The SDSS Coadd: 275 deg^2 of Deep SDSS Imaging on Stripe 82
  We present details of the construction and characterization of the coaddition
of the Sloan Digital Sky Survey Stripe 82 \ugriz\ imaging data. This survey
consists of 275 deg$^2$ of repeated scanning by the SDSS camera of $2.5\arcdeg$
of $\delta$ over $-50\arcdeg \le \alpha \le 60\arcdeg$ centered on the
Celestial Equator. Each piece of sky has $\sim 20$ runs contributing and thus
reaches $\sim2$ magnitudes fainter than the SDSS single pass data, i.e. to
$r\sim 23.5$ for galaxies. We discuss the image processing of the coaddition,
the modeling of the PSF, the calibration, and the production of standard SDSS
catalogs. The data have $r$-band median seeing of 1.1\arcsec, and are
calibrated to $\le 1%$. Star color-color, number counts, and psf size vs
modelled size plots show the modelling of the PSF is good enough for precision
5-band photometry. Structure in the psf-model vs magnitude plot show minor psf
mis-modelling that leads to a region where stars are being mis-classified as
galaxies, and this is verified using VVDS spectroscopy. As this is a wide area
deep survey there are a variety of uses for the data, including galactic
structure, photometric redshift computation, cluster finding and cross
wavelength measurements, weak lensing cluster mass calibrations, and cosmic
shear measurements.
