The Bolocam Galactic Plane Survey: Survey Description and Data Reduction

We present the Bolocam Galactic Plane Survey (BGPS), a 1.1 mm continuum survey at 33 \u27\u27 effective resolution of 170 deg(2) of the Galactic Plane visible from the northern hemisphere. The BGPS is one of the first large area, systematic surveys of the Galactic Plane in the millimeter continuum without pre-selected targets. The survey is contiguous over the range-10.5 \u3c = l \u3c = 90.5, vertical bar b vertical bar \u3c = 0.5. Toward the Cygnus X spiral arm, the coverage was flared to vertical bar b vertical bar \u3c = 1.5 for 75.5 \u3c = l \u3c = 87.5. In addition, cross-cuts to vertical bar b vertical bar \u3c = 1.5 were made at l = 3, 15, 30, and 31. The total area of this section is 133 deg(2). With the exception of the increase in latitude, no pre-selection criteria were applied to the coverage in this region. In addition to the contiguous region, four targeted regions in the outer Galaxy were observed: IC1396 (9 deg(2), 97.5 \u3c = l \u3c = 100.5, 2.25 \u3c = b \u3c = 5.25), a region toward the Perseus Arm (4 deg(2) centered on l = 111, b = 0 near NGC 7538), W3/4/5 (18 deg(2), 132.5 \u3c = l \u3c = 138.5), and Gem OB1 (6 deg(2), 187.5 \u3c = l \u3c = 193.5). The survey has detected approximately 8400 clumps over the entire area to a limiting non-uniform 1s noise level in the range 11-53 mJy beam(-1) in the inner Galaxy. The BGPS source catalog is presented in a previously published companion paper. This paper details the survey observations and data reduction methods for the images. We discuss in detail the determination of astrometric and flux density calibration uncertainties and compare our results to the literature. Data processing algorithms that separate astronomical signals from time-variable atmospheric fluctuations in the data timestream are presented. These algorithms reproduce the structure of the astronomical sky over a limited range of angular scales and produce artifacts in the vicinity of bright sources. Based on simulations, we find that extended emission on scales larger than about 5\u27.9 is nearly completely attenuated ( \u3e 90%) and the linear scale at which the attenuation reaches 50% is 3\u27.8. Comparison with other millimeter-wave data sets implies a possible systematic offset in flux calibration, for which no cause has been discovered. This presentation serves as a companion and guide to the public data release (http://irsa.ipac.caltech.edu/Missions/bolocam.html) through NASA\u27s Infrared Processing and Analysis Center (IPAC) Infrared Science Archive (IRSA). New data releases will be provided through IPAC-IRSA with any future improvements in the reduction. The BGPS provides a complementary long-wavelength spectral band for the ongoing ATLASGAL and Herschel-SPIRE surveys, and an important database and context for imminent observations with SCUBA-2 and ALMA