First‐Year Wilkinson Microwave Anisotropy Probe ( WMAP ) Observations: Beam Profiles and Window Functions

Knowledge of the beam profiles is of critical importance for interpretingdata from cosmic microwave background experiments. In this paper, we presentthe characterization of the in-flight optical response of the WMAP satellite.The main beam intensities have been mapped to < -30 dB of their peak values byobserving Jupiter with the satellite in the same observing mode as for CMBobservations. The beam patterns closely follow the pre-launch expectations. Thefull width at half maximum is a function of frequency and ranges from 0.82degrees at 23 GHz to 0.21 degrees at 94 GHz; however, the beams are notGaussian. We present: (a) the beam patterns for all ten differentialradiometers and show that the patterns are substantially independent ofpolarization in all but the 23 GHz channel; (b) the effective symmetrized beampatterns that result from WMAP's compound spin observing pattern; (c) theeffective window functions for all radiometers and the formalism forpropagating the window function uncertainty; and (d) the conversion factor frompoint source flux to antenna temperature. A summary of the systematicuncertainties, which currently dominate our knowledge of the beams, is alsopresented. The constancy of Jupiter's temperature within a frequency band is anessential check of the optical system. The tests enable us to report acalibration of Jupiter to 1-3% accuracy relative to the CMB dipole.