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Last modified: 24 February 2022


Bugs: wcs_update

Table of Contents



NOM and PNT keywords not updated consistently with WCS when scaling is applied

The RA|DEC_NOM and RA|DEC_PNT keywords are not updated the same way as the WCS parameters (ie the CRVAL keywords). The exact difference is not known at this time, but it only appears to be an issue when there is a scale factor applied. This could be from using method=rst when running wcs_match or if using the scalefac parameters.

Having these keywords out of sync may lead to some very small errors when computing coordinates, eg going from SKY(X,Y) to either off-axis angles or to celestial coordinates. Using dmcoords users will see a warning such as

# dmcoords (CIAO 4.14): WARNING: RA_NOM DEC_NOM different from WCS

# dmcoords (CIAO 4.14): RA_NOM DEC_NOM =     173.26163587 25.90709570  WCS =    173.26300329     25.90586571 


The only work around is to avoid using the method=rst or specifying a scale value other than scalfac=1.0.

Cannot use same file for infile and wcsfile

Users cannot use the same file for both the infile and the wcsfile parameters. Attempting to do so will cause the tool to quietly exit without making any updates to the infile.

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.