Number 1 September 1999


1) FUSE Cycle 5 Proposal Deadline is September 19
2) FUSE Sensitivity Update
3) Next generation CalFUSE Calibration Files Available
4) New Attitude Control Software Proves Itself as Gyro Fails
5) Some Notes on Event-Bursts and Their Impact

1) FUSE Cycle 5 Proposal Deadline is September 19

        Just a reminder that the proposal deadline for FUSE cycle 5 proposals is
coming up on September 19.  Approximately 6000 kilo seconds of on-target 
exposure time will be allocated to Standard and Legacy programs in Cycle 5. An 
additional 2400 ksec is expected to be allocated to Survey and Supplementary 

        Starting in Cycle 5 a new proposal category, "Legacy Programs", is 
available.  This proposal category provides the opportunity for large, coherent 
projects of general and lasting importance to a wide astrophysical audience.  
As in cycle 4, the "standard" and "survey" proposal categories are also 

For further information please consult the following  web sites:

FUSE Cycle 5 program description and proposal instructions:

Technical information is available from the FUSE mission web site:

General programmatic information for the ROSS-2003 NRA and its FUSE Cycle 5 

2) FUSE Sensitivity Update

        The FUSE calibration team has recently produced two new sets of 
flux-calibration files.  Dated January and April of 2003, respectively, the 
files are applicable to data taken since the fall of 2002.  Combined with 
earlier observations, these files show that, while the sensitivity of FUSE was 
roughly constant for the first two years of the mission, it is now declining at
a rate of some 5 to 10% per year, depending on the channel.

        We have installed the April 2003 flux-calibration files in the on-line
FUSE Exposure Time Calculator (Version 5.01) and Count Rate Tool (Version 1.91).
Be sure to use these versions (dated 2003 Aug 08) when estimating count rates 
and S/N ratios for your Cycle 5 proposals.

For more details, please see the FUSE flux-calibration White Paper at

3) Next generation CalFUSE Calibration Files Available

        We are pleased to release a new set of calibration files for use by 
CalFUSE, the FUSE calibration software pipeline.  The new inverse-sensitivity, 
scattered-light (background), and high-voltage calibration file will be 
available available by September 2, from the FUSE FTP site:

See the file README.NEW_CAL for details.

4) New Attitude Control Software Proves Itself as Gyro Fails

        The X-axis ("yaw") gyro channel in our backup Inertial Reference Unit 
(IRU-B) failed at 07:30 UT on July 31st, 2003.  The new software installed in
mid-April switched autonomously to using the "gyroless attitude estimator" for 
the X axis and the observation continued with no interruption.  FUSE has 
continued to operate very successfully with two functioning gyro axes since 

        Since that time, some observations have been adversely impacted by 
acquisition problems.  However, we are refining the acquisition procedures 
to handle the various problem cases as we encounter them so this situation 
is improving rapidly.  It should be noted that once a target has been
acquired the pointing stability is better without the gyros than with them!

        We plan to continue operating with the two gyro channels in IRU-B for 
the indefinite future.  If an additional channel fails in IRU-B we may switch
back to operating with IRU-A (which still has two functioning gyro channels).
The system has been demonstrated to work with no gyros at all, but operations
are more efficient with even limited gyro data, so we will make use of only a 
single IRU at a time to maximize the total available lifetime.  The success of 
the new control software bodes well for the long-term health of the mission.

5) Some Notes on Event-Bursts and Their Impact

        There have been an number of questions about event bursts recently, so
we provide a quick review of what is known about them and what effect they have
on the data. We're working on a white paper which will provide more details for
those who are interested; it should be available on the FUSE web site within a
few weeks.

        Event bursts are sudden increases in the count rate which are seen on 
all detector segments. They can last anywhere from a few seconds to many 
minutes, and peak count rates can be thousands of counts per second. The 
pattern on the detector varies, but often shows a checkerboard-like image of 
the grid wires. The pulse height distribution suggests that the source is 
charged particles rather than photons.

        We don't know what causes the bursts, but we do know something about 
their properties. They are more likely to occur soon after orbital sunrise, and 
they often repeat every orbit. Since a typical observation contains multiple 
exposures which occur at the same time in the orbit, if one exposure of an 
observation contains a burst, it is likely that some of the others do, too. 
A slew to a different target often makes them go away, so we believe they are 
somehow related to the geometry of the observation.

        If TTAG data contains bursts, the CalFUSE pipeline should screen them 
out.  However, if a particularly large burst occurs, the detector will drop to 
its safe "SAA" voltage level in order to protect itself. At the moment, it 
takes a command from the ground to return the high voltage to its nominal 
value. Now that the extended mission phase has begun and the Spacecraft 
Control Center (SCC) is not staffed around the clock, there is the chance that 
a significant time could elapse before the high voltage is restored. Thus, 
onboard scripts have been written to allow the Instrument Data System to bring 
the voltage back without ground intervention. That new code, which will limit 
the down time from a large burst to 20 minutes, should be up and running within 
the next few weeks.

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. Randy Ewing, 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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