The Solar Aspect System (SAS) for the High Energy Solar Spectroscopic Imager HESSI.

The HESSI SAS is a set of three Sun sensors, which shall provide high bandwidth (I 128 Hz) information on the solar pointing of the rotating spacecraft (15 rpm). The precision of less than or equal to 0.4 arcsec relative (on a lo level) is necessary in order to obtain the HESSI imaging resolution of 2 arcsec; the absolute accuracy of 1 arcsec is required for comparison with other measurements. Each SAS is based on focussing the Sun through a narrow bandwidth filter (670 nm) onto a 2048-element x (13 mu)(2) linear CCD. A digital threshold algorithm is used to select N pixels that span each solar limb for inclusion in the telemetry. Determination of the 6 limb crossing locations provided by the 3 subsystems (over-) defines the position offset of the Sun in the rotating frame. In this paper we describe the mechanical and electronic configuration of the SAS FM and the results of the first test measurements.