29-30 June 2004

Chandra Users' Committee

Attendees:  Monique Arnaud, You-Hua Chu, Jack Hughes, Vassaliki Kalogera, Bill Latter (by telephone), Julia Lee, Knox Long, Kazuhisa Mitsuda, Chris Reynolds, Alan Smale (ex-officio), Greg Taylor, Allyn Tenant (ex-officio).

• Harvey Tananbaum announced that new developments associated with the proposed bakeout of ACIS had resulted in new concerns regarding the likely effectiveness of the bakeout and that until these were resolved (see below) the bakeout would not occur.
• Ed Mattison described the status of the spacecraft.  There had been very little change in spacecraft status and there are no serious concerns at this time.  The efficiency of observing remained high, typically 65%, as measured by the faction of time the observatory was locked on guide stars
• Harvey Tananbaum reviewed the approved director's discretionary time for Cycle 5; he had used less that half of the time at his disposal.  The character of the proposals that had been approved was similar to previous cycles.
• Belinda Wilkes briefly summarized the CXC's responses to our previous report.  For the most part, she identified portions of later presentations where the response would be elucidated.  The CUC had suggested that the CXC find a way to allow for "High risk/high return" programs.  (These would be programs that are safe from a technical perspective, but high risk/high return if successful from a scientific perspective).  She said this would be considered as part of preparations for the next proposal cycle.  The CUC would like to hear the outcome of this for the next cycle. Since it is not clear how many proposals of this type would be submitted, it is possible that this should be included as a category under DD time. However, making these programs part of the regular call for proposals gives them a visibility that they would not have if they were somehow announced as being available under the DD time program.  Therefore, the CUC encourages the CXC to make these proposals officially part of the Call for Proposals.
• Paul Green gave an update on the performance metrics for Chandra.  The total numbers of publications and citations are impressive. The data obtained in small proposals tend to be published more quickly than that from large proposals. At present, the citation rate per kilosecond for small proposals is somewhat, but not a lot higher than from large proposals.  The CUC believes that it is useful to continue the effort to accumulate metrics, even though the interpretation of the metrics, at least as it applies to setting proposal policies, is not straightforward.
• Kathy Lestition described the EPO and educational activities of the CXC.  The degree and quality of activities was impressive, especially those associated with educational activities.  The CUC was generally unaware of the educational activities and suggests that the CXC make a larger effort to advertise these types of activities to the community of observers.  It is not that the CUC was unaware of opportunities to participate in education-related activities, but that it was unaware of what was coming out of those activities, and believes that wider advertising could lead to greater use of the materials and more interest in participation. 
• Paul Plucinsky summarized the huge amount of work that had been carried out to understand the ramifications of a proposed bakeout of the optical blocking filter in front of ACIS.  The CUC had previously stated that it felt the bakeout was, in principle, a good idea if it was just safe and likely to succeed in substantially increasing the effective area of the observatory at low energies. The project had been proceeding toward a recommendation for bakeout in the fall. In fact, a major review had been conducted at MSFC several weeks before in which the work to date was presented. But a new concern about the nature of the material being deposited on and/or the distribution temperature of the blocking filter had arisen.  Basically, the thermal models currently show that the middle of the windows are warmer than the edges, and this had been assumed to account for the fact that less material has been deposited in the center of the filters.  But if that is correct, then one expects the current rate of deposition on the center to be lower in the center than the edge, and this does not appear to be the case.  One possible explanation is that multiple contaminants are being deposited and that only one of them has a strong temperature dependence.  If so, it increases the possibility that the deposited material will not be removed efficiently during the bakeout.  Until this is resolved, no one, including the CUC, believes it is prudent to proceed given the high quality of science that is currently being conducted with Chandra as it is.  Further a nalysis is underway, but it is unclear when or if these will actually resolve the uncertainties.  Regardless of the outcome, the CUC commends everyone associated with the effort for the hard work they have performed to investigate the bakeout. 
• Belinda Wilkes described user support activities at the CXC.  These are extensive and seem to be well received by the community.  CUC members have received no complaints regarding user support, indicating that the CXC is fulfilling community expectations in this regard.  There are some services that are no longer being used by the community, including individual visits by users and contact scientists.  Presumably this is because the community is fairly knowledgeable about Chandra at present.  The CUC feels that some thought should be given to formally terminating activities that are not used.  While this may not save much money, it simplifies the user interaction.
• Martin Elvis described plans for Ciao 3.1 and also an internal study carried out to identify holes in analysis procedures.  One very nice feature of this plan was that the priorities that had been set up all involved an output product. The latter is particularly important because total staffing for the CXC is declining as the Chandra moves into its second 5 years of operation.  The big changes in Ciao 3.1 are to incorporate gain corrections into the ACIS pipeline (which could previously be done via scripts) and to incorporate a new mechanism for making rmfs for ACIS.  Two tools were demonstrated -- one for source count extraction using apertures with corrections for off-axis psfs and the other a flexible interface for abstracting the portions of time-resolved data set that satisfied various characteristics, such as hardness and count rate.
• Pat Slane described the scheduling process with emphasis on the effect of constrained observations on scheduling process.  At present, the percentage of constrained observations (30%) exceeds the maximum advertised in the call for proposals (20%).  They also exert effort to accommodate preferences expressed by proposers. At present, the number of constrained observations is not causing overall observing efficiency to decrease, but, in the judgment of the planning staff this is the limit that can be supported.  It was not clear that this number could be reduced without impact to science, but obviously it is an issue that the CXC should continue to monitor.
• Larry David described the priority plan that had been developed for calibrations of Chandra intended to reach the goals that had been established.  It appeared that there was a reasonable prospect of reaching all of the goals in the calibration plan, with the possible exception of those that involve HRC degap model These affect, astrometry with HRC, some types of image reconstruction, a higher quality dispersion solution and accurate determination of FWHM with the LETG.  The CUC appreciates the activity to systematically set calibration priorities and then to implement plans that will reach both the original calibration requirements as well the new goals that have been set. The CUC believes it should continue.  Since it seems unlikely that the bakeout will proceed in the fall, it presumes that some of the resources planned for that could be diverted toward the HRC degap model.
• Herman Marshall described the problems that had been encountered both in internal calibrations on Chandra and on trying to compare with XMM-Newton.  Significant increases in understanding have been obtained in specific areas, and some of these are being translated into products that the user community can use, e.g. a more consistent and presumably better calibration of the QE of the FI/BI CCDs.  However, the CUC was somewhat concerned that this activity is still too much of a research project, and that there is not clear path to a better declarative statement regarding internal consistency of Chandra calibration as well as cross-calibration with XMM. 
• Diab Jerius described improvements in knowledge of the HRMA, and the release of elliptical psfs for HRMA.  He asked for advice on what other HRMA related studies should be undertaken.

