Update #1

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

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.


William Blair, Chief of Mission Planning
(on behalf of the FUSE Project at JHU)


- 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:


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

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:

  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.