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ASCA Guest Observer Facility

GOF Status Report

N. E. White


The NASA Research Announcement (NRA) for AO5 is now released, with proposals due by close of buisness 1996, August 6. Proposals will be considered from all astronomers located within the United States, ESA member states and Japanese proposers, who should apply to their respective national agencies. AO5 covers a 1 year interval starting 1996, December 15.

Observations of AO4 targets had been progressing well approximately along the long-term timeline until April 23 when a problem with one of the gyros developed. About 20 days were taken to understand the cause of the trouble, to replace the degraded skew-2 gyro and to test the newly reconfigurated attitutude control system. For this period observation time equivilent to about one week was lost. The long-term observation plan is being revised for the latter half of the AO4 period.

The gyro problem on April 23 caused an offset in the pointing of approximately one degree, which happened after large manoeuvers. After a few days study, this was found to be due to the degradation of the skew-2 gyro (a 0.5% increase of pulse weight that may be caused by a slowing-down of the rotation rate due to increased of friction). During the time between April 23 and May 7 ASCA was operated with a two step manual control of manoeuvers (i.e., coarse and fine manoeuver), which reduced observation efficiency to about a half. On May 7 - 10 the ASCA team performed an operation to replace the degraded skew-2 gyro to a spare Z gyro and tested the reconfigrated new attitude control system with a combination of X-, Y- and Z-gyros. After May 11 ASCA returned to regular operations, with observations follwing the orginal schedule. In parallel, the Skew-2 gyro is being tested. The current degradation of gyro is not fatal and there is evidence that its function has been gradually recovering. Currently, the Skew-2 can be used again if a failure happens in one of the current gyros (although probably with some reduced observation efficiency).

The orbit of the ASCA satellite has decayed about 3% from 527/613 km (perigee/apogee) at launch to 518/596 km. This traces quite a similar decay history as Ginga and assuming the same trend ASCA will stay in orbit beyond 2002. There are no consumables, so ISAS expects to continue operations of ASCA at least until the launch of Astro-E (currently planned for Feb/March 2000).

The current NASA approved funding for participation in the ASCA program ends in September 1997, and a senior review will be made this summer to determine the funding levels for 1997 and 1998, and recommended funding through 1999 and 2000. The ASCA GOF and US users group will prepare a proposal for the senior review and we hope to hear a successful outcome by the end of the summer. Part of the US GOF activities over the coming two years will be to create the final archive. In this we plan to reprocess all the data utilizing the best knowledge of the calibrations. We will also add higher level of data products that will include spectra, response matrices and lightcurves for each target and bright serendipitous sources. This reprocessing will, subject to the senior review, probably start early in 1997.

This issue of ASCAnews includes important articles on the calibration of the ASCA instrumentation, in particular the SIS. The radiation damage to the CCD spectrometers is now quite significant and must be taken into account during analysis. The recent FTOOLS release contains an updated response matrix generator, which can take into account the time dependent resolution of the SIS. This article also outlines how the SIS in-flight calibrations were made, and the current uncertainties. Other articles on the SIS include the use of grade 6 events and the effects of flickering pixels on the background.

It has been recently noticed that the gain correction of GIS3 is no longer accurate, and that a few percent deviation is evident in recent observations. The GOF expects to have a patch to correct for this effect and details will be provided to the ascanews mail exploder. Please check the ASCA Calibration Uncertainties WWW page for the latest information on this, and other calibration problems /docs/asca/cal_probs.html

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