`reproject_events' changes the sky coordinate tangent point
in an event file.
There are two modes of operation: (1) the easy version in
which the new sky coordinates are evaluated assuming the old
aspect solution
was correct, and (2) the serious version in which the user supplies
a new aspect solution and the sky coordinates are calculated from
scratch, independent of the original aspect solution.
The first version (with "match=matchfile" and
"aspect=none")
is helpful in preparing to merge event lists with slightly different
aimpoints.
The second version (with "match=none" and
"aspect=@aspectfiles")
is helpful when you need to fix an old event list which was
processed with a bad aspect solution, and you have somehow obtained
(or hacked together) a new aspect solution file.
Each event in the input file is copied to the output. If
aspect=none, its sky pixel coordinates are recalculated by
evaluating the corresponding celestial coordinates using the
old sky pixel values and the coordinate system in the header,
then calculating new sky pixel coordinates using the tangent
point from the match file (or the tangent point values explicitly
supplied in the parameter string, see examples). Note that
this won't give you good results if the positions in
the file were wrong to start with, but it's fine if you want
to say "make this event file compatible with that one".
If an aspect solution file, or stack of aspect solution files,
is supplied in the aspect parameter, the new
focal plane DETX,DETY and sky X,Y
pixel coordinates are calculated from the CHIPX, CHIPY coordinates
using the aspect solution. The DETX,DETY depend on the SIM offsets,
which change with time due to telescope bending and which are
provided
in the aspect solution file as determined from observations of the
instrument fiducial lights. The X,Y are calculated from DETX,DETY
using the telescope pointing direction supplied in the aspect file.
If a match parameter is supplied, the X,Y are evaluated relative
to the specified match tangent point; if match=none, the original
tangent point in the input file is used.
Pixel randomization may also be applied, if an aspect
solution is used to reproject. The CXC standard data
processing (SDP) applies a 0.5 pixel randomization to event
locations to blur instrumental features on scales smaller
than the instrumental resolution. For comparison to SDP
data, this randomization should be applied. This tool does
*not* apply this randomization by default.
As a special case, if the input event file does not have a
time column, times are assigned internally to each photon randomly
within the GTI for its chip. This allows users to reproject background
event files whose photon times are not matched to the actual
observation time.
NOTES
- reproject_events is *not* meant to be used on solar
system objects to remove orbital motion. The 'sso_freeze' program should be used for this purpose
Parameter=infile (file required filetype=input stacks=yes)
Input dataset/block specification
Input event filename.
If the value begins with '@', the input
files will be obtained from a stack list.
Parameter=outfile (file required filetype=output stacks=yes)
Output dataset or stack name. If input is a stack, output
must be a stack with the same number of entries.
Parameter=match (file required filetype=input default=none)
Coordinate system to match to. Can be (1) a filename (not a stack),
in which case the coordinate system in the file is used;
(2) "none", in which case the coordinate system in the input file
is used (useful when reapplying a new aspect file); (3) a pair of
numbers separated by space or comma, interpreted as the new tangent
point RA and Dec in degrees.
Parameter=aspect (file not required filetype=input stacks=yes)
Aspect file, value can be "none" or a stack.
The aspect solution files for Chandra have names like
pcadNNNasol1.fits. There may be more than one asol1 file
for an observation, in which case use an ascii file
asol.lis in which each line is the name of one of the
asol1 files and set "aspect=@asol.lis".
If present,
the program uses the aspect file stack to recalculate the DETX,DETY
and X,Y values. DETX,DETY (focal plane coordinates) are
calculated using the fiducial light-derived SIM offsets
to determine the position of the mirror relative to the
chips, and the PIXLIB parameters to determine the locations
of the chips on the SIM. X,Y (sky pixel coordinates)
are then calculated from DETX,DETY using the RA, Dec and roll
for the appropriate event time and the grid point specified
by the "match" parameter.
Parameter=random (integer not required default=-1)
Random seed for 0.5-pixel randomization. The randomization is
performed by default for CXC Pipeline processed data to remove
instrumental artifacts on scales smaller than the actual spatial
resolution. A reprojected file should have this randomization for
accurate comparison to data produced by the standard pipeline. Any
positive integer value is allowed. Also, random = 0 will use the time
of the event and random = -1 will not randomize.
The name of the Pixlib Geometry parameter file.
Parameter=verbose (integer not required default=0 min=0 max=5)
Controls screen output information, value is 0 (no info)
to 5 (most info).
Overwrite output if it exists? (yes/no)
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