Number 20 June 20


1) FUSE 3-Year Launch Anniversary on June 24!
2) FUSE Sky Soverage, Present and Future
3) Observing Efficiency High.
4) A-axis controller 
5) Additional Observatory Programs Selected
6) CalFUSE News 
7) Cycle 4 Update
8) FUSE Operations Presentations Given at the AAS, Albuquerque Meeting

1) FUSE 3-Year Launch Anniversary on June 24!

        On June 24, we will celebrate the third anniversary of the FUSE launch.
As the events of the last six months have shown, on-orbit operations is not a
guaranteed thing.  We take this opportunity to recognize our dedicated 
operations staff as well as all of the scientists and engineers responsible
for the recovery of science operations after the reaction wheel anomaly.  
Here's to many more years of scientific productivity with FUSE!

2) FUSE Sky Coverage, Present and Future

        As we learn more about how to use the new attitude control system, 
implement new software, and optimize parameters, we are regaining more and more
of the sky as accessible to FUSE observations.  Presently, about 75% of the
sky is available over the course of a year.  This includes all points above
absolute declination ~40 degrees, as well as substantial parts of the sky at 
low declinations.  We have since April successfully observed several low 
declination targets, including sources at 8 degrees declination.

        Due to overlapping restrictions cause by lack of magnetic torque 
authority, low beta angle avoidance, and RAM zone avoidance, a region of about
1 steradian at low declination around RA=12h remains unavailable at present.  
We are working on several ways to improve the sky coverage further and do 
expect to regain 100% availability at some time in the future.  The main 
modifications we foresee that will allow us to further increase the sky 
availability are:

- Smaller RAM Avoidance Zone
By 2004, three years past Solar Max., the atmospheric density at FUSE's 
altitude is expected to have dropped by a factor ~100, allowing a significant 
reduction - or elimination - of the RAM avoidance zone.  This alone will 
improve sky coverage to nearly 100%.

- Improved Momentum Unloading Algorithms
The Magnetic Torquer Bars now provide both attitude control along 
one axis and momentum unloading for the remaining reaction wheels.  Improved 
algorithms for this dual role are expected to increase the instantaneous sky 
coverage by up to a factor 2.

- Spacecraft Roll Offsets
By offsetting the spacecraft roll by up to ~15 degrees, larger windows of 
positive torque authority will become available.

- Use of Partially Stable Orbits
For orbits where the only loss of fine pointing control occurs during target 
occultation by the Earth, normal observations may still be possible and will 
further extend the sky availability.

We still encourage FUSE users to give priority to high absolute declination 
targets, since low declination observations will remain a more limited resource 
and require more manual intervention in planning.  Prospective cycle 4 
proposers should note that we will post an update to the mission capabilities
(as described in appendix A of the NRA), as an NRA amendment no later than 30 
days prior to the proposal submission deadline (see item 7 below). 

3) Observing Efficiency is High.
        Even as the project has been working to recover full sky coverage, we 
have encouraged observers to consider target changes to higher absolute 
declinations.  This is partly because operations are more straightforward 
closer to the orbit poles.  However, a hidden bonus is that targets tend to 
have longer visibility intervals and shorter occultations, making observing 
more efficient.  This effect can be clearly seen in recent science observing 
efficiency numbers.

Over the first two cycles of science operations, the FUSE project achieved an 
average of 28% science efficiency. (This is on-target observing time against 
the wall clock.)  This is an excellent number for a low-earth orbiting mission, 
because the earth typically blocks targets from view more than half the time.  
Since operations resumed after recovery from the recent reaction wheel 
problems, our efficiency has shot up to 35% over the last 4 months!  Over 300 
unique targets have been observed during this time, for a total of some 3.2 
million seconds of science data.  This is roughly 640,000 seconds more science
data than would have been achieved over the same period at 28% efficiency!  
This indicates that higher declination observations have a benefit above and 
beyond making operations easier.

4) Pointing Drift due to Magnetic-Axis Controller 

        FUSE pointing is now controlled by reaction wheels on two axes
and magnetic torquer bars on the third.  Pointing performance for the 
axes controlled by reaction wheels is unchanged at 0.3 arcsec RMS.  
However, magnetic axis performance is somewhat different in two regards.
a) The high-frequency component (jitter) is roughly twice as large 
(about 0.6 arcsec RMS).  This has a negligible effect on the instrument 
spectral resolution.  b) There is also a larger, low-frequency component 
of motion with a period of half an orbit.  The magnitude of this component
varies with the angle of the line-of-sight from the orbit pole, from about 
1 arcsecond near the poles to a worst case of about 6 arcseconds (12 arcsec 
peak-to-peak) at pole angles near 45 degrees. This motion occurs in a direction
45 degrees to the dispersion direction, and can in principle affect the
derived spectral resolution slightly.  The effect is correctable for LWRS 
time-tag data and should be small enough to have little impact on LWRS
histograms, which are kept short anyway because of thermal effects.

