## HRC Deadtime Corrections with Telemetry Saturation

### Abstract

The original
DTCOR
and hence
EXPOSURE
values were not correct
for HRC observations which experienced
telemetry saturation.
The `hrc_dtfstats` tool was updated
to use a simple mean instead of a variance-weighted mean,
which corrects this problem.
`chandra_repro` run `hrc_dtfstats`
so data that has been reclaibrated have the correct DTCOR and
EXPOSURE values. All HRC data in the archive
have been reprocessed to correct this; however, users with
old versions of data need to study this caveat.

Of ~1800 HRC ObsIDs checked (both I and S), changing how we calculate dtcor will change the values by less than 5% for 93% of the obsids. 96% will change by less than 10%. Only about 1% of the obsids will have dtcor changed by more than 50%.

The HRC deadtime has two components, event processing time and telemetry saturation (Juda and Dobrzycki 1999). The deadtime correction factors are calculated in 2 second increments over the course of an observation using the telemetered events rate, the valid events rate, and the total events rate. These are available in the dtf1 file associated with each observation.

The CIAO tool, `hrc_dtfstats`, is used to
compute the overall deadtime correction to the exposure time
(see the CIAO thread Computing
Average HRC Dead Time Corrections thread). This tool
computes the variance-weighted average deadtime factor,
DTCOR. DTCOR and hence EXPOSURE are not correct for observations
which are subject to telemetry saturation.

For example, average count rates for Cas A, which should be a constant source, are highly variable.

The most egregious point, at ~50 months since 1/1/1999, is for ObsID 3698. The rate in the figure is computed with the DTCOR value from the std_dtfstat1 file, 0.972:

unix% dmlist hrcf03698_000N003_std_dtfstat1.fits"[col dtcor]" data --------------------------------------------------------------------- Data for Table Block DTFSTATS --------------------------------------------------------------------- ROW DTCOR 1 0.97192718881085

A simple mean of the DTF, however, is 0.766, which, if used instead, would lead to a rate much more in line with the others.

unix% dmstat hrcf03698_000N003_dtf1.fits"[col dtf]" DTF min: 0 @: 1 max: 1 @: 2 mean: 0.76564416062 sigma: 0.270005164 sum: 2081.0208286 good: 2718 null: 0

The cause of the discrepancy are the DTF errors, and in
particular the relative sizes of the errors in the unsaturated
vs. saturated telemetry cases. The errors during telemetry
saturation are ~10 times greater than those during unsaturated
intervals. Since `hrc_dtfstats` uses a variance-weighted
average of the DTFs to compute DTCOR, the contribution of DTFs
during saturated intervals is much reduced.

The `hrc_dtfstats` tool was
updated in CIAO 4.4 to use a simple mean instead of a
variance-weighted mean. Creating a new DTF file for this
dataset in CIAO 4.4 returns the expected result:

unix% hrc_dtfstats hrcf03698_000N003_dtf1.fits 3698_dtf_new.fits gtifile='hrcf03698N003_evt2.fits unix% dmlist 3698_dtf_new.fits data -------------------------------------------------------------------------------- Data for Table Block DTFSTATS -------------------------------------------------------------------------------- ROW DTCOR 1 0.75534347952970