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Polarization Efficiency Analysis: February 2007 run

For the February 2007 observing run, as for previous runs, we measured the polarization efficiency and HWP zero angle with two wire grids in series using a differential measurement with a cold and warm load. We used the 350 μm HWP and filter for these tests. The optical path was

Tests were also taken by placing 1/16-inch of nylon between the cold load and aperture. However, we forgot to reset the A/D gains to "hi" (all_gains hi) during this test so there was insufficient S/N for an accurate measurement. The differential measurement was undertaken by "leveling the hardware" of the SHARC-2 camera with the warm load (a piece of eccosorb) and in front of the aperture and then recording data with the cold load. The leveling procedure was repeated each time the HWP was moved. Details of the data analysis for this data can be found in my document on the July 2006 polarization efficiency measurements.

I carried out two different fits for this data. The first assumes the signal varies only sinusoidally with HWP angle and may include an offset term. The results, shown in the figure below, yield P = 93 ± 1 %, δ = 5.5° ± 0.3°, and an offset of ~ 4.1%. The uncertainties reported here (but not in the figure) are estimated by fitting two subsets of the data, each with half the data points.

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The second fit assumes that the reason for the non-zero offset term is an unpolarized component induced by the fact that the RGM used for data reduction is not accurate. The signal reaching the bolometers in the case of no nylon attenuation is likely large enough to change the bolometer loading such that the RGM is modified. I have assumed that this can be corrected by a single scale factor between the H and V halves of the array (f=Gv/Gh). The mathematical form can be found on webpages for previous runs (December and November 2006). The fit to the data below yields P = 93 ± 1 %, δ = 5.4° ± 0.2°, and f = 0.91 ± 0.02. As before the uncertainties reported here (but not in the figure) are estimated by fitting two subsets of the data, each with half the data points.

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If the fit is forced to have f = 1 (or offset = 0) then P = 91%. This is consistent with the difference we observed in December with the 450 μm HWP. When the nylon was inserted, so that there was minimal loading, we observed P = 98%. But this dropped to 96 % without the nylon.

The polarization efficiency of P = 93 ± 1% measured here is also consistent with the meausurements from November and July 2006.

Using a value of δ=5.4°=95.4° (measured from the vertical) from the above data we find

V_null_encoder = 95.4°.
V_null_rel = 95.4 - 50 = 45.4°
HWP zero angle = 2 × 45.4 = 91°

Last updated by John Vaillancourt. 2007-Feb-27.
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