Last year the AXAF program underwent a major restructuring in response to immense budget pressures, leading to a faster, lower-cost program with two flight elements. AXAF-I is built around the High Resolution Mirror Assembly which has a resolution of approximately 0.5 arcsec, to the extent that it can be characterized by a single number. AXAF-I will carry two imaging instruments (the High Resolution Camera and the AXAF CCD Imaging Spectrometer) and two sets of gratings (High Energy and Low Energy Transmission Gratings). Hardware development is well underway, with outstanding X-ray performance demonstrated in September 1991 for the two largest of the 8 flight mirror elements. AXAF-I is scheduled for launch in September 1998 into a high earth orbit of at least 10,000 by 100,000 km, possibly with a perigee above 60,000 km.
AXAF-S is built around the X-ray Spectrometer which will have an energy resolution of approximately 10 eV over the 0.3-10 keV band. Two options, both optimized for high throughput, are under consideration for the AXAF-S optics: an improved version of the foils developed for the successful Broad-Band X-ray Telescope shuttle experiment, and a replicated set of metal mirrors. The anticipated mirror resolution is 30-90 arcsec, although both larger and smaller numbers are still possible and acceptable at this time. AXAF-S will be boosted into a low-earth, sun-synchronous orbit, with launch scheduled for December 1999.
Future newsletters will provide more specifics about the contractor and principal investigator teams (AXAF-I has TRW for its prime contractor, and AXAF-S is being developed in-house at Marshall Space Flight Center); about technical features of the spacecraft, optics, and science instruments; about AXAF scientific capabilities; and about program status, budgets, and schedules. During the past year a major emphasis for both AXAF-I and -S has been on mirror work and on the formal statement and review of requirements for the spacecraft and science instruments.
The AXAF Science Center (ASC) is charged with assisting users and helping to maximize the scientific return from the AXAF-I and -S observatories. In response to a NASA request for proposals, SAO organized a team which includes MIT, TRW, The University of Chicago, The University of Hawaii, and Stanford University, and we were extremely pleased to be selected in 1991 to develop and operate the ASC. Our primary responsibility is to support you, the AXAF users: to make it easy for you to write proposals; to help NASA organize a fair and competent peer review; to help the AXAF Operations Control Center with the scheduling of successful proposals; and to develop an ASC Data System to receive, process, archive, and distribute the data. We are also working with the contractor and principal investigator teams to support the development and ground calibration of AXAF-I and AXAF-S, to create an effective interface between the instrument developers and the eventual scientific users, and to plan for on-orbit calibrations. ASC scientists also carry out their own scientific research, so as to increase their knowledge and understanding of AXAF and sharpen the skills needed for a first-rate ASC which can provide expert assistance to users.
Articles in this first newsletter provide preliminary information about calibration and the expected satellite orbits. An AXAF Users' Committee has already been formed; its membership list and a report of the first meeting are presented. Concepts for communications between users and the ASC, and for an interface between users and the ASC Data System, are presented through two pilot software facilities available for your use and comment: ASCinfo and ASSIST. At future AAS meetings, in subsequent newsletters, through the AXAF Users' Committee, and in other ways, we will describe our plans for the ASC. More importantly, we will interact with the astronomical community to obtain your inputs, reactions, criticisms, advice, etc., as we strive for a successful AXAF program.