HRC Event Time Correction, Screening and Filtering

Revised November 19, 2001

HRC Event times are misplaced in the event stream so that the proper time for an event is actually contained in the susequent event (provided it gets into the telemetry stream). For high precision timing, it is necessary to shift the pipeline event times by one event to correct for this problem. However, it is also necessary to have conducted the observation with all on-board event vetos disabled, so that all events get to the telemetry stream. Finally, because the typcial HRC count rate exceeds the telemetry limit (due to cosmic ray background) it is necessary to observe in the HRC-S (imaging) mode to be assured that no events are missed, allowing the time correction to be correct.

When the above conditions are met it is still necessary to make the time corrections using an off-line tool "screen_time_filter_gti" which is provided by the HRC Team. This tool will "fix" the times of events and then screen and filter the Level 1 data to produce an event list correcponding to a Level 2 event list.

The HRC Team has developed an event screening procedure that will greatly reduce the background in HRC images by flagging events with a non X-ray charge cloud distribution. The details are being written up in an HRC Technical Memo, and will also be presented at the July 2000 SPIE conference in San Diego. When available these documents will be linked on these HRC WWW pages.

The current version of event screening is the program screen_time_filter_gti, which uses the new SAORD Funtools libraries. Statically linked executable code for either Solaris or Linux can be obtained by clicking (with the shift key depressed) the appropriate link:

screen_time_filter_gti Version 1.0

Readme explains the use of this tool. There is no parameter file needed, all input is via the comand line. Executables are available for Sun Solaris (2.7 or greater) and for Linux (based on RH 7.1). After downloading the appropriate executable (statically linked), you may wish to rename the file to screen_evt1 without the appened OS name.

The typical command is:

screen_time_filter_gti -i inputfilename -g inputgtifile -o outputfilename -H1

screen_time_filter_gti "fixes" event times, screens the events based on the shape of the charge distribution measured on the three tap amplifiers of each axis. This is the default behavior. It will also screen on pulse heights (-p and -P options), and can be used to screen on saturated tap amplifier signals (-s,-S, and -t options). See the README file for details. A summary of the screening is reported by the program upon successful completion.

Source code

screen_time_filter_gti Version 1.0

Building requires that the Funtools libraries be available locally. These can be obtained via the SAO ftp site sao-ftp.harvard.edu. For convenience, here is a link to the funtools source code:

Performance

Screening will reduce the total number of events in an observation by about 40% (typical) which is mainly the background from charged particles. True X-ray events are reduced by only a few percent maximum (and most often not at all). A useful benefit of screening is the virtual elimination of the ``ghost'' images that are present in the raw data (see ghosts). The background map is significanly different for screened data than unscreened. Variations in background across the detector are reduced by about a factor of 2, and these are characterized mainly by a linear gradient in one dimension with a peak variation of about 5%.