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Waveplates and Wire Grids

Michelle contains two half-wave plates, one for the N-band window and one for the Q-band window, along with two wire grids. Together these allow polarimetric observations.

The transmission of the half-wave plates is measured to be about 30%, so the signal-to-noise ratio for a given polarimetry observation is about 3 times smaller than that for an observation of the same total on-source time in the normal imaging mode. A small image shift is observed from one wave-plate position to another. While these shifts are less than 1 pixel, they are still enough to induce a spurious polarization "signal" if the individual wave-plate images are not registered.

When imaging polarimetry observations are done all four wave-plate positions are observed twice in a single nod position. To keep the time per nod small enough to get a good atmospheric correction, the time per wave-plate position is only about 6 seconds. It is found that the moving of the wave-plate takes about 2 seconds, and with other operational overheads the result is that the on-source efficiency is about 40 percent lower for polarimetry mode than for normal imaging mode.

More technical details to follow.