Filter calibration

Our OBF calibration plan consists of five stages. At the University of Wisconsin Synchrotron Radiation Center (SRC) we map the transmission of all flight-like filters and many of their witness samples at five energies using a beam of 0.76 mm square. At the National Synchrotron Light Source at Brookhaven National Laboratory (BNL) we measure filter witness samples at hundreds of energies, but only two positions per filter. At NIST's SURF II, a UV synchrotron, we measure the optical/UV leakage through the filter. At Penn State, we measure changes to the transmission in filter witness samples as a function of temperature. Finally, we will combine the uniformity data from the SRC with the energy and temperature dependence data from BNL, SURF II, and PSU to make a functional prediction of the transmission as a function of position on the filter, energy of the radiation, and temperature of the filter.

The SRC data are critical as they are the only measurements of the flight filters as separate entities, before the filter response is convolved with the response from the CCDs and the AXAF mirrors. These data allow us to search for any large light leaks or manufacturing defects, to measure spatial variations, and to make normalizing bulk thickness measurements to carry over to the high-resolution BNL energy data, which in turn provide detailed transmission models including the effects of EXAFS (Extended X-ray Absorption Fine Structure).