We discovered that the August 2000 CTIO data had a number of problems with the default WCS in the header: (a) the rotation was considerably off (approx 1 degrees) from what was stated, and (b) there were a number of very bad (1.5" peak-to-trough) higher order wiggles. We finally concluded that it was easier just to scrap the default WCS and redefine it. Subsequently we found a similar problem with the default WCS for the Kitt Peak Wolf-Rayet filter exposures (in that case the header file wasn't being written correctly).
We use order 4 with msctpeak, and make sure that the projection is set to "tnx". We then run mscsetwcs, and finally check again with msctpeak.
In addition, the CTIO Mosaic data is written with a single default WCS solution regardless of filter. Comparision between filters shows that this almost works, but that the "I" exposures are all systematically distorted by 2 pixels in the lower left corner, and -1 pixels in the upper right corner, compared to the other filters we use.
We discovered a bug in msctpeak, in which
the ":rotate " parameter in fact does NOT rotate the expected coordinates
around the center of Mosaic correctly; it looked like assumed rotation axis
was somewhere else. This meant that we had to do an endless number of
"l...l" within msctpeak to get stars and expected positions to match.
Parameters for MSCtpeak:
PACKAGE = mscfinder TASK = msctpeak images = obj1043 List of WCS calibrated Mosaic images coordina= N6822.usno List of ra(hr), dec(deg), optional id database= N6822.db Database for astrometric fit (extname= ) Extensions (epoch = 2000.) Coordinate epoch (update = yes) Update image header WCS following fit? (autocen= no) Center catalog coords when entering task? (boxsize= 9) Centering box fullwidth (project= tnx) Sky projection geometry (fitgeom= general) Fitting geometry (functio= polynomial) Surface type (xxorder= 4) Order of xi fit in x (xyorder= 4) Order of xi fit in y (xxterms= half) Xi fit cross terms type (yxorder= 4) Order of eta fit in x (yyorder= 4) Order of eta fit in y (yxterms= half) Eta fit cross terms type? (reject = 3.) Rejection limit in sigma units (interac= yes) Enter interactive image cursor loop? (frame = 1) Display frame number (marker = circle) Marker type (omarker= plus) Overlay marker type (goodcol= blue) Color of good marker (badcolo= red) Color of bad marker (fdimage= ) (fdcoord= ) (mode = q) PACKAGE = mscred TASK = mscsetwcs images = obj* Mosaic images database= N6822.db WCS database (ra = ra) Right ascension keyword (hours) (dec = dec) Declination keyword (degrees) (equinox= equinox) Epoch keyword (years) (ra_offs= 0.) RA offset (arcsec) (dec_off= 0.) Dec offset (arcsec) (extlist= ) (mode = ql)