j_dead_time_calc

The dead time used in standard scientific analysis is calculated from the housekeeping counters that keep track of the fate of every event processed by the on-board software. The time to read in all the detector signals for a single event depends on how quickly the event is discarded due to grey filtering, buffer loss or particle rejection. The time taken for each sort of processing is stored in JMXi-DEAD-MOD (see Table in Appendix ). The read-in time is summed up for all the events to give DEADTIME in JMXi-DEAD-SCP - this deadtime only measures the time that the hardware is occupied with event handling and cannot take in new events and dead time due to buffer losses. A further dead time DEAD_EFF is calculated that includes the effect of grey filter. This factor can be used to correct fluxes and countrates for all detector processing losses. These values however have only 8 secs resolution since this is the frequency of housekeeping packets, therefore, very rapidly varying strong sources must be treated offline for dead time.

Most analysis tools use the DEADTIME value in JMXi-DEAD-SCP and then add the effect of grey filter losses by looking up the grey filter value in the the JMXi_INST-STA instrument status table. These grey filter values have a single event resolution that allows for somewhat better determination of the instantaneous deadtime. This procedure does not however, replace the need for an offline deadtime analysis for very rapidly varying sources.