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Each event detected with the HRI is time tagged with precision of
61 
s relative to the ROSAT spacecraft clock. The processing time
for determining the position of an event is dependent upon the event
location between the readout taps (the fine position) in the
crossed grid charge detector and varies from
0.36 to 1.35 ms. For an event rate of 100 counts s 
, the dead time
losses will vary from 4 to 15 percent. The standard dead time correction
formula can be used: 
where 
is the true
rate, n is the observed rate, and 
is the dead time per event.
The dead time versus fine position varies linearly, but since source
photons will be recorded over a distribution of fine positions due to
spacecraft motion and the point spread function of the telescope, there
is no single value of dead time for a particular source observation. As
a first approximation, it is possible to use a typical value of
ms.
A separate scaler on the HRI counts preprocessed events with a dead time
of only 30 s. The contents of this scaler are entered into the
telemetry once each second as secondary science. Thus if the HRI is
observing a bright source, the intensity as derived from the processed
events in the image can be corrected for the dead time using the data
rates determined from the secondary science scaler.