!Converted with LaTeX2HTML 95.1 (Fri Jan 20 1995) by Nikos Drakos (firstname.lastname@example.org), CBLU, University of Leeds >
The AXAF freight train is roaring ahead and there are many positive things to report. First and foremost, all of the figuring and polishing of the glass elements are completed! Not only that, but the surface quality appears to be superb - well below the requirements and near our goals. Typical surface roughnesses have been measured to be of order 3Å. Since X-ray scattering scales with the square of the roughness, this is a significant enhancement when compared with the 7Å rms specification. Not only were the glass elements excellent, they were completed significantly ahead of schedule. The AXAF Project has taken advantage of this windfall by accelerating other activities in order to increase the amount of slack and thus make the program more robust against any problems we may encounter in the future. I can also report that the first two elements (P6 and H6 -- the smallest paraboloid and hyperboloid) have also been successfully iridium coated. X-ray measurements, performed at SAO by Suzanne Romaine on accompanying witness samples, indicate that the coating exceeds specifications and yields somewhat higher reflectivity than we had baselined (see Reflectivity Verification of the Optical Coatings in this newsletter).
All of the preliminary design reviews and several of the critical design reviews have been completed. In particular, all of the Science Instrument (SI) critical design reviews were completed by August 1995. Thus far, the reviews have been very successful. Work continues on the X-ray calibration facility, and we are putting the final touches on the scientific calibration requirements document. Once completed you may access the document through the World Wide Web via the AXAF Project Science homepage at MSFC --- http://wwwastro.msfc.nasa.gov/xray/axafps.html. The document's contents can be changed (via a formal process) and comments are welcome to us via e-mail. Project Science has been particularly active in a number of arenas. First, at the urging and with the help of Gordon Garmire, the ACIS Principal Investigator, we have introduced a `magnetic broom' as part of AXAF in order to prevent low-energy electrons from reflecting through the telescope into the focal plane. There is strong evidence that EXOSAT experienced such phenomena, and we simply do not want to take any chances with AXAF. We have also been charged with designing, testing, and calibrating the `ground-to-orbit' flux transfer standard which we introduced in order to maintain the calibration accuracy against the effects of minute levels of molecular contamination. The device consists of a number of radioactive sources which are mounted to the forward contamination door and which illuminate the optics when the door is closed. Measurements -- with ACIS -- will be made on the ground at the XRCF and once again on-orbit, just prior to opening the door. Together with Tony Armstrong (SAIC), Kurt Dietz has completed a full modeling of the effects of radiation (photons, particles, activation, etc) for the entire AXAF. Finally, we have been working very closely with TRW to be sure that AXAF is adequately shielded to X-rays not passing through the telescope. (Ultra-light-weight spacecraft can create some problems!)