The fourth meeting of the AXAF Users' Committee was held 30, 31 May 1996, at SAO in Cambridge Massachusetts. Eleven committee members attended as well as A. Bunner and F. R. Harnden Jr. from NASA Headquarters.
H. Tananbaum and R. Brissenden gave brief reports on the status of AXAF and the ASC. The AXAF mirrors have been coated and are being bonded to the support structure. Three (out of 8) are now in place but the third is slightly out of tolerance in tilt. The cause is not yet understood, but the situation is under control. Things are moving well towards calibration which calls for a dress rehearsal in mid-August. Mirror calibration starts in October or November of this year. The ASC has finished CDR1 and is headed for CDR2 in July. The Users' Committee was invited by A. Tennant to review documents being prepared for this review. The first software required for calibration has been delivered. NASA has decided to locate the Operations Control Center (OCC) in Cambridge and the ASC is in the process of finding a suitable facility. 110 FTE people are now at the ASC (including TRW and MIT). Fifteen more are expected in the next five years.
Plans were presented for proposal submission, and for proposal planning tools. The NRA will be a modified XTE NRA and a draft will be ready in March. The Committee will review Appendix C, which contains rules for submittal and review, and criteria for selection. The Committee requested that the requirement stating that ``observations be relevant to the capabilities of AXAF'' be written with enough detail so that it will be useful to the peer review.
The description of instruments and spacecraft, Appendix F, will be taken from the Science Instrument Notebook (SIN) which is now available on the WWW. This will be updated in August 1996 and again after calibration in August 1997. The Committee requested that hard copy of this be distributed since printing from the WWW was slow and figures were sometimes difficult to get via FTP. It was considered most reasonable to send this in advance to libraries and then to individuals on request. Automatic distribution to everyone who gets the NRA is a waste of paper.
The Remote Proposal Submission (RPS) software used for ROSAT, ASCA, and XTE will be used for AXAF with appropriate modifications. In response to specific questions from the Committee: Latex can now be used in the abstract and there is a ``save'' button that allows users to save their input. A budget form will be supplied at the appropriate time but allowance will be made for organizations that insist on using their own form. The Committee discussed the review of budgets. There was unanimous agreement that a two stage review worked best. Evaluation of funding results will be a topic at the next meeting.
The ASC plans to modify PIMMS and XSPEC, two existing software tools familiar to the community, for use in the proposal planning process. The committee requested that the ASC generate a schedule for development of other proposal planning tools showing effort required and the ASC ranking of importance. The Committee noted that grating spectra were going to be particularly difficult to predict usefully. They recommended that simulated data be put on the WWW for inspection by proposers. The Committee also strongly recommended that roll constraints be part of the program which calculates target visibility.
There was discussion about targets of opportunity (TOO) discovered in the AXAF data. Should the ASC search for TOO in AXAF fields and actively pursue follow up observations? The Committee thought that it would be helpful for the ASC to contact observers if unexpected phenomena were seen in the fields without, of course, losing sight of the proprietary nature of these data. All fields will, as a matter of fact, be inspected to verify that instrument operation was normal. As part of this process, sources with unexpected strengths will be flagged and unusual phenomena investigated to verify that they are not instrumental, so the ASC might recognize an unusual event in a timely manner that might not be achieved by the observer.
Plans for analysis software were presented. Key features of IRAF and XSPEC such as parameters, files, regions, and filters will be used and AXAF data and ASC software will be compatible with IRAF and FTOOLS. IRAF tools, FTOOLS, and XSPEC will all work on AXAF data. The Committee stated that compatibility with IDL was important and that compatibility with European analysis environments should also be investigated.
The Committee emphasized the obvious importance that the basic data processing be correct and that effort should not be diverted from this for the ``bells and whistles'' that are attractive to scientists. A. Tennant again invited Committee members to review the documents prepared for CDR2 which will give software design details.
The Committee also recommended that warning messages be incorporated in the analysis software. Users should be warned when, e.g., parameters are obsolete or due to be recalibrated or when pileup effects might distort the data.
After initial data processing, it is customary to search for sources in the field and to automatically give the observer source locations, strengths, light curves, and spectra. Since standard analysis software is used for this, it takes very little effort to generate these data. There are however, two dangers. Seasoned X-ray observers know that automatic analyses are often incorrect, but an observer new to the X-ray business can think this analysis, done by the ASC, is suitable for publication. The second problem occurs for fields in which the computer finds a great many weak (often false) sources. A stack of useless paper can be generated.
The Committee felt that products which are not reliable should not be sent to observers. Automatic analysis, however, sometimes does flag a source as being interesting. The consensus was that source locations and light curves were the most valuable. Pulse height spectra should also be generated for the stronger sources. The ASC should use its own judgement in determining a cut-off to prevent automatic analysis of many very weak sources.
Results of the survey described on pages 16 and 17 were presented. The Committee suggested a second survey to ask what computer the user was planning to use to analyze AXAF data and to ask why CDROM and FTP were unacceptable to the 10 percent who gave this answer. If lack of a CD reader was the reason, AXAF proposal money might be used to buy CD readers, which are not expensive.
Results of a workshop held at the San Diego AAS meeting
(described in pages 17 to 19), and of a
study to determine the level of effort, were presented by N. Evans. The
proposal is that the ASC sponsor deep optical observations of all deep
AXAF fields. The optical images would be placed in the public archive
for use in archive studies. In the first year, 
ACIS images
are expected with exposure greater than 
sec.
