exomerge -- merge compatible EXOSAT LE event files
exomerge @evtlist outfile
This task merges event files obtained from a single observation with the EXOSAT CMA detector and it corrects the reference coordinates if data were taken with the star tracking mode STR1. It is intended to work for EXOSAT CMA raw data files as stored in the HEASARC archive. The content for each observation includes a catalog file, an orbit file and several event and housekeeing files. Within an observation, a new event and housekeeping file was created either after a few hours of observing time (ranging between 2-4 hr) or because of changes in one or more observational parameters. The task checks that all the input event files share the following modal parameters: pointing mode (STAR_PNT), detector (INSTRUMENT), energy filter (FILTER), on board computer mode (OBCMODE), and the same grating setting (GRATING). An error is reported for any of the above modal parameter differences found. The task will stop execution and no merged file is created. Before restarting the process discard in the list of input event files those with different modal parameters. A warning instead is given, if the data were taken with different background filters, known as the diamond filter. This filter masks part of the detector FOV. The merged event file is created but it is usable only within the diamond region (e.g. outside the exposure will be underestimated). The 'exomerge' output file will have all the extensions merged, the data sorted in time and the timing header keywords updated.
EXOSAT could be programmed to maintain the spacecraft attitude in two modes, start tracking mode 1 (STR1) and 2 (STR2). In STR1 the spacecraft attitude was maintained using one star in the star tracker field of view (FOV) and the position of the sun determined using the fine sun sensor (FSS). STR2 instead used two stars in the star tracker FOV. In STR1 mode a correction to the reference coordinates is needed to account for the roll error in the spacecraft attitude. The correction modifies the RA_NOM, DEC_NOM, TCRVL2, TCRVL3, TROTA3 and TCROT3 keywords in the merged event file. This is calculated using routines from the EXOSAT Interactive Analysis software package and the information stored in the housekeeping and orbit files. The calculation uses the housekeeping values corresponding to the midpoint of the time interval between the first and the last input event file. A warning is given if there are no housekeeping data available at the midpoint observation time (e.g. for a data gap). In this case the correction is based on the housekeeping values at the closer midpoint time.
Exomerge can be run standalone or using two different scripts which create the input list for exomerge and run exomerge. The first, tkexomerge, is a graphic user interface using Tcl/TK which displays the content of the catalog file and allows the user to create the input list by mouse selecting the event files to merge. The other, automerge, is a perl script which creates a list of event files on the user response to a few input paramaters (e.g.detector, filter type, and the grating setting).
Create a merged event file, out.evt, using the event files listed in the file evtlist.
> exomerge evtfile=@evtlist outfile=out.evt