The first meeting of the AXAF Users' Committee was held on June 24th and 25th, 1993, at SAO, Cambridge, MA. Attending were J. Bechtold, J.P. Caillault, W. Cash, L. Cominsky, K. Flanagan, S. Kahn, J. Patterson, C. Sarazin, F. Seward (chair), H. Spinrad, A. Tennant, and M. Watson. Click here to see a full list of committee members and their contact addresses.]
The meeting started with a presentation by Harvey Tananbaum on the status of AXAF and the AXAF Science Center (ASC).
Last year, AXAF was restructured and divided into two missions; AXAF-I (imaging) and AXAF-S (spectroscopy). The emphasis of AXAF-I is on high resolution imaging (0.5 arcsecond resolution) and grating spectroscopy. The high resolution mirror assembly (HRMA) contains four mirror pairs (reduced from six). There are two imaging instruments in the focal plane and two sets of gratings. One imaging instrument is the AXAF CCD imaging spectrometer (ACIS), similar to the CCD array now taking data on the Japanese spacecraft ASCA. The second imager, the high resolution camera (HRC), is similar to the HRI instruments carried by Einstein and ROSAT. The two sets of transmission gratings are made so that efficiency peaks at high energies (HETG) and at low energies (LETG). It is planned to put AXAF-I in a 10 by 100 thousand km elliptical orbit, where observing efficiency will be much higher than in low-earth orbit and continuous observations longer than a day can be made. Launch is scheduled for September 1998.
The AXAF-S mission will carry one instrument, a state of the art, cryogenically-cooled, X-ray bolometer. The telescope will be capable of moderate spatial resolution (~1 arcmin). A near-earth, sun-synchronous, polar orbit is planned. The lifetime will be limited to about 5 years by the cryogen. The launch of AXAF-S is planned for December 1999.
The role of the ASC is to coordinate key activities and help astronomers from different backgrounds observe with AXAF. The ASC's responsibilities include the review and assessment of AXAF calibrations, observation planning, development and operation of data processing software and procedures, development of proposal review procedures, development of data analysis software, and the preparation of documents and software to aid users in proposal preparation and data analysis. Most current work concerns detailing requirements for an ASC data system which will support these functions.
The Users' Committee asked about the lifetime of the ACIS CCD detectors in high-earth orbit. The answer depends on the orbit, which is not yet definite. The CCD detectors on ASCA will supply useful data concerning radiation damage, and steps might be taken to protect the ACIS detectors during solar flares.
Fred Seward reviewed the function of the AXAF Users' Committee, which is the official interface between the ASC and the user community.
The primary function of the committee is to inform the ASC of the users' needs and concerns. This should happen naturally since committee members are all users and have contacts, friends, and collaborators. Comments, suggestions and complaints can be passed easily to the ASC via the committee. The ASC expects to be informed of any problems which are brought to the attention of committee members. Anything that affects the users is the concern of the committee, particularly the interaction of the ASC with the users.
There are important policy issues that the Users' Committee and the ASC will develop together. For example, possible conflicts of interest between ASC staff and the external user community. These issues will be identified, discussed, and resolved. The charter says the ASC will do many things ``in consultation with the Users' Committee''.
The Users' Committee is advisory to the Director of the ASC, to whom the chair of the committee will make a report after every meeting. If there are actions required of the ASC, the ASC will report on these at the next meeting of the committee or sooner. We expect to resolve most issues informally.
Before each meeting, the ASC will publicize the names of the Users' Committee members and a list of topics to be discussed.
A preliminary list of topics which the committee will discuss before policy decisions are made was presented at the first meeting. When complete, this list will also contain the time frame when each topic will be discussed, and the list will be made generally available so that comments and additions can be received and incorporated.
There are beta test sites at Stanford University and the Universities of Hawaii and Chicago. The ASC-developed software will be exercised first by these institutions, and changes will be made before general release. The Users' Committee suggested that the role of the beta test sites be clearly delineated and that the principal testers present their findings to the Users' Committee at the appropriate time.