Using a prediction of ACIS sensitivity, and the goal of
finding 70 percent of the AGN optical counterparts in a 
sec
field it was estimated that images three magnitudes fainter than the digitized
Palomar Survey would be needed.
The Committee had mixed feelings about ASC involvement in this. On the plus side: it is good science and would be an asset to archival research. There is also good potential for serendipitous discovery. On the negative side: there is no need to collect optical data early, since ground-based observatories are permanent facilities. Probably, only a few fields are necessary to achieve the main science goals. It might be a better use of the time to extract the extended X-ray sources and get optical spectra. ASC resources are better used elsewhere.
The HRC
The energy
resolution will, of course be low, similar to that of the ROSAT HRI.
Three things will limit the counting rate: 1) There is a telemetry
limit of 184 events/sec. 2) Since an HRC pore takes about a second
to ``refresh'', the HRC gain becomes non linear at rates >1/s /pore,
which is 
5 c/s for a point source. 3) There is a permanent
gain drop when the integrated rate reaches 500/pore. This will permit
observation of point sources only up to 200 c/s or 
40 Uhuru c/s.
Sources such as Sco X-1, Cyg X-1, etc. cannot be observed unless the
telescope is defocussed.
The Committee suggested a zero-order block so bright sources could be observed with the gratings. This is incompatible with the dither and there are no plans to block the zero order. Observations of bright sources might be accomplished with offset pointing.
There is also a problem of order overlap using the LETG/HRC
combination. The problem arises because the energy resolution of
the HRI is not good enough to separate higher order events from the
first order events of primary interest. This was illustrated
with a coronal spectrum calculated for a continuous emission measure
distribution peaking at 
K having strong Fe L
complex at 8-17Å. The first order here is not completely resolved
so the third order at 25-50Å cannot be uniquely predicted. There
is also noise which makes higher order flux predictions inaccurate. The
ambiguity could be helped with a reflective filter. This could be
placed to use 15 percent of the active area (vertical space) of
HRC-S. Mirrors coated with Cr and C operating at incidence angles of
would reflect only wavelengths > 37Å and
>55Å, respectively. An engineering study is now underway and a
decision concerning incorporation of such a mirror will be made in a
matter of weeks. Although 85 percent of the HRC-S will be available
for use without this mirror, there was some concern on the Committee
that this would limit the low energy response of the HETG/HRC
combination.
The ACIS
There are
two clocking modes. The usual timing mode has a duration of from
0.3 to 3 sec depending on the size of the area to be read out. This
determines the time resolution of the data. Three types of event
processing set maximum count rates of 93, 175, 385 c/s. A continuous
readout mode allows time resolution of 0.003 sec but
only 1-dimensional spatial
information is obtained. Maximum rates for continuous readout are
400 and 650 events /s. The requirement for accurate spectra and pulse
pileup set another count-rate limitation. Two photons in one
pixel array (which contains most of the energy from a focussed
point source), during one readout interval, will be registered as one
event with energy equal to the sum of the two photons. The resulting
count rate limit can be overcome by defocussing the telescope, or, for
not-too-bright sources, by using a subarray mode.
Visibility
After peer
review selection of targets, a long term science plan will be
developed and released to the community. The ASC and OCC will also
develop short term time lines weekly. Observations during the 64 hour
elliptical orbit are limited
by the radiation belts which render instruments inoperative for 
percent of the time and
by sun/moon/earth viewing constraints.
The Committee recognized that roll constraints
(through a solar power/solar panel orientation limit) will narrow the
observing window. It was recommended that tools be available for use
by potential grating users to calculate viewing limitations caused by
restrictions on roll angle. The Earthblock problem could be
bothersome but some observations in the blocked area are possible.
Because the orbit precesses slowly, the Earth blocks the same region
for a long time. A target in the worst Earthblock position, however,
can be observed for 
hours in any one orbit.
The schedule for the Huntsville calibration, which extends from November 1996 to April 1997, was reviewed. Considerable time was spent discussing AXAF's ambitious calibration requirements. The Committee thought that 1 percent was unrealistic, and this should not be advertised to the scientific community if not feasible. Also, the goals might interfere with choices which have to be made during the Huntsville calibration. The ASC's and Project Science's responses were that all of the required calibration accuracies appeared feasible and could be met.
The Committee was told that the flight ACIS and the HRC-S mirror (if used) will be in place for the ground calibration. The Committee remarked that molecular contamination could produce broad band effects not detectable at only one energy. The Committee was also shown a plan for three days of in-flight science calibration. A series of targets spanning most astrophysical fields of interest would be observed using both instruments and gratings. These observations would go immediately to the public archive and serve as examples of AXAF observations to be used in proposal planning. The Committee endorsed this concept and suggested additional in-flight calibrations simultaneous with XMM and AstroE. It was noted that early science observations should be suitable for PR purposes.
There was a brief presentation of the ASC education and outreach plan. The Committee suggested that AXAF material might be incorporated into CLEA, a set of astronomical interactive teaching materials. The ASC requested that education ideas be sent to Kathy Lestition or to Fred Seward.
The future of the Users' Committee was discussed. The present committee will be retained until launch, unless any members wish to be replaced. At that time, the AXAF and ASC insiders will be replaced with outside people representative of the General Observer community. Members will serve staggered 3-5 year terms. The Committee thought that the group should be technically focussed and that new members should be actively working with AXAF data. The Committee also recommended closer ties to the HEASARC.
Fred Seward