SIRTF Fine Guidance Sensor Analysis: Focal Plane Configuration Issues

AbstractWe examine the interrelationship of the issues of sensitivity, image centroidingprecision, and field of view and how they interact with the SIRTF pointing requirement todetermine a FGS focal plane configuration. While the examples are specifically orientedtoward SIRTF, our approach is general and can be applied to other CCD -based star sensorsystems.IntroductionIn its baseline configuration, the Space Infrared Telescope Facilityl (SIRTF) contains acharge coupled device (CCD) -based fine guidance sensor (FGS) at the visible wavelengthfocal plane of the liquid helium -cooled infrared telescope. The FGS focal plane is locatedinside the liquid helium -cooled volume but it probably will operate at a temperatureconsiderably above that of the scientific instruments. It is desirable to reduce the FGStemperature as much as possible, both to reduce the thermal load on the SIRTF cryogen andto simplify the approach needed to ensure essentially complete isolation of photons emittedby this relatively warm volume from the scientific instrument infrared detectors. As isclear from its location deep inside the SIRTF cryogenic volume, servicing of the FGS,particularly its on -orbit replacement, would be extremely complex, and very high relia-bility is required of this device.While the thermal design of the SIRTF FGS will be a challenging problem, first andforemost the FGS must fulfill its requirement to sense the position of the SIRTF line ofsight with sufficient precision such that gyro drifts are kept within a tolerable level.We describe below an approach that begins with the SIRTF requirements and derives FGSsensitivity and field of view requirements. In combination with the precision with whichthe centroid of a image can be determined, we derive a suitable focal plane configuration.SIRTF pointing requirementsDuring many of the various SIRTF modes of operation, the infrared line of sight must bepositioned on the sky with a mean offset (accuracy) of 0.15 arcsec from the desired loca-tion and an rms jitter (stability) of 0.15 arcsec about the mean position.2 Observations ofa celestial object that is a source of both visible and infrared radiation (e.g. a planet)will enable the SIRTF users to calibrate the angular offset between an arbitrary locationon the FGS visible wavelength detector and the center of an infrared detector array. Theuncertainty associated with this angular offset measurement will contribute to the SIRTFpointing accuracy, as will low frequency changes in the positions of the FGS or a scien-tific instrument relative to the SIRTF telescope.The primary function of the SIRTF FGS is to measure the low frequency drift of the SIRTFgyroscopes. As such, the FGS detector will be read out only about once per second; highfrequency perturbations of the line of sight will be sensed by the gyros. These twoeffects are not entirely independent, however, since jitter of the line of sight will tendto smear out an image in the FGS. To the extent there is a non -zero mean to the jitter, itwill affect the FGS measurement of the gyro drift; a substantial amount of jitter coulddegrade the FGS signal -to -noise ratio.The SIRTF requirements also specify that the FGS must be able to act as a gyro- drift-corrector even when the telescope is pointed at locations in the sky where there is arelatively low density of visible wavelength stars. Specifically, the FGS must have a 95percent probability of detecting two (or more) stars at any location in the celestialsphere. (The second star is needed to sense roll.) This requirement translates directlyinto a FGS field of view requirement that is primarily a function of the faintest stars theFGS can detect.