8.2 Payload System Improvements

  Based on lessons that we have learned from the previous flight, we are making a number of improvements in IMAPS. These changes, outlined in sections that follow, should bring about a substantial gain in the overall productivity of IMAPS and the quality of the data.

The observing duty factor of IMAPS on ORFEUS-SPAS I was 18.5%, and this figure included orbits where we were commissioning the instrument and performing trouble shooting. With the time lost in transferring data to the Astro-SPAS tape recorder and turning on the camera for each exposure, our highest possible duty factor could only have been (34 s exposure interval)/(99 s interval between exposures) = 34%, a calculation that does not account for the additional time lost during maneuvers between targets and the inefficiencies of target non-availability at certain times. We expect this (highest attainable) 34% on-target efficiency figure to increase to 85% with the new, much faster (6 s) data transfer to a flash memory system (§8.2.1 below) that will be internal to the IMAPS electronics system. This increase, when combined with the longer mission duration and the expected performance of an undamaged photocathode, should increase our product of observing time and effective area by a factor of about 20.