The Panchromatic Hubble Andromeda Treasury
  The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST
Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6
filters, from the UV to the NIR. The full survey will resolve the galaxy into
more than 100 million stars with projected radii from 0-20 kpc over a
contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W
and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and
F160W with WFC3/IR. The resulting wavelength coverage gives excellent
constraints on stellar temperature, bolometric luminosity, and extinction for
most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9,
F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the
uncrowded outer disk; however, the optical and NIR data are crowding limited,
and the deepest reliable magnitudes are up to 5 magnitudes brighter in the
inner bulge. All pointings are dithered and produce Nyquist-sampled images in
F475W, F814W, and F160W. We describe the observing strategy, photometry,
astrometry, and data products, along with extensive tests of photometric
stability, crowding errors, spatially-dependent photometric biases, and
telescope pointing control. We report on initial fits to the structure of M31's
disk, derived from the density of RGB stars, in a way that is independent of
the assumed M/L and is robust to variations in dust extinction. These fits also
show that the 10 kpc ring is not just a region of enhanced recent star
formation, but is instead a dynamical structure containing a significant
overdensity of stars with ages >1 Gyr. (Abridged)
