Chandra Announcement: SAMP values in HRC-I evt1 and evt2 files for observations processed with DS 10.8 and DS 10.9

[Tara Gokas]

Note: SAMP VALUES IN HRC-I EVT1 AND EVT2 FILES FOR OBSERVATIONS PROCESSED WITH DS 10.8 AND DS 10.9

In DS 10.8, released on June 18, 2019, the eventdef structure for evt1 and evt2 files was changed to include the SAMP values that are used to compute the gain corrections for HRC-I observations. This addition to the evt1 and evt2 file was done with the SAMP values incorrectly normalized (the wrong SAMPNORM value was applied). The reported PI values are computed correctly and the gain maps are unaffected. Only the values in the SAMP column in the event files are in error. 

Although the PI values and gain maps in the Chandra archive are correct, users who want to re-process data (for example, to correct for gain drops with newer CALDB files) using hrc_process_events or chandra_repro will need the correct SAMP values in the evt1 files.

Until the fix to the evt files is implemented, users working with impacted observations can compute the correct SAMP values by using the following expression:

SUMAMPS = AU1+AU2+AU3+AV1+AV2+AV3
SAMP = SUMAMPS*2^(AMP_SF-1))/148

Where AU1, AU2, AU3, AV1, AV2, and AV3 are the position signals from the evt1 file, and AMP_SF is the event amplitude scale factor.

A note regarding this issue has been added to the V&V reports of impacted observations.