  We describe the Submillimeter Array (SMA) Polarimeter, a polarization
converter and feed multiplexer installed on the SMA. The polarimeter uses
narrow-band quarter-wave plates to generate circular polarization sensitivity
from the linearly-polarized SMA feeds. The wave plates are mounted in rotation
stages under computer control so that the polarization handedness of each
antenna is rapidly selectable. Positioning of the wave plates is found to be
highly repeatable, better than 0.2 degrees. Although only a single polarization
is detected at any time, all four cross correlations of left- and
right-circular polarization are efficiently sampled on each baseline through
coordinated switching of the antenna polarizations in Walsh function patterns.
The initial set of anti-reflection coated quartz and sapphire wave plates
allows polarimetry near 345 GHz; these plates have been have been used in
observations between 325 and 350 GHz. The frequency-dependent
cross-polarization of each antenna, largely due to the variation with frequency
of the retardation phase of the single-element wave plates, can be measured
precisely through observations of bright point sources. Such measurements
indicate that the cross-polarization of each antenna is a few percent or
smaller and stable, consistent with the expected frequency dependence and very
small alignment errors. The polarimeter is now available for general use as a
facility instrument of the SMA.
