The Advanced X-ray Astrophysics Facility (AXAF), scheduled for launch in 1998 1, will provide high-angular-resolution imaging spectroscopy and transmission grating spectroscopy in the 0.1-10 keV band. We, together with colleagues at the MIT Lincoln Laboratory and the Pennsylvania State University, are developing the AXAF CCD Imaging Spectrometer (ACIS), one of AXAF'S two focal plane scientific instruments. 2 ACIS employs X-ray-photon-counting CCDs designed and produced at MIT Lincoln Laboratory. 3 AXAF scientific objectives call for extremely accurate knowledge of instrumental response; for example, the goal for detection efficiency knowledge accuracy is of order 1%, and the corresponding goal for energy scale knowledge is of order 0.1%.
Even to approach these calibration goals, which are unprecedented in X-ray astronomy, requires an extensive, carefully planned calibration program. ACIS calibration, itself a subset of the AXAF Observatory calibration effort, will occur in several phases. In this paper, we describe the first and most time consuming phase of ACIS calibration, in which individual CCD detectors are calibrated before they are installed in the instrument. ACIS flight detector calibration has been underway since early this year, and a complete set of calibration data has been acquired for some fifteen devices. We believe we have acquired data which will meet or nearly meet each of our ambitious calibration goals. Our objective here is to describe the overall calibration approach and to present sample results which illustrate the quality of the data. A series of companion papers [4,5,6,7] describe various aspects of the instrumentation and analysis tools developed to implement the calibration plan.
Following a brief description of the ACIS instrument in section 2, we discuss the calibration strategy in section 3. In sections 4 and 5 we describe the objectives, measurements and selected preliminary results of our program to determine the CCD spectral response function and detection efficiency.