The UVCS has a sun sensor consisting of four photodiodes located in the sunlight trap. These diodes detect the position of the patch of direct sunlight which passes through the entrance aperture, and hence provides information on instrument pointing relative to the solar disk. The system is used for safety monitoring and for fine pointing control.
Safety monitoring of instrument pointing is done for both sun-center-pointing
and offset-pointing type observations. The purpose is to ensure that direct
sunlight does not reach the spectrograph entrance slits. Overexposure of XDL
detectors to direct Ly-
radiation could damage the detectors.