The CTI correction applies a scalar correction to the event list set, on an event by event basis, in order to correct for loss of charge which has occurred in reading out the CCD array.
Note that the task just requires the PHA values be corrected. There is no flag or other data carried forward to the RMF-generating routine for example, to account for a ``noise'' factor in the correction process.
We expect to correct for a fixed amount of charge loss, and a fractional amount of charge loss, which depends on the number of row and column transfers completed. The early analysis of ground calibration data shows some possible detailed effects that need to be accounted for. For example:
Note that the intended use of the correction occurs in the pipeline at , so that the rawX,rawY attributes are related to the co-ordinates range (1,600), whereas if the same routine were to be used to correct diagnostic data, then a conversion is needed. This would be invoked on looking for the PUT_XY flag setting. At the moment the latter case is hardly conceived as needing some CTI correction therefore ignored and assume a future SEPARATE CAL routine might be needed. Use in also implies we can only accurately correct a modification for a background light contamination has occurred, and also that in principle we should correct ALL E components of an event, and not just the central pixel value.
Additionally we note that in principle CTI may change with mode due to the differences in clock timings employed, so that a mode dependence in the CAL state and the CCF files must be carried (TBC from ground data).
Note in the current context of applying the correction to a whole array there is not a concept of handling a frame-dependent count rate dependence to modify the CTE values. The best method for doing this should be established. For example should actually ingest a frame file, and calculate from that some count rate estimate - not easy if the dependence is really column dependent.
A detailed in-flight trend-analysis of the MOS Charge Transfer Inefficiency (CTI), using the two main emission lines (Mn and Al) from the internal calibration sources, spanning three years of operations has shown that :
(1) at a given energy the energy loss per transfer due to CTI (Charge Transfer Inefficiency) is a linear function of time, in time intervals between major discontinuities due to solar flares or change of CCD operating temperature.
(2) the energy losses per transfer scale as a power law function of energy, by comparing the time evolution (degradation) of the CTI at Mn (6keV) and Al (1.5 keV) energies.
Assuming that the energy losses at other energies can be extrapolated from these two energies (Al and Mn emission lines from the internal calibration source), the MOS CTI has been successfully modeled and coded as follows in the CAL CtiCorrector.cc with ALGOID = 2 (in the latest set of CCFs) for a pixel at CCD coordinates (RawX,RawY) :
The elapsed time since launch is computed from the date indicated in the FITS header keyword REF_DATE of the first binary extension.
The plots of MOS parallel and serial CTI trends since
launch for all CCDs at Mn and Al energies are available on
the internal web at: