Visible stray light and bright stars in the RGS avoidance fields can alter the zero energy level (CCD electronics offset or pedestal) measure by the RGS CCDs. This effect, if not properly taken into account, can affect the energy calibration of X-ray events.
It is ensured procedurally during the operations of the instrument that DIAGNOSTIC mode data of all CCD's are available at the start and at the end of each observation. Furthermore, if enough telemetry bandwidth is available, DIAGNOSTIC mode data are also transmitted interleaved with science data (this type of DIAGNOSTIC data is called Q-memory). Q-memory data are not guaranteed to be present for all CCDs in all observations, due to the uncertainties in the predicted event rates (hence in the telemetry requirements).
DIAGNOSTIC and Q-memory data can be used to determine the offset and noise levels of the CCD electronic chains at the actual telescope pointing. Q-memory data could be used to devise a time dependent correction of the offset levels by interpolation. This will only be implemented at a later time, if in-orbit experience warrants it.
rgsoffsetcalc accepts a list of DIAGNOSTIC datasets (refer to the ODF ICD . For each DIAGNOSTIC dataset rgsoffsetcalc calculates the mean and standard deviation of the pulse height distribution. This calculation is performed by CCD node. The mean and standard deviation are calculated by fitting a Gaussian to the pulse height distribution.
Before being processed each diagnostic dataset is examined to determine if it is likely to yield a good fit. The dataset is ignored if any of the following conditions is false:
rgsoffsetcalc stores the results of the computation in the dataset specified by outset. The output dataset can be used as a CCF constituent, and this is usually how rgsevents will use it. Because the output dataset is a valid CCF constituent, it can only contain data relative to one CCF scope . If the list of input datasets contains more that one CCF scope (this is identified through the attribute INSTRUME), rgsoffsetcalc will process only the datasets belonging to the first CCF scope identified (this is the scope of the first dataset in the input list). A warning is issued.
rgsoffsetcalc can also operate on a list of datasets that do not conform to the ODF ICD. The only requirement is that the arrays be of type integer16. In this case the output dataset is not formatted as a valid CCF constituent. In the output table, null values are used to indicate information that could not be obtabined from the input datasets.
Optionally, rgsoffsetcalc can store the pulse height distributions for each input dataset and node in a dataset (see the description of withhistogramset and histogramset).