Submillimeter array polarimetry with Hertz

We describe the characteristics of the 350 mu m polarimeter Hertz learned from laboratory tests and recent observations at the Caltech Submillimeter Observatory. Hertz contains a pair of 32 element arrays with 18 " pixel spacing and 20 " resolution. The instrument has been improved since initial observations in 1994 and 1995; the detector noise is now below the sky background noise. In excellent weather conditions on Mauna Kea, the noise-equivalent flux density (NEFD) for the measurement of polarized flux is 3-4 Jy Hz(-1/2). The subtraction of correlated sky noise accomplished by the two-array design is crucial for achieving this performance. A method for analysis of our polarization data in the presence of the correlated noise is described. The instrumental polarization of Hertz is less than 0.5% across the detector array. Systematic errors in the measurement of polarization are less than 0.2%. We present a 350 mu m polarization map of Sgr B2 with 140 detections at greater than 3 sigma significance. For our current database of all 350 mu m polarization measurements, the median polarization is 1.1%.