTF telescope are diffraction limited only at 6.5 microns and longward, they yield a complicated point spread function at visible wavelengths. We present operational solutions to these and other challenges that allow the PCRS to meet its accuracy requirement with minimal impact on the rest of the SIRTF mission.