All RGS CCDs are operated in the so-called “frame store” mode, in which half of each CCD is exposed to the sky. The contents of this half are transferred to the second shielded half, which works as a storage area before readout, while the first half is acquiring the next frame. Thereby, the two-dimensional dispersed photon distribution is stored.
The standard science mode of operation of the RGS instrument is called “Spectroscopy”. It consists of a two-dimensional readout of one or more CCDs over the full energy range. Each RGS1 CCD is read via two nodes, resulting in an accumulation time of 4.8 s when reading the eight CCDs. Starting in August 2007, RGS2 CCDs are read via a single node. RGS2 frame times are therefore twice as long as RGS1 frame times (accumulation time is 9.6 s when reading the eight CCDs).
|Spectroscopy||2-D readout of up to all CCDs over the full energy range with 4.8 s accumulation time for RGS1, 9.6 s for RGS2, for eight CCDs. Each CCD readout takes 0.6 s in RGS1 and 1.2 s in RGS2. Diagnostic images are taken in parallel.|
|Small Window||Similar to Spectroscopy, but only the central of the CCD rows in the cross-dispersion direction is read. Each CCD readout takes 0.15 s in RGS1 and 0.30 s in RGS2.|
The on-chip binning (OCB) factor in this mode is 3x3 pixels, after which other operations are performed on board to reduce the data rates to within the RGS telemetry bandwidth. After applying a low signal threshold, hot columns and hot pixels defined by a look-up table are rejected. The remaining pixels are treated by a programmable processor (DPP). Pixels exceeding an upper signal threshold and pixels with complex structures due to cosmic rays are rejected, and only events which fit in a pattern are transferred to the ground including information on their shape.
Each individual chip or any combination of chips can be read out. The energy range covered depends on which CCDs are read out and on the positioning of the source within the field of view. A rough estimate of which energies are sampled if a given CCD is read out is provided in Table 9.
In parallel to the recording of science data, diagnostic images are transferred to the ground. One complete CCD of each RGS is read in this way every 1500 s. The on-board data processing is bypassed and the entire CCD image or “Q-dump” is transferred to ground. These data are used by the instrument and calibration teams for dark current and system noise level verification with the aim to identify possible instrument degradation with time and/or changes affecting the health of the RGS instruments. These data are now routinely used to improve the calibration of the CCD offsets.
Telemetry bandwidth limitations can be an issue in observations of very bright sources. In the cases in which the estimated count rate is so high that this problem is expected, the telemetry requirements can be reduced by disabling the download of the diagnostic frames. This is the so-called High Count Rate (HCR) mode. The few cases that would require the use of this mode will be identified and evaluated by the SOC during the proposal enhancement process.
The Spectroscopy mode has been used for almost all observations and is the recommended configuration. For the few 20 brightest X-ray sources, the Small Window mode (§ 18.104.22.168) may be considered.
Some basic characteristics of the RGS science modes are listed in Table 12.