  We describe the Carnegie-Spitzer-IMACS (CSI) Survey, a wide-field, near-IR
selected spectrophotometric redshift survey with IMACS on Magellan-Baade. CSI
uses a flux-limited sample of galaxies in Spitzer IRAC 3.6micron imaging of
SWIRE fields to efficiently trace the stellar mass of average galaxies to
z~1.5. This paper provides an overview of the survey selection, observations,
and processing of the photometry and spectrophotometry. We also describe the
analysis of the data: new methods of fitting synthetic SEDs are used to derive
redshifts, stellar masses, emission line luminosities, and coarse information
on recent star-formation. Our unique methodology for analyzing low-dispersion
spectra taken with multilayer prisms in IMACS, combined with panchromatic
photometry from the ultraviolet to the IR, has yielded high quality redshifts
for 43,347 galaxies in our first 5.3 sq. degs of the SWIRE XMM-LSS field. A new
approach to assessing data quality is also described, and three different
approaches are used to estimate our redshift errors, with robust agreement.
Over the full range of 3.6micron fluxes of our selection, we find typical
redshift uncertainties of sigma_z/(1+z) < 0.015. In comparisons with previously
published spectroscopic redshifts we find scatters of sigma_z/(1+z) = 0.011 for
galaxies at 0.7< z< 0.9, and sigma_z/(1+z) = 0.014 for galaxies at 0.9< z< 1.2.
For galaxies brighter and fainter than i=23 mag, we find sigma_z/(1+z) = 0.008
and sigma_z/(1+z) = 0.022, respectively. Notably, our low-dispersion
spectroscopy and analysis yields comparable redshift uncertainties and success
rates for both red and blue galaxies, largely eliminating color-based
systematics that can seriously bias observed dependencies of galaxy evolution
on environment.
