Photometric Cross-Calibration of the SDSS Stripe 82 Standard Stars
  catalogue with Gaia EDR3, and Comparison with Pan-STARRS1, DES, CFIS and
  GALEX catalogues
  We extend the SDSS Stripe 82 Standard Stars Catalog with post-2007 SDSS
imaging data. This improved version lists averaged SDSS ugriz photometry for
nearly a million stars brighter than r~22 mag. With 2-3x more measurements per
star, random errors are 1.4-1.7x smaller than in the original catalog, and
about 3x smaller than for individual SDSS runs. Random errors in the new
catalog are ~< 0.01 mag for stars brighter than 20.0, 21.0, 21.0, 20.5, and
19.0 mag in u, g, r, i, and z-bands, respectively. We achieve this error
threshold by using the Gaia Early Data Release 3 (EDR3) Gmag photometry to
derive gray photometric zeropoint corrections, as functions of R.A. and
Declination, for the SDSS catalog, and use the Gaia BP-RP colour to derive
corrections in the ugiz bands, relative to the r-band. The quality of the
recalibrated photometry, tested against Pan-STARRS1, DES, CFIS and GALEX
surveys, indicates spatial variations of photometric zeropoints <=0.01 mag
(RMS), with typical values of 3-7 millimag in the R.A., and 1-2 millimag in the
Declination directions, except for <~6 millimag scatter in the u-band. We also
report a few minor photometric problems with other surveys considered here,
including a magnitude-dependent ~0.01 mag bias between 16 < G_Gaia < 20 in the
Gaia EDR3. Our new, publicly available catalog offers robust calibration of
ugriz photometry below 1% level, and will be helpful during the commissioning
of the Vera C. Rubin Observatory Legacy Survey of Space and Time.