General conclusions:

The CUC is generally pleased with the status of Chandra and the level of user support provided by the CXC.  The science productivity of Chandra is very high, and this is in large measure due to the dedication of its staff.  With the exception of continued question concerning the effect of contaminant buildup on the Optical Blocking Filter of ACIS, there are no major technical problems that are under intensive investigation.

The CXC has been responsive to the recommendations of the CUC in the past, and for that we are grateful. In particular, we see continued progress in terms of the rationalization and systemization of a calibration program.  We also applaud the effort to attempt to find holes in CIAO analysis procedures, using this to establish priorities for development, and especially the focus on what products are needed to close holes that have been identified.  We do not detect any specific areas where user complaints are concentrated. In fact, very few members of the community have complained to us, or indicated that they were unhappy in our informal questions to them. 

Specific Recommendations

• Bakeout of the optical ACIS optical blocking filter:  A bakeout has the potential to significantly improve the scientific performance of Chandra.  The committee had been impressed by the quality and detail of the work that has been done to data.  However, like others in the project, the CUC is concerned by the new information that was developed between the MSFC review and the CUC meeting.  The committee supports the CXC Director in terms of assuring that we fully understand the issues before proceeding, given that this is the premier X-ray observatory in the U.S. arsenal.  It is important to continue to improve the understanding of the probable CTI degradation as well as to obtain a better understanding of the volatility of the material.   Assuming that these issues are resolved in the next 3 - 6 months, the CUC would like a full review of plans. If there needs to be a special meeting of the CUC to accomplish this, we are prepared to support this. If a higher degree of confidence is not obtained in the next six months, then the project needs to assess whether further study of this issue is ever likely to result in a recommendation to proceed.
• CIAO software installation: The timescale of individual Chandra research projects is often longer than the time between builds (or patches), users must decide when and how often to update their version of CIAO (or to advise systems personnel to do so).  As one might expect, there is often a progression of S/W capabilities from ad hoc tools to address individual problems that are not part of a standard release to incorporation of a process in the default pipeline.  The CXC could aide users in this process if it would provide clearer advice on the importance of specific S/W releases to users, especially those users who are the middle of a project and want to understand whether their previous work is affected by a change.  In particular, it is important to note what communities are unlikely to be affected in a significant way by a patch or upgrade.  The release notes that have been provided in for example the recent bug patch were too unspecific to allow an easy assessment of this. Our impression is that the release notes for Cal DB are somewhat better organized for science assessment, and so these might serve as an example of what is needed.
• DS9 and other visualization tools:  The committee was generally impressed with the tools demonstration, particular the demonstration involving DS9 and software to create elliptical contours that reflect the 90% contours for counts as a function of off axis angle.  The development of off-axis psfs that were the basis of the regions reflects an earlier request of the CUC and we are grateful that this activity was accomplished.  However, no one on the committee was aware of how to attach tools to DS9 and was only vaguely aware that some at the CXC seemed able to do so.  So the question this raised in the minds of the committee was what attitude the CXC ought to be taking regarding the interrelationships between a visualization tool and ciao scripts. We are excruciatingly aware that the issue of interfaces is controversial. One of the advantages of writing the threads as scripts is that they involve minimal interaction with a users choice.  This said, users would like easy access to tools, including GUIs, which are likely to be useful.  Therefore we suggest that some threads be developed that taking advantage of some of the facilities of various image display tools, especially DS9, which is widely used by the X-ray community.  It is possible that one should also consider creating a web page that provides information on how to get started on spiffy interfaces, and how they might be utilized.
