Number 28, August 2003
Contents: 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 programs. 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 available. For further information please consult the following web sites: FUSE Cycle 5 program description and proposal instructions: http://fusegi.pha.jhu.edu Technical information is available from the FUSE mission web site: http://fuse.pha.jhu.edu General programmatic information for the ROSS-2003 NRA and its FUSE Cycle 5 component: 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: ftp://fuse.pha.jhu.edu/fuseftp/calfuse/ 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 then. 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.
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