Because of the complexity of the optics' spectral and angular response, the difference between conditions during ground testing and on-orbit operation, and the finite amount of calibration time available, it would not have been practical to describe the optics' performance based solely upon measurements made at the XRCF. Instead, we use the calibration results as ``anchor points'' for very detailed models of the system so that we have some confidence in the models' ability to predict performance in areas which were not tested. It is thus fairly important that the models agree with the data. While the performance of the optics is outstanding and the overall agreement of the models to the measurements are quite good, there are a few areas requiring further study. Much of the work over the last year has concentrated on improving our understanding of the measurements and the limitations of the models.
One of the most important measures of the optics' performance is their effective area. Effective area measurements at the XRCF were done with two different detectors (flow proportional counters and a solid-state detector). Understanding the subtleties of the instrumental setups and detector physics was challenging, to say the least. After much work, we now have agreement to within between the detectors as to the measured on-axis effective area at the XRCF. (Please note as well that these effective area values are not what will be seen on orbit. They include many effects peculiar to the environment at the calibration facility.)
You'll note that while there is good agreement between the Flow Proportional Counter (FPC) and Solid State Detector (SSD) measurements, the raytrace predicts an excess of effective area above 2keV. This is the snark we continue to hunt. We have recently been focusing our attention on the off-axis XRCF measurements, as they have previously given us clues to the alignment of the mirrors. Additionally, we are investigating the manner in which we describe the mirror surfaces and their iridium coating. Measurements of the surface characteristics of the uncoated optics were performed by Hughes-Danbury Optical Systems (HDOS). During the coating process a number of witness mirrors were in place around the optics. Measurements of their reflectivity as a function of energy and angle by the AXAF Synchrotron Reflectivity group as well as the HDOS data form the basis of our treatment of the optics' surface. Our early approach assumed a simple semi-infinite layer for the optical surface. We have substantially improved our models to incorporate a multi-layer surface with interior grading and are currently studying deviations from a perfect multi-layer surface description indicated by the synchrotron measurements.
Significant progress was also made in the absolute calibration of the FPC and SSD detectors. The detectors' performance had been measured at the BESSY synchrotron facility in Berlin, Germany. We now have determined the absolute quantum efficiency of two of the FPC's and the SSD. The primary impact of these measurements is upon measurements of the combined performance of the HRMA and the Science Instruments; the direct measurements of the HRMA's performance relied upon the relative QE of the detectors, a quantity which was initially determined at the XRCF. We find that the relative QE determined at BESSY is in very good agreement with that determined at the XRCF.
Overall our models are in good agreement with the measurements. To ensure that the on-orbit calibrations are well understood, it is imperative, as noted before, that we achieve as high a consistency as possible, thereby increasing our confidence in predictions in areas where no ground testing was possible.
- Diab Jerius