RHESSI Aspect System

The RHESSI spacecraft spins at about 15 rpm around an axis close to Sun center.  Images are reconstructed by using the modulation patterns seen on each of the detectors behind the bi-grid rotating collimators.   In order to reconstruct images with 2 arcsec resolution, precise knowledge of the spacecraft pointing and roll angle is needed. To meet these requirements, RHESSI has a Solar Aspect System (SAS) that yields sub-arcsec knowledge of the radial pointing with respect to Sun center, and two redundant roll aspect systems,  a CDC-based Roll Aspect System (RAS) and a Photo-Multiplier Tube Roll Aspect System (PMTRAS), that provide precise knowledge of the roll angle of the spacecraft.  In practice, the PMTRAS has proved to be the more reliable of the two.