To: FUSE User Community
From: Bill Blair, Chief of Observatory Operations
Re: Reassessment of Unobserved FUSE Targets and Target Pool Enhancement
Date: Mar. 4, 2002

Summary:
The FUSE satellite is back on-line and performing science observations after the recent downtime due to reaction wheel failures. The "new FUSE" maintains the same scientific capabilities as before in terms of sensitivity and spectral resolution. However, there are aspects of operations that have become much more complicated and in some aspects limiting. For all users with unobserved targets awaiting execution, we are requesting that you reassess your target selection and consider modifications that may be both in your best interest and the best interest of the project.

Background:
Briefly, we have modified our attitude control system to operate using the two operational reaction wheels and a third axis of motion that is controlled by magnetic torquer bars (MTBs). The MTBs are cycled so that we can "push" and "pull" against the earth's magnetic field to reorient and steady the satellite. In stable regions, the pointing quality has been demonstrated at sub-arcsecond levels, which is excellent. The MTBs are not as strong as the reaction wheels, however, and so we have one "weak" axis which is not strong enough at all attitudes to hold the satellite steady. "Steadiness" is a complex function of time and boresight pointing direction, the former because the magnetic field varies substantially around the orbit and the latter because it affects the orientation of the MTBs with respect to the field and the "weak" axis direction they must try to control.

Since operations were recovered on January 28, 2002, we have been performing a mix of science observations and tests to help us better understand how to operate the new FUSE. The primary disturbances felt by the satellite are due to gravity gradient torques, which are minimized near the orbit poles. Tests to date have validated pointing stability within a region about 25 degrees from the orbit pole. The FUSE orbit is inclined by 25 degrees from the celestial equator, and the orbit precesses in about 60 days. This means that presently we can access targets above declination 40 degrees (or below -40 degrees) at one time or another during the 60-day precession cycle. However, even within this region, time-critical (or otherwise constrained) observations will be more limited than they were previously.

For targets within 40 degrees of the celestial equator, all hope is not lost, but we are still learning. Our current models predict that there will be times (but perhaps very limited times) when operations at lower declinations will be stable. We will be working over the next several months to understand this situation more thoroughly and perform the necessary tests to extend operations to lower declination targets. However, even if observations are possible in this region, they will always be limited either in terms of the amount of time we can observe (or the efficiency if you prefer) or pointing jitter may make these data more difficult to analyze and interpret. The FUSE project is looking at ways to incorporate pointing information into the data reduction process for time-tag data to correct for any significant pointing jitter.

Target Changes:
The bottom line is this: we anticipate that operations and pointing stability will henceforth be better for observations of targets at declinations above |40| degrees. In addition, targets in the range |50| < dec < |80| are in or near the continuous viewing zone (CVZ) at one point or another and become very efficient. To the extent that we can maximize the availability of targets at higher declinations, timelines will be more efficient and expected data quality will be higher. It is in your interest and ours to enhance the target pool at higher declinations and reduce the pool of targets at lower declinations.

Toward that end, we ask the user community to assess two things:

1) Evaluate your programs for possible target changes, reducing the number of low declination targets and increasing the pool of higher declination targets. 2) Consider submitting high declination "Observatory Program" targets to enhance the target pool above |dec| = 40 degrees.

These two aspects are described in detail below.

1) Evaluate your programs for possible target changes.

For any unobserved targets in your programs that are at |dec| < 40 degrees, consider whether there are other targets at significantly higher declinations that would work for your program science. Note, however, that we are looking for significant changes here. Moving from a target at 38 degrees declination to one at 42 degrees is not worth the trouble, because the dec=40 line is not a hard limit. However, swapping a dec=38 target for one at dec=55 should be considered!

At the present time, we are looking for target changes that stay within your assigned program time allocations. Target-for-target swaps, 2-for-1 swaps (two short observations out for one longer observation coming in), etc., are what we are looking for right now. Proprietary rights for these targets will be maintained as if they were part of your original Phase 2 submission. Options involving a change in program observing time allocation must be cleared directly with NASA Project Scientist George Sonneborn (george.sonneborn@gsfc.nasa.gov), and will be considered on a case-by-case basis.

