Solar Aspect System
SAS Lens Diagram
The SAS provides (1) high-resolution, high-bandwidth aspect information for image reconstruction, (2) real-time aspect error signals for spacecraft pointing, (3) monitoring of the relative twist of the two grid trays, and (4) full-Sun white-light images for coalignment with ground-based images. The SAS is similar to the aspect system on HEIDI that demonstrated 0.5-arcsec performance at balloon altitudes. It consists of three identical lens-filter assemblies mounted on the forward grid tray to form full-Sun images on three 2048 x 13-µm linear diode arrays mounted on the rear grid tray. Simultaneous exposures of three chords of the focused solar images are made every 10 ms by each of the arrays. A digital threshold algorithm is used to select four pixels that span each solar limb for inclusion in the telemetry. These digitized pixel outputs allow six precise locations of the solar limb to be obtained on the ground by interpolation, thus providing knowledge of Sun center in pitch and yaw to 1.5 arcsec (3 sigma). 

When the SAS is pointing to within ~0.2° of Sun center, simple algorithms using the limb pixel numbers also provide real-time error signals with ~<10 arcsec precision to the spacecraft Attitude Control System (ACS). Use of the SAS in this way for both imaging and pointing avoids problems of coalignment. The SAS is also used as a solar acquisition sensor with an effective radial field of view of 46 arcmin by the detection of a single limb in any one of the three diode arrays. 

Although the aspect solution itself is independent of twist, the internal consistency of the three independent solutions possible with the built-in redundancy provides a continuous, highly sensitive measure of the relative twist of the upper and lower grid trays during flight. The SAS aspect requirement of 1.5 arcsec corresponds to a sensitivity to relative twist of 0.4 arcmin. During prelaunch coalignment tests, this SAS twist measurement is calibrated against that provided by the Twist Monitoring System (TMS). The SAS was built by the Paul Scherrer Institute in Switzerland.