The successful coating of the first two flight optics (P6 and H6) has just been completed, so it is an appropriate time to give a brief update on what has been happening.
Many readers may not be aware of the Process Selection Study (SAO report: SAO-AXAF-DR-92-016 by Pat Slane) which was carried out a few years ago under the recommendation of a coating working group comprised of members from TRW, the MSFC Project Office, MSFC Project Science, and the SAO Mission Support Team (MST). This study gathered information needed for the selection of materials, vendor, coating techniques, etc., for the coating of the AXAF optics. Based on the study, DC magnetron sputtering was recommended to be used as the coating technique, and iridium was chosen as the coating material. Also, at this time, Optical Coating Laboratories Incorporated (OCLI) of Santa Rosa, CA was chosen as the coating vendor by TRW via competitive procurement. The coating process includes a chromium undercoat which is first sputtered onto the optical surface to insure the adherence of the iridium layer which follows. The nominal thickness of the layers is 100Å of chromium and 350Å of iridium.
Based on the data gathered during the coating process selection study, a minimum performance criterion for the X-ray reflectivity of the coatings was specified to be used as an evaluation tool. The SAO/MST reflectivity laboratory in Cambridge is a facility dedicated to the AXAF program to provide quick turnaround reflectivity measurements for this evaluation. SAO/MST has been working with TRW, MSFC, and OCLI to provide the X-ray reflectivity measurements of the sample coatings.
The coating program with OCLI, which began in June of 1993, has three main phases: Scaleup, Validation, and Production. The activities during the Scaleup phase included the building and testing of the chamber in which the flight optics are coated. This phase also included several test runs to coat sample substrates in each of the mirror geometries (P6, H6, P4, etc). These coatings were then characterized to provide data to set the correct coating parameters for each geometry. This was completed in February 1995.
The Validation phase of the coating program (completed in April 1995) was used to show the reproduceability of coating parameters and coatings in each of the geometries.
The Production phase, which is underway now, denotes the period of time dedicated to the coating of the flight optics. This phase of the coating program began in June 1995, and approximately one flight optic per month is being coated until expected completion in February 1996.
During the production phase, a qualification coating run takes place several days prior to the coating of each optic. During the qualification run, several substrates (6" x 2" x 1") are coated to give us a representative sampling of the coating geometry for each optic. These samples are measured at SAO to check that the coating meets the requirements and to check for linear and azimuthal coating uniformity. Within 48 hours of receiving the qualification samples, SAO/MST must verify that the coatings meet spec before OCLI can proceed with the coating of the flight optic. Along with the coating of the flight optic, six witness samples are coated and sent to SAO. These samples are then analyzed in the same way as the qualification samples.
Figure 1. X-ray reflectivity as a function of grazing angle. Solid line is theoretical data for iridium and squares are experimental data for a sample from the production run for the smallest of the iridium-coated hyperboloid mirrors (H6). The X-ray energy is 8.03 keV.
Figure 1 shows X-ray reflectivity data from one of the coated witness samples from the H6 coating run. These data were taken with an 8.03 keV X-ray source, and one can see that the critical grazing angle for this energy occurs near 34 arcmin. A measure of the reflectivity near the critical angle is a sensitive probe of the density of the coatings, and a measurement of reflectivity below the critical angle provides information on the density gradients near the surface. For these reasons, the requirements were chosen such that the reflectivity be 82% at 20 arcmin grazing angle and 50% at 34 arcmin (at 8.03 keV) for the coatings to be acceptable (memo from Steve O'Dell to Steve Hixson, 11 June 1992).
Six of the flight optics have now been received at OCLI. The remaining two, P1 and H1, are scheduled to be shipped from Eastman Kodak Corporation (EKC) to OCLI in August '95. The order of coating is: P6, H6, P4, H4, P3, H3, P1, H1. As the optics are coated, they will be shipped in pairs to EKC for assembly, which is scheduled to begin in October '95.
Several dozen samples were characterized during the scaleup and validation phases of the program. The reflectivity of the coatings to date has generally exceeded specification, with only three cases of subpar reflectivity which was traced to improper cleaning of the substrate. The success of this phase is due to the cooperation and collaboration of OCLI, MSFC, TRW and SAO. In particular, we are fortunate to have a dedicated team of people at OCLI, headed by Jerry Johnston, the program manager, and Bob Hahn, OCLI's chief engineer for this project. OCLI is under subcontract to TRW, the spacecraft prime contractor, and much credit goes to Beth Barinek, the technical TRW contract manager, for her ability to keep the program running smoothly and for keeping the momentum in the forward direction!