The mechanism for executing these changes is as follows:

a) Check the list of pending observations for targets in your program or programs. A list of all pending observations is available on-line at this link:

Lists sorted by program ID and by increasing RA are both available.

b) Assess potential target changes to higher declination (|dec|>40 degrees). When new potential targets are identified, check the on-line lists of previously approved FUSE targets, also available at the above link, to avoid duplications. (Again, lists of previously observed targets, ordered on program ID and RA are available at the above link.)

c) Contact fuse_support@pha.jhu.edu with a brief description of the desired changes.

    ----------------------------------------------------------------
   |   It is IMPERATIVE that you include your four-digit program    |
   |     identifier in the subject line of any messages to the      |
   |                    fuse_support account!!!                     |
   |                (One message per program please.)               |
    ----------------------------------------------------------------

FUSE User Support will determine the best method to process your changes. The submission method will depend on the extent of the desired changes and whether your program has been partially executed or not.

In general, if observations have not yet been obtained for your program (most Cycle 3 programs), you will be requested to modify and resubmit your entire Phase 2 program. (A current copy will be provided to you.) Any safety/feasibility issues for new targets can be addressed in the text blocks, and the "p2test" and "p2submit" method of proposal processing can be utilized, as with any Phase 2 submission. After verification, your new Phase 2 inputs will supercede your earlier submission.

If your program is partially complete and the desired changes are not extensive, it will likely be easier to handle your cahnges by e-mail submission of the changes only. As part of this process, yopu will be asked to provide a brief paragraph of explanation for the changes you are making and address any feasibility/safety issues for any new targets. CLEARLY IDENTIFY the target that is to be removed. Then provide Phase 2 format target/observation keywords. for each new target at higher declination.

Responses to this opportunity are needed as soon as they are available, but no later than Mar. 20, 2002.

We want to highlight that YOU DO NOT HAVE TO CHANGE low declination targets at this time, pending our on-going testing of operations closer to the orbit plane. But operations near the plane will be a limited resource at best, and in the interest of getting your data sooner rather than later, we encourage users to assess their programs and decide for themselves. At a later time, when more information is available, we will reassess this situation and contact you again with more information.

2) Consider submitting high declination "Observatory Program" targets to enhance the target pool.

Operating at higher declinations has two competing aspects: higher scheduling efficiencies and fewer potential targets in the existing pool! To address this issue, we are encouraging the submission of a new class of target.

Observatory program targets should be any target above an absolute declination of 40 degrees that is not currently in an approved observing program and for which a FUSE observation will provide useful information to the general community. These targets will be of lower priority for scheduling and any data obtained on such targets will become public immediately upon archiving. Observations of these targets are not guaranteed, as they will be used at the discretion of timeline schedulers to make the best use of observing capabilities as Mission Planning Schedules are created. Any such targets submitted as part of this category will not be excluded from consideration in Cycle 4 requests.

Since the idea is to increase the pool of targets substantially, we are particularly interested in "survey" programs, where reasonable (less than 20 ksec) observations of a number of potential targets in a particular category are requested. These could be viable data sets in and of themselves or, for fainter targets, they could be considered "snapshots" to assess whether a longer observation in Cycle 4 would be warranted. Individual targets or small numbers of targets will also be considered.

The current listing of Observatory program targets is available at:

Lists sorted by program ID and by increasing RA are both available. These lists will be updated regularly as new submissions are added.

The mechanism for submitting this class of target is being streamlined. Since we are already operating, the sooner we increase this pool of targets the better. If you have interest or ideas about target categories to include in this program, you are encouraged to contact B-G Andersson at fuse_support@pha.jhu.edu for the current status of this effort.

Epilogue:
We stress that our primary interest is in doing the best science we possibly can with the FUSE satellite. We appreciate any efforts the community as a whole can make toward this goal, and we thank you for your efforts and your support.