The AXAF aspect camera is a 4.4-inch diameter Ritchey-Chrétien telescope, with a 3-element refractive corrector, which images about 2 square degrees of sky onto one of two red-sensitive 1024 x 1024 CCDs. The optics of the camera are deliberately defocussed, so that the FWHM of star images is about 9 arcsec, well spread out relative to the CCD scale of 5 arcsec per pixel. All acceptable guide stars are brighter than about 10 mag in the aspect camera system, which is related to magnitudes in conventional astronomical systems by
Patches of 16, 32, or 64 pixels are read out near each guide star,
with the readouts taking 1.025, 2.05, or 4.1 sec respectively. In the
nominal (32-pixel) configuration, the position of a guide star at the
limiting magnitude can be measured to better than 
arcsec
in each coordinate at every camera read-out. The
location of the mean position of an asterism of 5 guide stars (the
nominal configuration) is then established to about 
arcsec
in a single readout, and to 
arcsec after smoothing the
results over a number of aspect camera readouts using the gyroscope
data and allowing for systematic errors.
Guide stars are selected from the AXAF Guide Star Catalog according to an algorithm that takes account of the field crowding, and the method by which the (unshuttered) aspect camera reads the data out. In general this process will be invisible to the user, but in special circumstances, for example if there is a scientific reason to choose a particular set of guide stars, it is possible to specify the guide star set entirely. In addition, one of the guide star data slots may be allocated to monitor the brightness of an object in the X-ray field (for example a star that is being observed for X-ray variability). That object may still be used as a guide star if it is sufficiently bright: if it is not, then the accuracy of the aspect solution is slightly reduced.
Three fiducial lights are normally imaged and tracked by the aspect camera at the same time as the five guide stars. The images of the fiducial lights are bright, so that their positions on the CCD can be measured with a smaller statistical error than those of the guide stars. The systematic errors in their positions are larger, however, so that the positions of photons in the X-ray detectors are known with an error that is approximately equally due to the guide stars and fiducial lights. Allowing for calibration errors and further systematic errors, the direction from which any photon arrives at the detector will be established to better than 0.5 arcsec (rms error circle diameter). This is the image reconstruction error, which determines the precision with which long observations can be registered to give good images, and should be attained or bettered over the first five years of the mission.
The absolute celestial location error of an image made by the Observatory depends on the quality of the aspect camera guide star positions (on the sky) and the calibration errors in the alignments of the X-ray and optical detectors. It is expected that the absolute coordinates will be known to better than 1.0 arcsec (rms error circle radius), allowing for all error sources, throughout the mission, with better results possible immediately after the (infrequent) boresight calibrations.
The gyroscope data are sampled every 64 msec on each of four axes. These data are inserted into telemetry for ground analysis and are used by the on-board computer for fine pointing control and for aspect information during slews. The pointing direction of the Observatory will drift during an observation both from the effect of external torques, and from a controlled dither pattern, designed to move images of X-ray sources by some tens of arcsec on the detectors. Even the fastest pointing drift during an observation will produce only a few gyro output counts per 64 msec.
The analysis of these data on the ground will proceed through an automatic pipeline that analyses the aspect camera imaging data to derive guide star centroids (using an algorithm that is designed to be insensitive to varying background levels, spoiler stars, and other effects), and produces an aspect solution with the implicit 64 msec granularity of the gyroscope data, although smoothing to longer timescales will be needed to overcome the quantization of angles in the gyroscope data on the sampling timescale.
Redundancy in the aspect determination system is provided in each of the critical electrical units. The aspect camera contains two CCDs, one of which is illuminated by a flip-in mirror if the other fails. Spare fiducial lights are provided near each X-ray detector, with only three lights needed to achieve full aspect solution accuracy. For the gyroscopes, two independent gyroscope boxes are provided, each containing two gyroscopes, each capable of providing rate information on two axes.
Futher variations on this basic system exist: for example, it is possible to use fewer fiducial lights and more guide stars (or vice versa). A variation on this basic scheme is used to check for sources found during slews between individual pointings. And it is possible to use other, lower-precision, pointing data (for example from the fine sun sensor) if necessary.