        We expect to be able to greatly reduce this effect in coming months
by tuning parameters in the attitude control system.  Also, the CalFUSE
pipeline is being taught to remove the effect on the data whenever it can.
However, users of MDRS and HIRS apertures may experience some impacts from
these additional motions and should check their data carefully.  If you have 
any questions, please contact

5) Additional Observatory Programs Selected

        As we discussed in the last FUSE Newsletter (#19), we have instituted a 
class of "Observatory Programs" (OP) to help maintain high scheduling 
efficiency of scientific observations.  In response to the call for further 
such survey class projects, we received several good suggestions.  Based on 
the submitted suggestions the Project Scientist have defined 5 further OPs.  
These are:

Z907  AGN/IGM extragalactic targets
Z908  Vela SNR 
Z909  Blue compact dwarf galaxies
Z910  Cataclysmic variables
Z911  Planetary nebulae
Z912  Completing the FUSE OB star atlas

        These programs contain approximately 50, 4, 5, 50, 60  & 24 targets, 
respectively.  We are currently in the process of ingesting these programs into 
the mission planning system.  A list of targets as well as a short abstract for
each OP can be found at:

        Note that all observations of OP targets go public immediately upon 
archiving and that you can track the performed observations either using the 
MAST web site at STScI, or by consulting our list at the above URL.
6) CalFUSE News 

- New Background Calibration Files

        A new set of background calibration files designed for observations
obtained since 19 Feb 2002, when the FUSE detector high voltage was last 
raised, are now available on the FUSE FTP site 
( ).  The data are stored as a
gzipped tar file, bkgd2002.tar.gz.  The file contains four calibration files, 
bkgd**, which should be placed in the directory calfuse/v2.1/calfiles, 
and a new master_calib_file.dat, which should be moved to 

        In addition, the file contains a new version of cf_make_ttag_bkgd.c,
which corrects a bug that caused the program to crash when RUN_MKBK and
RUN_BKGD are set to 'NO' in the parm*.fit files.  The file should be placed in 
the directory calfuse/v2.1/src/fuv.  To install the file, type "make install".

        NOTE: The new background files for segments 1A, 1B, and 2B work well.
The file for segment 2A is less satisfactory because the high voltage was 
changed several times between February and mid-April.  After mid-April, the 
voltage of segment 2A was returned to the pre-February value.  In our tests, 
neither the old (version 008) nor the new segment 2A background file gives an 
exceptional fit to data obtained after mid-April, but the new one does a 
slightly better job, and we recommend its use.  We plan to refine this file as 
we obtain more data.

        For more information on the change in detector voltage with time, see 
the FUSE white paper "Time-Dependent FUSE Calibration Effects" at .

- Time-Dependent Flux Calibration

        Repeated observations of standard white-dwarf stars indicate that the
sensitivity of FUSE remained remarkably stable over the first two years of the 
mission. Since the middle of 2001, however, we have noticed a slow degradation 
in the effective area of the FUSE spectrograph. The decline is more or less 
wavelength independent and affects all channels at roughly the 10% level 
(though it may be higher for LiF 2A). A time-dependent version of the flux 
calibration that takes into account this degradation is now in development.  
We will provide more information as it becomes available.

- New FUSE Data to Be Processed with CalFUSE v2.1

        Beginning in early July, we will reduce all new FUSE observations with
CalFUSE version 2.1.6.  The pipeline will include all software updates made 
available since the general release of v2.1 as well as a jitter-correction 
module to counter the effects of spacecraft motion during an exposure.  Users 
should see a dramatic improvement in the quality of FUSE data obtained from 
MAST once the new software is in place.  Reprocessing of the entire FUSE 
dataset with CalFUSE v2.1.7 will begin in early autumn.
7) Cycle 4 Update

        The FUSE cycle 4 NRA is in the final approval process at NASA HQ.  The
preliminary cycle 4 schedule looks as follows:
Cycle 4 NRA Release:        mid-July 2002
Cycle 4 Proposal Deadline:  early October 2002 
Cycle 4 Proposal Selection: February 2003 
Cycle 4 Observations Begin: April 2003
The nominal Cycle 4 period is from 1 April 2003 through 31 March 2004. 

8) FUSE Operations Presentations Given at the AAS, Albuquerque Meeting

        Several members of the FUSE Mission Operations team gave talks at the 
recent AAS meeting in Albuquerque on the recovery of the mission after the
reaction wheel anomalies.  For those of you who could not attend the meeting,
we have posted these presentations on the FUSE/JHU web site:

The Observer's Electronic Newsletter is published by the FUSE project and is 
aimed at the FUSE user community.

Editor: B-G Andersson, FUSE Guest Investigator Officer.

The FUSE Project is managed by Johns Hopkins University's Center for 
Astrophysical Sciences in Baltimore, MD, for NASA's Goddard Space Flight 
Center.  The FUSE Principal Investigator is Dr. Warren Moos, the FUSE Project 
Manager at JHU is Mr. J.B. Joyce, and the NASA Project Scientist for FUSE 
is Dr. George Sonneborn.

Further information about the FUSE Guest Investigator Program can  be
obtained from:  Dr. George Sonneborn;

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