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Last modified: 24 October 2023


Bugs: dmcoords

Table of Contents




dmcoords does not work correctly on the ACIS blank-sky background files

The pointing (PNT) header keyword values in the background files are set to zero:

unix% dmlist bgevt2_c7.fits header |grep PNT
0058 RA_PNT                                  0           Real8        Pointing RA
0059 DEC_PNT                                 0           Real8        Pointing Dec
0060 ROLL_PNT                                0 [deg]     Real8        Roll (fake)

These values indicate where the optical axis was during the observation. Having zero-values will cause the CIAO tool dmcoords to produce incorrect results when run with the background file.


Follow the workaround in the The ACIS "Blank-Sky" Background Files thread to update the PNT header keyword values.

Switching between celfmt=deg and celfmt=hms can lead to a misleading error message.

When switching between using celfmt=hms to celfmt=deg, dmcoords may already have some RA/Dec values set: ra, dec, ra_zo, dec_zo, etc. Unless these parameters are explicitly set to a blank string, the tool will complain about an "Invalid entry" and will prompt for a new value.

unix% punlearn dmcoords
unix% dmcoords /export/byobsid/635/repro/acisf00635_repro_evt2.fits asol= op=sky x=4096 y=4096
unix% pget dmcoords ra dec
unix% dmcoords /export/byobsid/635/repro/acisf00635_repro_evt2.fits asol= op=sky x=4096 y=4096 celfmt=deg
Invalid entry '16:27:17.957' - try again!
ra_zo[ deg ] : 


To work around the problem, simply

% punlearn dmcoords

between uses.


Incorrect values from CAR transform.

The WCS library that the DM uses has a problem computing coordinate transforms that involve the CAR transform.