This information serves as an addendum to the Computing Average HRC Dead Time Corrections thread. It describes how to remove time periods with a DeadTime Factor (DTF) less than a chosen threshold, and then update the filtered event file with the correct values of DTCOR, LIVETIME, and EXPOSURE in the file header.
The DTF threshold used in this example, 0.90, usually yields a file with a sufficiently accurate livetime, although the required degree of precision will vary depending on the analysis goals. An increased degree of telemetry saturation, indicated by small DTF values, leads to larger uncertainties in livetime calculations. During periods of telemetry saturation caused by background flares the spectral S/N will of course be lower, but the S/N of a particular spectral feature during the observation as a whole may or may not be improved by removing the time period in question. Users will have to do their own calculations to determine which is the case, and what if any filtering should be applied.
Detector background also tends to become less uniform during background flares, with some crowning in the middle across the cross dispersion direction and lower rates in the "T" region of the HRC-S uv/ion shield, which has a thicker layer of Al. (Usually the top of the T is outside the background extraction regions.) Depending on the duration and strength of background flaring, the nonuniform background may cause small errors in determination of the background in the spectral extraction region.
The filtering described here is often used in calibration analyses with a DTF threshold of 0.98 in order to minimize systematic uncertainties; usually a calibration data set is large enough that any loss in statistical S/N is negligible. Users analyzing a single observation are less likely to find that discarding any of their data is beneficial.
dmgti "primary/hrc_examp_dtf1.fits" dtf090.fits userlimit="dtf>0.90"
dmcopy "evt2_old.fits[EVENTS][@dtf090.fits]" evt2_tmp.fits
dmappend "evt2_old.fits[REGION][subspace -time]" evt2_tmp.fits
hrc_dtfstats infile="primary/hrc_examp_dtf1.fits" outfile=dtf_stats.fits gtifile="evt2_tmp.fits[EVENTS]"
dmlist "dtf_stats.fits[cols DTCOR]" data,clean
dmhedit evt2_tmp.fits file= op=add key=DTCOR value=from_above
dmcopy "evt2_tmp.fits[time=:]" evt2_new.fits