Complex sentence: An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper--to monitor and determine the satellite attitude, and in the process to gather photometric and astrometric data of all stars down to about 11th magnitude.
Split into simpler sentences: An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper. Its purpose was to monitor and determine the satellite attitude, and in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude.