ASC Calibration Plans

The frontier astrophysics which the AXAF-I and AXAF-S missions will accomplish requires that the telescopes, science instruments, and other spacecraft systems (such as the aspect star cameras) be fully calibrated on the ground and in orbit. The ASC is responsible for collecting and archiving the calibration data, coordinating the analysis of the data, archiving the results of the analysis, and making the data and the analysis results available to AXAF users.

For the science instruments and the telescope, the calibration activities are conducted in three sequential stages:

• Subassembly Calibration: This stage involves the separate calibration of the science instruments and the telescope. Laboratory calibration of each Science Instrument (SI) is conducted by the instrument team as part of the SI development activities, under Marshall Space Flight Center (MSFC) cognizance. The calibrations include determinations of the detectors' quantum efficiencies, spatial uniformity, and count-rate linearity. The periods of the Low-Energy and High-Energy Transmission Gratings will also be measured. The SI teams are working with the AXAF Calibration Task Team (which is chaired by the NASA AXAF Project Scientist, Martin Weisskopf, and includes members from the MSFC AXAF Project, the SAO Mission Support Team, the SI teams, TRW, and the ASC) to formulate laboratory calibration plans for AXAF-I and -S. The ASC will collect and archive the laboratory data, together with algorithms and software for their analysis, and will evaluate the calibration results. We expect most of the laboratory test data on the AXAF-I flight instruments will be obtained in 1995, but with significant amounts of data available in 1994.

  As part of the telescope calibration, the SAO AXAF Mission Support Team is conducting a program of synchrotron measurements to determine the reflectivity function of the AXAF-I mirror coating, while Hughes Danbury Optical Systems (HDOS) is characterizing the properties of the mirror surface. Kodak will measure the alignment of the mirror pairs. The ASC will archive these telescope calibration data and the analysis results. We will begin collecting the synchrotron measurements and the HDOS metrology in 1994.

• Calibration at the X-ray Calibration Facility: The SIs and the telescope for AXAF-I will undergo calibration at the MSFC X-ray Calibration Facility (XRCF). This calibration is currently scheduled from October 1996 to May 1997. At the XRCF, X-rays from a particular target (e.g., aluminum), located 1700 feet from the telescope, are focussed by the telescope onto a detector. The XRCF calibration will determine the effective area of the telescope/detector combination, the instrument scale, the spectral resolution, and the point response function as a function of energy and angle from the telescope axis. The ASC will receive and archive these calibration data and the analysis results, and make them accessible to users; included in the results are models which will be used in the ASC-developed software to assist users in planning proposals and analyzing data from observations. The mirror assembly and X-ray Spectrometer (XRS) for AXAF-S will undergo an integrated ground calibration to measure the instrument alignment, the point response function, and the effective area.

• On-orbit Calibration: The ASC performs the initial and on-going on-orbit calibrations. The on-orbit calibrations will involve observations of celestial sources and the bright earth, as well as observations where the instrument is not at the telescope focus, in order to measure instrument backgrounds and characteristics.

eshepard@asc.harvard.edu