|FUSE Observer's Advisory Committee|
|Meeting Minutes No. 6|
|19 March 2002|
Present: Bregman, Dinerstein, Federman (chair), Harper, Leighly,
Starrfield, St. Louis (replacing Robert), Wannier
NASA: Hasan, Sonneborn
JHU: Andersson, Blair, Friedman, Kaiser, Kruk, Moos, Sankrit, Weaver
Blair opened the meeting with a report on spacecraft status, recent performance, and planning. The Yaw Reaction Wheel experienced a catastrophic failure, probably involving the bearing. When combined with the failure of the Pitch Reaction Wheel (from bubbles in the Kapton tape), the Magnetic Torque Bars were utilized to maintain pointing the solar panels at the Sun. (The third reaction wheel has never experienced anomalies that may have foreshadowed the failures noted above.) Within 2 months (from early January to early March), FUSE resumed science operations as the primary activity. Mission Operations proved that the spacecraft can move between targets in the northern and southern skies, developed a tool for determining good torque stability beyond declination 40 degrees, showed that pointing jitter was at or below 0.6 arcseconds (the original requirement), and verified that the sensitivity has not changed. The restriction on declination translates into observing 36% of the sky, where 50% of the FUSE targets lie. (Subsequent tests have shown that successful observations below 40 degrees are possible.)
Kruk described the status of the Gyros and the plans for gyroless operations. The Y axis of Inertial Reference Unit-A (IRU A) failed in May, 2001, and IRU B has been utilized since then. Subsequent laser intensity warnings appeared in the X axis (August 2001) and Y axis (October); as a result, the Team developed a gyroless observing mode. The Fine Error Sensor (FES) will acquire rate information while observing, while the TAM's (Three-Axis Magnetometers) will maintain attitude during occultations. Drift rates in the absence of FES data will increase from 0.01 arcseconds/sec to about 10 arcseconds/sec. Slew errors and drifts during occultations may reach +/- 2 degrees. Key components of the new operations include autonomous FES processing of images and tracking of guide stars, an enlarged guide star finding table, and dead-reckoning slews with the Attitude Control System. There is a potential problem with offset acquisitions if a bright object appears in the field of view. Gyroless operations will be possible by 10/02.
Van Dixon brought the FOAC up to date on matters of calibration. Version 2.0.5 was released in late Fall 2001, and V2.1 became available in April. The latest version includes better logic, which recognizes day/night observations and allows time sequences, updated bad-pixel maps, automatic worm identification in LiF1B (but not corrected), and improved background files. There are now 5 GIF images available for viewing aspects of the data, including detector images and count rate plots. All new data will be processed with V2.1. There is new documentation on background subtraction, wavelength and flux calibration, and error propagation. Future plans include improved treatment of astigmatism and jitter correction (large-scale sinusoidal variation) over the near term, as well as 1-dimensional flat fields, which for bright targets will be the automated shifting and combining of exposures.
Sonneborn led the discussion about the Project. He noted that the FUSE Mission is at peak productivity, with over 1200 objects observed over 17 Msec and 66 refereed papers appearing through 2001, among other highlights. Much of the remaining discussion dealt with the transition to the Extended Mission. One particularly important item involved how best to populate the target pool under the current pointing constraints. The 'Observatory Programs', which do not have reserved targets, was described. Moreover, while changes to target lists for Cycle 2 and 3 programs are allowed, the consensus among the participants was that no changes were possible once the NRA for Cycle 4 was announced.
The upcoming NRA and the Extended Mission (EM) was the focus of most of the remainder of the meeting. Sonneborn noted that about 6000 ksec will be available in Cycle 4. There will be two categories of programs: Standard Programs (~ 70% of the available time), where all targets will be observed (including constrained and unconstrained observations); and Survey/Supplementary Programs (~ 50 to 60% of available time), with only unconstrained observations possible in an oversubscribed pool of targets. A proposer can request the latter program, or a proposed Standard Program may be converted during the review process. The funding level for Survey/Supplementary Programs was not yet final. A constrained observation is one which has a total visibility of 3 weeks or less, and it may involve coordination with another spacecraft, targets of opportunity, and declination limitations. A key aspect for Cycle 4 will be the ability to observe brighter objects, either using only the SiC channels or defocusing LiF2 and SiC2. (The time schedule appeared in the most recent FUSE Newsletter.) Andersson mentioned that a new Users' Guide will be available for the upcoming NRA. Furthermore, a new web-based tool is available which permits the user to calculate windows of visibility (availability) over a specified time range for a list of targets. Target availability is subject to the traditional constraints of beta (anti-sun), ram, and moon avoidance, as well as the torque authority limitations imposed by the new magnetic control mode.
Blair discussed the activities leading up to the EM. The necessary capabilities for the EM are being developed during the Prime Mission, and many of them are now in place. These include automation of ground station passes, more robust scripts, and improved scheduling of alignments. A main constraint in this execise is the decrease in staffing. The main effects to scientific productivity will be the increased recovery time from anomalies and fewer channel alignments.
Friedman brought the FOAC up to date on the proposal for the Senior Review at NASA Headquarters. The main thrust was a request for an EM through Cycles 6 and 7, two years beyond the current plan. The discussion revolved around several topics. Foremost, the FOAC noted that the broad range of science, including unexpected results such as H2 on Mars, needed to be stressed. The Committee suggested that augmented funding for Cycles 4 and 5 should be requested. This would allow a smoother transition between current activities under the Prime Mission to those in the EM. The most efficient complement of observing programs was also discussed. The key is to have an adequate target pool throughout the sky. For instance, the Survey/Supplementary Programs now being implemented could evolve into 'Legacy' programs that highlight the need for FUV spectroscopy. (The proposal was submitted in May.)
Finally, Graham Harper will become Chair of the FOAC at its next meeting.
FOAC Recommendations -