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epexposure (epexposure-0.16) [xmmsas_20211130_0941-20.0.0]


There is a need to calculate the fractional exposure FRACEXP column entries in the CCD specific exposure extensions (see [2] and SSC_LUX_TN_0038) per frame which will be addressed by higher level SAS tasks for the computation of exposure times or exposure maps. In addition one needs to create GTIs for timing analysis, e.g. light-curves or for determination of absolute quantities like fluxes.

The plan is to use the counter and discarded line info stored within the EPIC pn Auxiliary (AUX) File ([1], p. 50) to check if and how many events have been lost during the on-board processing and telemetry or have been flagged as bad in epframes.

A problem arises when comparing these counters with the number of events in the actual CCD specific events list: counters are given every $m$ CYCLEs and they come per quadrant, i.e. not per CCD. Here $m$ gives the number of cycles which have been integrated prior to the creation of the counter information. It is a programmable parameter defaulted to 20 to reduce the housekeeping telemetry and needs to be extracted from the periodic housekeeping data (PMH) (parameter name is An_MAXFRC, for quadrant $n=0..3$, see Table 1). For a description of all HK-parameters see [3].

Table 1: Counters used for livetime computation
HK Parameter
Storage Values Meaning
An_MAXFRC PMH 0 - 255, def. 20 number of cycles after which count info is sent
An_CCD0SWISTAT PMH ON/OFF status of CCD 0 of this quadrant
An_CCD1SWISTAT PMH ON/OFF status of CCD 1 of this quadrant
An_CCD2SWISTAT PMH ON/OFF status of CCD 2 of this quadrant
An_CHOP PMH 0 - 31 number of frames excluded from the onboard
      processing after one was included
NDISCLIN AUX 0 - 16383 number of discarded columns
NEPDH AUX 0 - 65535 number of events which have been transmitted
      from Event Analyser to Data Handling Unit

XMM-Newton SOC -- 2021-11-30