OFFSET and GAIN are needed to calculating the energy of each detected RGS event. Early in the mission, a single OFFSET value was used in the SAS for each CCD node before the introduction of dynamic values calculated for individual pixels from diagnostic data averaged over three XMM revolutions and supplied with the ODF. The structure of one of these files, whose names follow the scheme ${rev}_${ObsID}_R[12]X00000OFX.FIT, is shown below.
The static values stored in this CCF are kept in reserve. The table OFFSET_GAIN contains individual values of gain and offset for each CCD readout node. Although it has not yet proved necessary, OFFSET values are formally parameterized as a linear function of observation time with the relevant parameters stored in the columns OFFSET_CONST and OFFSET_SLOPE to be used in conjunction with the value of the keyword REFTIME.
The columns REJTHRESH and
ACCTHRESH hold the thresholds in units
of PI that are applied during event reconstruction in rgsevents.
These thresholds are scaled for event
by using the
values of the keywords QSPLITR2/3/4, which are empirically
determined charge-splitting ratios for grades between two and four.
The table ADUCOEFF is for QLA use only where no CTI corrections are performed.
The CCF data are compensated for this. The
formula used is
