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 fsupport@pha.jhu.edu 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.