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 on 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 about 4.8 sec (0.6 sec
per CCD). Starting in August 2007, RGS2 CCDs are read via a single
node. RGS2 frame times are therefore twice as long as RGS1 time frames.
Readout time for RGS2 is 9.6 sec, i.e. 1.2 sec per CCD.
| Mode | Description |
| Spectroscopy | 2-D readout of up to all CCDs over the full energy range with a long duty cycle (4.8 sec readout time for RGS1, 9.6 sec for RGS2). Each CCD readout takes 0.6 sec in RGS1 and 1.2 sec in RGS2. 3x3 pixel On Chip Binning is applied. Some events are rejected on-board. Diagnostic images are taken in parallel. |
| Spectroscopy HCR | Similar to Spectroscopy, but no diagnostic images are taken.
For high count rates ( 70 cts s ). |
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.
For extremely bright sources, non-standard read out schemes can be employed, such as reading only a limited set of CCDs to reduce pile-up or using a read out sequence like CCD 1, 3, 2, 3, 4, 3 etc. for some special timing purposes.
In general, most RGS sources will not suffer from pile-up. However,
when photon rates approach 0.04 (0.02) photons/cm
/s in RGS1
(RGS2) around the wavelength of maximum effective area (15 Å),
effects of potential pile-up should be considered. The effect of
pile-up will be more severe in RGS2 due to the longer frame time.
Pile-up may result in first order events ending up in second order. On locations on the CCD where multiple-pixel events are more likely, pile-up will have a higher chance to result in ``complicated patterns'' which will be discarded by the on-board processing. An important diagnostics tool to asses pile-up is the comparison between fluxed first and second order spectra. The effective area calibration ensures that the fluxed first and second order spectra of non-piled-up sources will be identical. Hence, looking at the ratio between the first and the second order can be a diagnostics tool to determine whether pile-up plays a role.
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 1500s. 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, e.g. in the case of visual light background.
This mode has been used for almost all observations and is the
recommended configuration. However, for extraordinarily bright
celestial sources with an expected RGS count rate larger than 70
counts s, the Spectroscopy HCR (High Count Rate) mode should be
used. This mode is the same as the Spectroscopy mode but without
diagnostic frames being taken in parallel to reduce the telemetry
rate.
Some basic characteristics of the RGS science modes are listed in Table 11.