Alan Bunner from NASA headquarters presented a summary of the guaranteed-time observations in the restructured program. Originally, there were 4,270 hours for guaranteed-time observers (GTOs) distributed as follows: Months 1-2, 100%; 3-20, 50%; 21-30, 20%; > 30, 0%. Considering the increased observing efficiency of AXAF-I, the fact that there will be two spacecraft available for most of the time, and the reduced area of both the I and S mirror systems, this allotment will have to be reworked by NASA and the AXAF Science Working Group, which is chaired by the NASA AXAF Project Scientist, Martin Weisskopf. One proposal is to increase the time to 5,148 hours and to keep the guaranteed time fraction the same as outlined above. The guaranteed time will be split between the instrument principal investigators (A. Brinkman, C. Canizares, G. Garmire, S. Holt, and S. Murray), the telescope scientist (L. Van Speybroeck), and the five interdisciplinary scientists (A. Fabian, R. Giacconi, J. Linksy, R. Mushotzky, and A. Wilson).
The Users' Committee was vocal on this topic. They endorsed the concept that the developers of the AXAF instruments should receive a large fraction of the early time. It is obviously beneficial to all if those responsible for proper operation of the instruments get the first data, and that this data should be scientifically exciting. The principal concern was that a very large number of targets could be reserved and made unavailable to others in the community. The committee remarked that some members of the community keenly desire the opportunity to write good proposals to get the specific targets they want, and would like to see peer-refereed scientific competition for all targets.
It would be a good idea if some of the guaranteed time were used for projects not likely to be awarded by peer review. It was noted that historically it has been difficult to get large blocks of observing time through peer review.
Several alternate ways of handling the guaranteed time were suggested. Some of these systems have actually been used in other missions.
Rich Simon and Dave Huenemoerder presented plans for the ASC data system and analysis software. The data system will sustain all ASC activities such as receiving, processing, and distributing AXAF data, and supporting mission planning, the proposal process, and data analysis. The immediate goal is to determine the requirements for the ASC data system by November 1993. There will be both a graphical user interface and a command line interface. The graphical user interface currently used, the ASSIST package, was demonstrated for some members of the Committee. Click here for more information on ASSIST.
The design goals and some current thoughts and issues related to the ASC data system were presented. Ways of using IRAF tools in other environments are being explored. The goal is to have a set of scientific functions that can be run in multiple environments -- a so called ``open architecture''. The ASC organization chart and science data processing and analysis plans were presented. The division of data products into level 0,1,2,3 was reviewed, and there was discussion of the data analysis tools to be developed for the general user.
The ASC expressed an interest in getting from members of the Users' Committee some examples of software tasks which are run sequentially to accomplish scientific analyses; examples of such ``scripts'' generated within the ASC were shown.
The committee had the following comments:
The ASC is presently generating documents to be used in the ASC Requirements Review this coming November. Allyn Tenant invited members of the Users' Committee to participate in this review. It was decided that the most convenient form of participation would be for those who are interested to read the documents and send their comments to the ASC and to Allyn. The ASC will forward a copy of the appropriate documents (particularly the Science Plan) to the committee members.
The calibration plans for AXAF were presented to the Users' Committee by Christine Jones, Ed Kellogg, Dan Dewey and Brian McNamara. The talks included science goals, descriptions of the instruments, layout in the spacecraft, and laboratory calibrations (quantum efficiency, spectral resolution, spatial uniformity, count rate linearity). Plans for ground and on-orbit calibration of the assembled mirror-detector system were described (point spread functions, alignment, spatial resolution, boresight, focus). Results of the testing of the first mirror pair were presented; the first analysis showed a large asymmetry in the image due to distortion of the mirror by gravity, but this was corrected analytically and mechanically and the final image had a deconvolved FWHM as small as 0.2 arcsec. The X-ray Calibration Facility (XRCF) at MSFC was described; it comprises an X-ray source, a 1700 foot evacuated pipe, and a target chamber. Instrumentation used during the mirror calibrations was described.
The calibration of the flight instruments and mirror assembly is a major project. The ASC estimates that 4 to 6 months of continuous calibration at the XRCF will be required for AXAF-I. Calibration at the XRCF cannot commence until the mirrors and instruments are assembled; the end date is driven by the overall schedule for the project. The more the launch is delayed, the more expensive the project becomes.
The committee had the following comments concerning calibration:
The meeting ended with a roundtable discussion concerning what kind of computers the members use and how they use them. Members were very modest about their skills and achievements, and unsophisticated computer users were in the majority.