Table of Contents
The RA_PNT & DEC_PNT in the aspect solution files have shifted ~180 degrees.
The RA_NOM & DEC_NOM and the RA_PNT & DEC_PNT are always updated using the same matrix as the rest of the data in the file (RA & DEC columns or the event file's WCS).
For reasons beyond the scope of this bugs page, the aspect solution files use a different convention than the other Chandra files. In the aspect soluiton files, the RA_NOM & DEC_NOM keywords are the mean pointing of the telescope during the observation, where as in all the other data products, the RA_PNT & DEC_PNT are used for the mean pointing.
Thus in the aspect solution files, the RA_PNT and DEC_PNT keywords have no meaning, and have been both been set to 0.0.
Since tangent plane projections are only valid for the hemisphere in which the tangent point lies, when RA,DEC=(0,0) is used with some WCS parameters, it may end up being shifted approximately 180 degrees to land in the correct hemisphere as the tanget point.
CIAO tools never use the RA_PNT & DEC_PNT from the aspect solution files so the large shift in these values after applying wcs_update is irrelevant.
Quaternion values, q_att column, in aspect solution are not updated
The quaternion values in the aspect solution, the q_att array column, are not updated by wcs_update to match the modified values of the ra and dec columns.
SAOTrace, the offical Chandra mirror model simulator used by ChaRT uses the quaternion values. For ChaRT v2 simulations performed before 2016-06-10 where uses uploaded a wcs_update adjusted aspect solution file, the location of the PSF will not be adjusted for the astrometric shift. Typically this shift is very small, < 1 arcsec.
All the CIAO tools and MARX use the RA, Dec, & Roll values from the aspect solution rather than the quaternions.
The wcsfile parameter doesn't accept the virtual file syntax (19 Aug 2011)
unix% pset wcs_update wcsfile="wcs.fits[bin sky=128]" # wcs_update (CIAO): dsDMTABLEOPENERR -- ERROR: Failed to open a DM table in dataset ''. DM Error: '[dmi_block_parse_syntax]: null arguments'. # wcs_update (CIAO): ERROR: Could not find WCS in wcsupdfile, and no separate wcsfile provided.
Copy the file before running wcs_update:
unix% dmcopy "wcs.fits[bin sky=128]" wcs.img unix% pset wcs_update wcsfile=wcs.img