The PMTRAS Roll Aspect System on RHESSI

High-resolution solar hard X-ray imaging on the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft is achieved by a set of rotating modulation collimators. The interpretation of the observed time-modulated X-ray flux in terms of high-resolution, accurately located images requires continuous, arc-minute roll aspect, which at present is provided by the `Photo-Multiplier Tube Roll Aspect System' (PMTRAS). This paper describes the PMTRAS operating principles, hardware implementation, calibration, performance and data analysis approach, with emphasis on its effect on RHESSI imaging.