The following information was sent to the FUSE User Community on Friday, January 7, 2000.
To: All FUSE Program and Guest Investigators From: The FUSE Project at JHU Re: Project Level Changes Date: 07 Jan. 2000 It is now somewhat over 6 months into the FUSE mission. Regular science operations for the PI team and GI programs began on 01 December 1999 even as checkout and verification activities continue to be interspersed. We have learned much about the way the FUSE satellite is performing on-orbit. This memo addresses project-wide changes that are necessary in order to improve operations and produce quality data sets for all science programs. These changes are summarized here, and a more detailed discussion is included under the double line below. 1) The DEFAULT observing aperture is now the LWRS (30 arcsec) aperture. Observations requesting MDRS will unilaterally be moved to LWRS. (If this is unacceptable to all/part of your program, you need to contact us as soon as possible--see below.) Observations requesting HIRS aperture are being held until we receive instructions from affected observers. Again, the sooner we receive your inputs, the sooner your targets can be released for scheduling. 2) The minimum assumed "requested" observing time is being increased to 4 ksec. This is being done for OPERATIONAL reasons, and will not affect official program allocations. (No action on your part is required.) 3) Targets with expected count rates up to 2500 cts/sec will be observed in "time-tag" mode; only targets with expected rates above this value will use HISTogram mode. (No action on your part is required.) 4) Additional program reviews and updates will be required for Cycle 1 programs on a longer time scale. You will be contacted about these changes separately in the near future. Sincerely, William Blair, Chief of Mission Planning (on behalf of the FUSE Project at JHU) -------------------------------------------------------------------- CONTACTS: - All Guest Investigator issues should be handled through the normal channels, using the firstname.lastname@example.org e-mail account, which is monitored by GI Officer Dr. B-G Andersson. - All PI team Investigators should address questions to their JHU program coordinator.
Detailed Discussion of these changes: 1) Change of default observing aperture from MDRS to LWRS. In-orbit checkout (IOC) activities have demonstrated that the detector backgrounds are low and that the scattered light properties of the instrument are excellent. Airglow lines or contamination of spectral features are not a problem for the vast majority of FUSE science programs, the primary exception being programs needing access to the core of the Lyman-beta line region. Pointing stability is also excellent, at or above pre-mission expectations. All of these point toward the idea that the spectral resolution for point sources observed in the LWRS (30 arcsec) aperture will be indistinguishable from similar data obtained with the MDRS (4 arcsec) aperture. Furthermore, we have discovered various thermally-induced motions in the alignment of the four primary mirrors, both as we move from target to target and, at a smaller level on an orbital timescale. We have characterized the target-to-target changes well enough that careful planning and pro-active alignment changes can keep the four channels aligned for observations in the LWRS aperture. However, orbital variations, especially on the SiC channels, appear to be large enough that maintaining alignment in the MDRS aperture is problematic. Orbital tests are still addressing how best to use this mode, and make take considerable time (months?) to come to fruition. Thus, until further notice, we are making the LWRS aperture the default observing aperture. Any observations specified in Phase 2 for the MDRS aperture will be changed to use the LWRS aperture. (Any observations requesting LWRS will of course also be performed using the LWRS aperture.) If there are specific targets in your program(s) or whole programs that need to be exempted from this change, it is the responsibility of the program PI to inform FUSE Mission Planning. For observations requesting the HIRS (1.25 arcsec) aperture, the situation is more complicated, because the selection of this aperture could have been for any number of reasons. Therefore, observations requesting HIRS will not be scheduled until we hear from the affected program PIs. For each such observation, we will need to know whether a move to LWRS is acceptable, whether the target should be observed with MDRS (with the potential loss of short wavelength data), or whether HIRS is strictly required. (See related discussion under item (4) below.) 2) Minimum Observation Time Increase In the current planning environment, it is impossible for the FUSE Project to handle the increased fractional overhead caused by numerous short requested observation times. On the other hand, the project held a substantial overhead in reserve in case target acquisitions took longer than expected. Target acquisitions have become fairly routine, and do not for the most part take a substantial fraction (>50%) of an orbital viewing interval. This allows us to make the following operational change with relatively little impact: As of now, SCIENCE OBSERVATIONS REQUESTING LESS THAN 4 ksec WILL BE SCHEDULED AS IF 4 ksec WAS REQUESTED. This will be accomplished without undue impact to program allocations, with the extra time coming out of the "acquisition reserve." Note: for consistency, stated program allocations are NOT changing, but program accounting will take these changes into account. This change has several immediate and positive impacts, some on operations and some on the quality of the resulting science data. It is our desire to maximize the production of quality science data sets while at the same time easing the operational burden imposed by scheduling many short observations. This change is effective immediately. Exempted from this change are any "safety snap" observations, which are very short by definition, and selected other very bright targets that are close to the bright limit of the instrument. These determinations will be made by FUSE mission planning personnel. 