The AXAF mirrors and science instruments will be calibrated at the X-ray Calibration Facility (XRCF) at the Marshall Space Flight Center (MSFC) from September 1996 to March 1997. The XRCF tests will characterize the X-ray performance of the AXAF High Resolution Mirror Assembly (HRMA), and calibrate the end-to-end performance of the combined configurations of the HRMA/Science Instruments (SIs) and the HRMA/transmission gratings/SIs. AXAF's mirror assembly is expected to have a point response function of less than 0.5 arcsec FWHM. The resolving power of the AXAF gratings is expected to range from about 1000 at the lowest energies to about 100 at the highest energies.
Before calibration at the XRCF, each AXAF subsystem will undergo extensive laboratory verification and characterization. Some key parameters, however, cannot be evaluated in the laboratory without the HRMA or other special equipment. The XRCF has been designed to allow measurements to 1% accuracy. An X-ray Source System (XSS) and system of monitors and detectors have been constructed to meet requirements derived from this level of accuracy. The XSS was designed to provide the required spectral purity, together with the required dynamic range of flux, spectral-line width, and source size. The XSS consists of three types of X-ray generator (an Electron Impact Point Source [EIPS], two rotating anode sources, and a Penning gas discharge source), two monochromators (a grating monochromator and a double-crystal monochromator), and a filter assembly. A later section will discuss the capabilities and operating principles of the XSS.
The specially designed detector system is called the HXDS (HRMA X-ray Detection System). The HXDS was designed by the SAO Mission Support Team to provide higher counting rates and finer spatial resolution than the science instruments, so as to characterize the HRMA at the highest possible resolution and sensitivity. The HXDS has two Beam Normalization Detector systems (BND-500 and BND-H) and a focal surface detector system called the HRMA X-ray Detection Assembly (HXDA). The HXDA will be used only for the characterization of the HRMA and HRMA/grating combinations, while the BND-500 and BND-H will be used throughout the XRCF calibration.
The XRCF calibration has unique features which complement subassembly laboratory calibrations and in-flight calibration. The XRCF offers the only opportunity to irradiate the entire AXAF optical system with X-rays of well-known spectrum and flux at the entrance to the HRMA. On orbit, there are no celestial sources whose absolute X-ray fluxes and spectral energy distributions are as well known. With the tunability of the monochromatic sources, the XRCF provides a unique opportunity to measure the X-ray properties of the entire system across the spectral edge-features of the HRMA, gratings, and SIs. The SIs will be illuminated by a focussed converging beam for the first time at the XRCF.
AXAF calibration at the XRCF has three phases, according to the current plan. Phase Zero will characterize the properties of the HRMA thermal control system. Phase One will characterize the HRMA and the HRMA/gratings with the HXDS. Phase Two is the end-to-end calibration of the HRMA/SI and the HRMA/grating/SI combinations. Before the Phase Zero calibration, there will be an integration, and a dress rehearsal with the HXDS, a simple grating assembly, and a pair of scale-model AXAF mirrors.
This article will now provide general information about the XRCF, plans for calibration, operations, and data analysis and archiving.