A high-accuracy, small field of view star guider with application to SNAP

To accomplish its mission, the spaceborne observatory SNAP( SuperNovaAccel- eration Probe) requires a pointing stability of < 0.03 arcseconds during exposures lasting up to 500 seconds. A Monte Carlo simulation of the photoelectron statistics from the guiding starinvestigates geometrical (such as the pixel size of the detector or the plate scale) and physical parameters (such as the magnitude of the star). It is shown that simple centroiding calculations can lead to the desired accuracy with guide starsas faintas magnitude 16. Availability of these stars is verified thanks to the HST Guide StarCatalog complemented with a statistical model of the distribution of stars. Thus a through-the-lens sensor that uses stars as faintas magnitude 16 to provide the necessary guiding signals is feasible. In one year of study, the SNAP( SuperNovaAcceleration Probe) satellite can discover, follow the light curve, and obtain spectra at peak brightness for more than 2000 supernovae. SNAPinstrumentation includes a wide field optical imager(field of view of one square degree) and a small IR imager, which, combined with filters, will allow photometry from 0.35 to 1.7 µm and a three arm spectrograph sensitive in the same range of wavelengths. SNAPinstrumentation will be used with a simple, predetermined observing strategy designed to monitor a 20 square degreeregion of sky both near the north ecliptic pole(16 h 25 min + 57 deg) and near the south ecliptic pole(4 h 30 min 52 deg), discovering and following supernovaethat explode in these regions. More information on the scientific background of this mission can be found at the URL http://snap.lbl.gov. The data from the wide field optical imagerwill serve both as search images and for follow-up photometry. The detection of supernovaeis ac- complished by a repeated comparison of fixed fields to reference images. The optical photometerobtains wide-field frames overlapping the positions of