Measurements with the Chandra X-Ray Observatory's flight contamination   monitor
  NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor (FCM), a system of 16 radioactive calibration sources mounted to the inside of the Observatory's forward contamination cover. The purpose of the FCM is to verify the ground-to-orbit transfer of the Chandra flux scale, through comparison of data acquired during the ground calibration with those obtained in orbit, immediately prior to opening the Observatory's sun-shade door. Here we report results of these measurements, which place limits on the change in mirror--detector system response and, hence, on any accumulation of molecular contamination on the mirrors' iridium-coated surfaces. 
R. F. Elsner, J. J. Kolodziejczak, S. L. O'Dell, D. A. Swartz, A. F.   Tennant and M. C. Weisskopf