3 ASCA Performance Update

This chapter summarizes the changes from the nominal, pre-launch performance of the satellite and the instruments as determined from PV and AO data. More detailed information is given in the relevant instrument chapters.

3.1 XRT Performance

So far there has been no detectable change in XRT response (although there have been improvements in models of the actual response). The PSF and effective area, on and off axis, are essentially as described in the first Technical Description.

3.2 GIS Performance

As far as we can tell, the GIS has been performing as expected, in general. The energy resolution, position determination, background and useable field of view are all very close to the values predicted from ground calibrations. Inflight observations have, however, emphasized the need to avoid placing point-source targets beneath the wires of the grid which supports the window. This is analogous to the gaps between the SIS CCDs, and both effects are taken care of in the same way, i.e., by offsetting the pointing position. Details of the in-flight performance of the GIS are given in Chapter 6; their impact on the feasibility of observations is discussed in Chapter 8.

3.3 SIS Performance

Details of the in-flight performance of the SIS are given in Chapter 7. Their impact on the feasibility of observations is discussed in Chapter 9. Here, we highlight those aspects which have changed since AO-5.

3.3.1 CTI, RDD, & spectral resolution

CTI, charge transfer inefficiency, of the CCDs have been found to be highly non-uniform. This causes a significant degradation of the spectral resolution. The FWHM resolution at 6.5 keV will have increased from  120 eV at launch to  320 eV by the end of the AO-6 period, in 1-CCD mode. Moreover, in 2-CCD mode, the RDD (residual dark distribution) effect will have caused additional degradation of spectral response. Even though we now provide a tool to correct the RDD effect, the result is not well calibrated yet in Faint mode and not expected to restore the resolution fully for Bright mode data.

Due to RDD as well as hot/flickering pixel effects (see below), 4-CCD mode is no longer recommended for general use (see S9.3).

3.3.2 Hot and flickering pixels

The rate of hot and flickering pixels continues to increase in 2-CCD mode, although it remains relatively constant in 1-CCD mode. Hot and flickering pixel occupy telemetry slots and can lead to telemetry saturation in certain mode/bit rate combinations. We have estimated count rate limits for an X-ray source in various mode/bit rate combinations.

Processing on the ground can eliminate all the hot pixels and the more active flickering pixels from the data. However, low duty cycle hot pixels cannot be eliminated, and this appears as an increased low-energy background: this is clocking mode dependent, and shows a secular increase.

3.3.3 Background

The SIS background remains at the level expected before launch, except for the flickering pixel component described above.

3.4 Satellite Operations Since the PV Phase

It is no longer possible to use 4-CCD mode and retain the low-energy response. If the full 22x22 arcmin SIS field of view down to keV is required, 2-CCD mode with double the exposure time should be used.