  We present photometry of images from the Wide-Field Infrared Survey Explorer
(WISE; Wright et al. 2010) of over 400 million sources detected by the Sloan
Digital Sky Survey (SDSS; York et al. 2000). We use a "forced photometry"
technique, using measured SDSS source positions, star-galaxy separation and
galaxy profiles to define the sources whose fluxes are to be measured in the
WISE images. We perform photometry with The Tractor image modeling code,
working on our "unWISE" coaddds and taking account of the WISE point-spread
function and a noise model. The result is a measurement of the flux of each
SDSS source in each WISE band. Many sources have little flux in the WISE bands,
so often the measurements we report are consistent with zero. However, for many
sources we get three- or four-sigma measurements; these sources would not be
reported by the WISE pipeline and will not appear in the WISE catalog, yet they
can be highly informative for some scientific questions. In addition, these
small-signal measurements can be used in stacking analyses at catalog level.
The forced photometry approach has the advantage that we measure a consistent
set of sources between SDSS and WISE, taking advantage of the resolution and
depth of the SDSS images to interpret the WISE images; objects that are
resolved in SDSS but blended together in WISE still have accurate measurements
in our photometry. Our results, and the code used to produce them, are publicly
available at http://unwise.me.
