  The PAU (Physics of the Accelerating Universe) Survey goal is to obtain
photometric redshifts (photo-z) and Spectral Energy Distribution (SED) of
astronomical objects with a resolution roughly one order of magnitude better
than current broad band photometric surveys. To accomplish this, a new large
field of view camera (PAUCam) has been designed, built, commissioned and is now
operated at the William Herschel Telescope (WHT). With the current WHT Prime
Focus corrector, the camera covers ~1-degree diameter Field of View (FoV), of
which, only the inner ~40 arcmin diameter are unvignetted. The focal plane
consists of a mosaic of 18 2k$x4k Hamamatsu fully depleted CCDs, with high
quantum efficiency up to 1 micrometers in wavelength. To maximize the detector
coverage within the FoV, filters are placed in front of the CCDs inside the
camera cryostat (made out of carbon fiber) using a challenging movable tray
system. The camera uses a set of 40 narrow band filters ranging from ~4500 to
~8500 Angstroms complemented with six standard broad-band filters, ugrizY. The
PAU Survey aims to cover roughly 100 square degrees over fields with existing
deep photometry and galaxy shapes to obtain accurate photometric redshifts for
galaxies down to i_AB~22.5, detecting also galaxies down to i_AB~24 with less
precision in redshift. With this data set we will be able to measure intrinsic
alignments, galaxy clustering and perform galaxy evolution studies in a new
range of densities and redshifts. Here, we describe the PAU camera, its first
commissioning results and performance.
