A Map of the Cosmic Microwave Background from the BEAST Experiment

We present the first sky maps from the BEAST (Background Emission Anisotropy Scanning Telescope) experiment. BEAST consists of a 2.2 meter off axis Gregorian telescope fed by a cryogenic millimeter wavelength focal plane currently consisting of 6 Q band (40 GHz) and 2 Ka band (30 GHz) scalar feed horns feeding cryogenic HEMT amplifiers. Data were collected from two balloon-borne flights in 2000, followed by a lengthy ground observing campaign from the 3.8 Km altitude University of California White Mountain Research Station. This paper reports the initial results from the ground based observations. The instrument produced an annular map covering the sky from declinateion 33 to 42 degrees. The maps cover an area of 2470 square degrees with an effective resolution of 23 arcminutes FWHM at 40 GHz and 30 arcminutes at 30 GHz. The map RMS (smoothed to 30 arcminutes and excluding galactic foregrounds) is 54 +-5 microK at 40 GHz. Comparison with the instrument noise gives a cosmic signal RMS contribution of 28 +-3 microK. An estimate of the actual CMB sky signal requires taking into account the l-space filter function of our experiment and analysis techniques, carried out in a companion paper (O'Dwyer et al. 2003). In addition to the robust detection of CMB anisotropies, we find a strong correlation between small portions of our maps and features in recent H$\alpha$ maps (Finkbeiner, 2003). In this work we describe the data set and analysis techniques leading to the maps, including data selection, filtering, pointing reconstruction, mapmaking algorithms and systematic effects. A detailed description of the experiment appears in Childers et al. (2003).