.For the April/May 2008 observing run we measured the HWP zero angle with two wire grids, parallel and in series, using a differential measurement with a cold and warm load. Cold and warm load files were taken at each HWP angle and then differenced in software (the data from the warm file immediately preceeding a cold file, was subtracted from the cold file). We used the 450 μm HWP for these tests and used both the 450 μm and 850 μm SHARC-II filters. The optical path was
The results, shown in the figure below, yield P = 105 ± 5 %, δ = 7.9° = 52.9° ± 0.5°, and an offset of ~ 5%. The uncertainties reported here (but not in the figure) are estimated by fitting two subsets of the data, each with half the data points and comparing them to the fits with all data points.
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The polarization efficiency of P = 105 ± 5% is obviously high, but is =100% within the uncertainties. Previous measuements of the 450 μm efficiency yielded ~ 98%. (e.g., December 2007).
Using a value of δ=52.3° (measured from the vertical) from the above data we find
V_null_encoder = 52.9°.
V_null_rel = 52.9 - 50 = 2.9°
HWP zero angle = 2 × 2.9 = 6°
Below is the data for the 850 μm filter. Note that data were
taken only every 20°, rather then every 10° as with the 450
μm filter. The results show a large offset (44%), but this is
consistent with low measured polarization efficiency as well
(P = 53 ± 6%). The angle is consitent with the angle
measured at 450 μm, as expected (δ = 7.4° =
52.4° ± 1.7°).
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Using a value of δ=52.4° (measured from the vertical) from the above data we find
V_null_encoder = 52.4°.
V_null_rel = 52.4 - 50 = 2.4°
HWP zero angle = 2 × 2.4 = 5°