align_evt - Generate dmhedit commands to align Chandra event files
for multiple observations of the same field, based on detected sources.
align_evt [options] <event file 1> <event file 2> ...
This program generates dmhedit commands to align Chandra event files
for multiple observations of the same field. This is done in the
following way:
-
Generate a list of detected X-ray sources within a specified radius of
the center for each event file using celldetect. This step is skipped if a
corresponding source file is already available (i.e. for an event file named
<root>_evt2.fits, the corresponding source file must be named
<root>_src2.fits).
-
A reference catalog of sources if taken from either the longest observation
or the first observation on the command (<event file 1>). This is controlled
with the -longest switch. The reference catalog is the baseline to which
the remaining event files are aligned.
-
The sources from each event file are cross-correlated with the reference
catalog. The RA and Dec offset for sources which match to within a specified
match radius are recorded. For each observation, the median of all the offsets
is used to create dmhedit commands to align the event file to the
reference event file.
-
The user can then cut and paste the dmhedit commands to align
the event files.
- -help
-
Print this help information.
- -quiet
-
Run in quiet mode to suppress the printing of processing messages
- -snr <Signal_to_Noise>
-
Specify the minimum signal to noise ratio for sources to be used in
cross correlation
- -det_rad <Detection_Radius>
-
Distance from center (arcmin) for sources to be used in cross-correlation.
Default value is 4 arcmin.
- -match_rad <Match_Radius>
-
Maximum radial offset (arcsec) for cross-correlation between a source and
a candidate match in the reference catalog. Default value is 3 arcsec.
- -force
-
Force program to run celldetect for each event file even if corresponding
source files are found.
- -longest
-
Use the longest observation for the reference catalog. Default is to use
the first observation in the command line.
To run align_evt you first need to be in the CIAO
environment so tools like dmlist and
celldetect are in your path.
Align the files acisf02234N001_evt2.fits and acisf02423N001_evt2.fits, using
the first one in the list for the baseline alignment:
align_evt acisf02234N001_evt2.fits acisf02423N001_evt2.fits
Align all the evt2 files in the current directory, using the longest
observation for the baseline alignment. Run in 'quiet' mode to suppress
printing processing information:
align_evt -longest -quiet *evt2.fits
Align all the evt2 files in the current directory, using only bright sources
with a celldetect S/N ratio of at least 10:
align_evt -snr 10.0 *evt2.fits
Tom Aldcroft ( taldcroft@cfa.harvard.edu )