XMM-Newton Science Analysis System


epexposure (epexposure-0.16) [xmmsas_20230412_1735-21.0.0]

Chopper dependent livetime factor FC3

Because of telemetry limits an exposure might be programmed such that not all frames will be transmitted. This info is stored as quadrant specific periodic housekeeping parameters An_CHOP (see Table 1). These chopper parameters (normally 0) control the number of frames excluded from the onboard analysis before one is analysed and sent to telemetry. The livetime due to such discarded frames is represented by the factor

  $$FC3 = 1/(1+An\_CHOP)\;.$$ (3)

As the chopper parameter and thus the associated livetime is constant for a specific exposure the values be stored in the keywords An_CHOP and AX_FC3 by epframes in the CCD specific EXPOSURE extension to be addressed by higher SAS tasks. If one is just interested in a correct count rate then it is sufficient to divide the rate by FC3.