  The Cluster Lensing And Supernova survey with Hubble (CLASH) is a 524-orbit
multi-cycle treasury program to use the gravitational lensing properties of 25
galaxy clusters to accurately constrain their mass distributions. The survey,
described in detail in this paper, will definitively establish the degree of
concentration of dark matter in the cluster cores, a key prediction of CDM. The
CLASH cluster sample is larger and less biased than current samples of
space-based imaging studies of clusters to similar depth, as we have minimized
lensing-based selection that favors systems with overly dense cores.
Specifically, twenty CLASH clusters are solely X-ray selected. The X-ray
selected clusters are massive (kT > 5 keV; 5 - 30 x 10^14 M_solar) and, in most
cases, dynamically relaxed. Five additional clusters are included for their
lensing strength (Einstein radii > 35 arcsec at z_source = 2) to further
quantify the lensing bias on concentration, to yield high resolution dark
matter maps, and to optimize the likelihood of finding highly magnified
high-redshift (z > 7) galaxies. The high magnification, in some cases, provides
angular resolutions unobtainable with any current UVOIR facility and can yield
z > 7 candidates bright enough for spectroscopic follow-up. A total of 16
broadband filters, spanning the near-UV to near-IR, are employed for each
20-orbit campaign on each cluster. These data are used to measure precise
(sigma_phz < 0.02(1+z)) photometric redshifts for dozens of newly discovered
multiply-lensed images per cluster. Observations of each cluster are spread
over 8 epochs to enable a search, primarily in the parallel fields, for Type Ia
supernovae at z > 1 to improve constraints on the time dependence of the dark
energy equation of state and the evolution of such supernovae in an epoch when
the universe is matter dominated.
