Bottom Line on the SDSS Comparison - For the r' filter the scatter is small and uniform. For the other filters there is structure in the chip-to-chip plot for the g', i', and z' filters. The r' filter is the only one without color terms and this could be the source of the variation in the other filters. Jim Rich saw a similar structure using a different set of images. He also found that the color term for the z' filter did not produce a good match, as we also discovered.
Observations - The SDSS coverage overlaps with our D2 and D3 fields. I used recently processed images of these fields, where the conditions as indicated by the SkyProbe data were photometric.
CFHT Photometry - Photometry was performed using standalone DAOPHOT to derive an object list and to perform aperture photometry using 25px radius apertures (sky from 30 to 40px radii). We assumed that this radius contains all the light to within 1% (see the curve of growth analysis). These magnitudes were then calibrated using the image header photometry keywords: PHOT_C, PHOT_X, and PHOT_K in combination with the header keywords: EXPTIME and AIRMASS. We note that PHOT_C is the zeropoint calculated for the given run, not the canonical zero point. We also note that the airmass correction was calculated at an airmass of 1 and the offset from 1 to 0 airmasses is included in the zero points.
SDSS Photometry - I used the SDSS DR3 catalog for the comparison. I used the object flags to remove the resolved sources, and then used the psfMag column which is the optimal measure for isolated point sources.
Star Matching - I used the STSDAS routine rd2xy to transform the SDSS ra,dec coordinates into x,y image coordinates. This routine only uses the linear part of the WCS and there were position residuals up to 15px. I used the DAOPHOT ancillary programs DAOMATCH and DAOMASTER to match and refine the SDSS image positions. Both star lists were sparse enough that this produced few mis-matches as checked on individual images.
Analysis -
The offset between the SDSS psf magnitude for each star was calculated and
a mean offset and scatter for each chip was recorded. We required that all
objects be brighter than 21.5 mag (SDSS) and that the combined photometric
error be less than 0.1 mag. We also clipped objects with an offset greater
than 0.2 mag (mis-matches). For each filter, the offsets and scatters are
plotted as a function of chip (see below).
measurements of the color terms using our culled tertiary standards.