3) TTAG vs. HIST Count Rate Boundary At Phase 2 submission, the count rate boundary between TTAG and HIST mode observations was stated to be 1200 cts/sec. Operationally, we have been using 1800 cts/sec, with exemptions made on a case-by-case basis. IOC activities have demonstrated that TTAG data are more readily correctable for spectral image motions. We therefore have increased the expected count rate threshold for which HIST observations will be invoked. All targets with predicted count rates below 2500 counts per second will automatically be performed in TTAG mode, while higher count rates will still require HIST mode. We are investigating whether on-board memory management and other operational concerns will allow us to invoke more HIST exposures per viewing interval to further decrease image motion affects on HIST data quality. These changes will occur with NO ACTION on your part. This new threshold is the now the default value. We report it here for your information. 4) Program Reviews and Revisions The Phase 2 submissions for your programs continue to be our primary source of information about your program's needs. However, there are any number of pre-mission assumptions about FUSE operations that are now known to be either incorrect or only partially correct. In addition, we have encountered a number of situations where users, when asked, have been willing to "loosen" their stated requirements in the face of on-orbit realities. This indicates to us that substantial review and revision of some programs may be necessary, a step that is not only costly in time but in human resources. One of the most difficult situations for the project to assess is your true spectral resolution requirements. Some people have stated Res=0.03 Ang (R=30,000) and they MEAN it, and others entered this value almost as a `default' for their S/N calculations. Some users specified the HIRES special requirement and/or the HIRS aperture specifically because the highest spectral resolution was required, and we are leaving such observation on "hold" for now. Sometimes the submitted text clarifies things, but many other cases are unclear. This is complicated by our current operational situation. We have demonstrated spectral resolutions of R=12,000 - 15,000 prior to even making a focus adjustment. In mid-December 1999 a focus adjustment was made and further focus adjustments are being carried out. However, assessing the changes and/or improvement have been difficult (as much because of difficulties in data reduction and manipulation as instrumental performance per se). We believe the current resolution being produced by the instrument is of order R=20,000, although in detail this is a function of wavelength and spectrograph channel (cf. FUSE Observers Guide, on the Web at http://fuse.pha.jhu.edu/support/guide/guide.html). This is the best we can say for now. Hence, we assume any requested observation for which Res=0.05 Ang or larger was specified in Phase 2 to now be under consideration for scheduling. In some cases, we have scheduled observations with stated Res<0.05 when it is clear (at least to us!) that this is allowable by the science. We suggest to all users that the time is right to perform an informal review of your submitted science programs to begin assessing your particular situation. We are not encouraging, nor are we prepared to handle, a full set of revised Phase 2 submissions for cycle 1. On the other hand, if you conclude that the current resolution is acceptable to your program but you have specified Res<0.05 Ang, you should contact us with specific requests to release the observation for scheduling. Since resolution and aperture choice are both variables in the equation of whether to schedule or not, we provide the following summary: requested aperture requested R change ----------- ------------- ------------------------------------------ HIRS >20000 observations to be be kept on HOLD (Res<0.05 A) awaiting advice from Program PI MDRS <20,000 change to LWRS; immediately available (Res>0.05 A) for scheduling MDRS >20,000 change to LWRS (if NOT desired, timely user (Res<0.05 A) input will change back); most observations on HOLD until focus improvements implemented or until user feedback received. LWRS any no change ------------------------------------------------------------------------ A some point in the near future, after the AAS meeting in January 2000, the project will institute a more systematic review of all programs, and systematic user inputs will be solicited. At that time, with improved instrument performance information available to users, it may be necessary to consider more dramatic revisions to Cycle 1 programs. This is (potentially) a big impact for the project, and we ask you to exercise good judgement in requesting changes. However, it is a task we take on in the interest of producing the highest quality science from FUSE in cycle 1.