Performance of the onboard compression algorithm for ACS

An extensive set of experiments was performed to test the performance of the onboard compression algorithm created for ACS by R. White. We determined the compression factor for realistic simulated images of different astronomical targets as well as of STIS images and ACS darks and flat fields. We present the results of such experiments and our recommendations for the compression settings of primary and parallel observations. We also estimated the time required to compress ACS images on-board, but concluded that these estimates are at present not reliable and therefore suggest an alternative strategy. Finally, for comparison purposes, we also list the compression factors achieved with the UNIX commands gzip and compress.