GOF Status Report
-- by N. E. White, GSFC
This newsletter is intended to inform the ASCA community, both in the US and Japan, of the latest happenings with the guest observer program. It will be issued approximately every six months, or more frequently if pressing issues arise. The first 10 months of the ASCA mission have been an outstanding success. First results from the performance verification, PV, phase have delivered the promised results. Examples of ASCA spectra are shown in the ASCA Gallery. The instrumentation and spacecraft are performing well, with relatively minor problems.
Before launch, it was feared that radiation damage might cause degradation in the spectral resolution of the two CCD spectrometers. To date, no degradation in spectral resolution has been found. There is some concern about hot and flickering pixels in the CCD, which are gradually increasing with time. While these events can be removed easily in the analysis, they are taking up telemetry. The 4-CCD mode may not be usable in a few months (at least for the full energy band). This situation only affects observations of extended sources that fill the entire field of view. The performance of the gas scintillation proportional counters, GIS, and the telescope have been excellent. The instrument in-flight performance to date is described in this issue, and will be updated in future issues.
The performance verification phase is now complete and the AO1 observations of the guest observer program began on 1993 October 26. The satellite operations are being performed by staff and students from ISAS and Japanese Universities. These operations are an intense, time-consuming activity, and the procedures are described in this issue of ASCANews. Because of the burden on the Japanese scientists and students, we ask all guest observers to try to minimize complexities and sudden changes in their observation requirements.
In November, shortly after the start of the guest observer phase, one of the gyroscopes began to show degraded performance, which was causing difficulties with acquiring the target after a maneuver. On November 29, the attitude system was switched to use a backup gyro, and normal operations resumed. There is one more redundant gyro and reduced gyro-modes are possible. In spite of the interuption in the observing program, it will be possible to perform all priority 1 and 2 observations.
The first data were sent to US guest observers on 1993 December 17. These are FITS format data, converted from the original telemetry at the GOF. All the telemetry data are converted, with no loss of information (attitude solution and detector gain/linearization are added at this stage). As the data processing and distribution becomes routine, we anticipate the delivery of data to US guest observers about one month after the date of the observation. The analysis software is working well and the analysis procedures described in The abc Guide to ASCA Analysis, which can be obtained from the GOF.
This FITS-based analysis software currently is being used both in Japan and in the US by members of the ASCA PV team. Basic data cleaning and selection is performed using the XSELECT/FTOOLS package. This procedure produces cleaned event files, spectra, images, and lightcurves, which then can be used with XSPEC, XIMAGE, XRONOS, IRAF/PROS and/or IDL. There are still a few minor problems with the FITS conversions (e.g. keyword-naming issues), but these do not prevent data analysis. There are now preliminary response matrices available for all instruments, but it is clear that there will be revisions in coming months. The improved resolution given by the CCD detectors has highlighted every quirk and uncertanity in the calibrations. The instrument teams are working hard to further refine the response matrices. We advise guest observers to work closely with the GOF science staff to ensure that they fully understand the current limitations in the calibrations. Until the calibrations reach a more stable state, the 1-year clock for the transfer of the data to the archive will not be started.