• Calibration: As noted earlier, we believe significant progress is being made toward systematizing the calibration needs of the community in terms of requirements and goals, and in terms of carrying out observations and analyses to realize these goals.  The projects scheduled for completion in September will be a significant step toward this goal.  Some areas that do need work include the following:
  • While the requirements are clearer than they have been, it is not always clear where they apply.  Although the CXC realizes that it is not easy to fulfill all requirements over the entire FOV of ACIS for example, it is important to attempt to do so as far as possible.  In any event, the statement of the requirement and the degree to which we have fulfilled the requirement should be clear on these points.
  • For spectral analysis, the Iridium edge of the HRMA has been a long-standing issue.  While one can ignore the region around the edge and continue with spectral analysis, this is an annoying work-around that should not really be necessary this long after launch.  We urge the CXC to give priority to understanding this problem and correcting the energy response functions appropriately so users do not have to ignore this spectral region.
• Pileup:  An area that was not addressed adequately at the meeting was pile-up.  This is a practical problem for users of Chandra.  Users need practical tools for photometry on and off axis and for spectral analysis.  The CUC is aware that there are some models to deal with pileup in MARX for imaging and in xspec for spectral analysis, but the applicability of these models is unclear to the CUC.  We would like a comprehensive review of the tools and threads that exist for handling pileup at the next users committee meeting, and assessment how well these tools work on and off axis.
• Joint proposals:  About 10% of Chandra proposals involve coordinated observations with other observatories.  In most cases, the Chandra TAC has not fully subscribed the time available on HST, NRAO, or NOAO.  Typically, only 50% of the maximum allocation is awarded.  In one sense this is a good situation, since it means all coordinated programs approved by the TAC do go forward.  Successful proposers are not placed in double jeopardy, was one of the main reasons for creating this proposal category. There are a number of reasons the coordinated time is not fully subscribed.  First, a significant number of coordinated proposals fail because the X-ray time is not considered of high priority, and coordinated observations are not awarded unless the primary X-ray time is awarded.  Secondly, because the amount of coordinated time is not large, proposers typically submit proposals with relatively modest requirements for the second observatory. The CUC fully subscribes to the statement that any program approved by the Chandra TAC should be excellent science, and that any coordinated observations should support the X-ray observations.  On the other hand, the current selection process may disadvantage programs that require roughly equally resources from Chandra and other observatories.  It is not entirely clear that anything can be done about this. Nevertheless, as arrangements for coordinated observations expand, which seems to be the trend, we urge the CXC to engage the other observatories on whether the current process could be improved.  In the current circumstance, it is also important for the CXC to reiterate to the TAC that the specific purpose of joint proposals is to enable supporting observations that will enhance the X-ray observations with Chandra; the committees should not assume that coordinated time to support Chandra observations would not be awarded by other TACs. Statements like "the HST time can be proposed for separately" or "Ground-based images can be obtained with still higher resolution with adaptive optics" are not the sorts of comments that should be sent to the proposer.
• Cross-calibration: Physical understanding of the instruments is important for doing cross-calibration. It was good to see that the CXC and the XMM-Newton support center are discussing the cross calibration.  But the general user is publishing data today and so we need better qualitative characterization of the errors.  We need on the one hand a practical guidance to the relative photometry and on the slope that are derived from simple spectra -- e.g. AGN, galaxy clusters.  A definite plan to produce output products is needed.   The recommendations regarding the cross-calibration should be consolidated in a single place on the web site.

Next time

The suggested time for the next meeting is Jan 25, 26th.  This is conveniently after the HST deadline, which is January 21st

Topics for the next meeting are not yet established, but, should include, as noted above a complete discussion of pileup tools and also a more extensive discussion of archival proposals, both as part of the proposal process and in terms of demonstrated science productivity.