Improved astrometry by Nikolay Kuropatkin

Zeropoints from Douglas Tucker:

The column names in the table are a bit strange, due to the inner workings of my matching routine.  Basically, you want to use FILENAME_2 (or EXPNUM_2 and CCDNUM_2) to match to the right fullcat catalog.  The zeropoint to use is MAG_ZERO_MEAN.  The statistical uncertainty in tying to the ATLAS-REFCAT2 catalog is MAG_ZERO_MEAN_ERR (it is in magnitudes).  The number of matched stars that went into calculating the photometric zeropoint is MAG_ZERO_NUM.

The plots are skyplots for expnum 908899, color coded for MAG_ZERO_MEAN, MAG_ZERO_MEAN_ERR, and MAG_ZERO_NUM.

To calculate DES calibrated mags in the 908899 fullcat file:
    mag_des_calib = -2.5*log10(FLUX_APER_08) + MAG_ZERO_MEAN
(although FLUX_APER_08 was used for the calculation of these photometric zeropoints,
the aperture offset from FLUX_PSF is small enough to use FLUX_PSF, as well, at least for
a quick look).