ME DEAD TIME CONSIDERATIONS
Following a general survey of the EXOSAT instrumentation timing
and dead time characteristics (Express No. 5, p.31) a number of
discussions have taken place at the Data Analysis Workshop and
elsewhere to try to resolve an outstanding discrepancy in the ME
dead time correction factor and its functional form. All observatory software and analysis routines use an empirical effective
sample rate to correct spectral counts to fluxes through the
known dead times of the HK QEP counts. Although this method
produces the 'correct' result, several users, notably C. Page at'
Leicester and A. Tennent at Cambridge, have pointed out that for
certain high time-resolution measurements precise knowledge of
the factors contributing to the total loss is important. This
note explores- in more detail the predicted loss of events from
sampling, inherent electronic dead times and subsidiary effects,
which taken together give good agreement with the observational data.
Figure 1 shows the distribution of time tags (raw channel counts) of a sample of ME events (245760 - mainly background), selected on board according to valid E (-= QEP) with a sample scheme of E, TT at 4K s-1 (sampling scheme eg. E, ID or E, ID, TT has no effect).
Note that the maximum time tag possible is 31, determined by sample interval/clock interval (244.1 rs/7.63