HWP Zero Angle Analysis: January/February 2009 run

For the January 2009 observing run, as for previous runs, we measured the HWP zero angle with one wire grid using a differential measurement with a cold and warm load. We used the 450 μm HWP and filter for these tests. The optical path was

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.

The results, shown in the figure below, yield P = 80 ± 1.5 %, δ = -37.7° = 52.3° ± 0.3°, and an offset of ~ 2%. 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 = 80 ± 1% measured here is a bit low, but not unreasonable for a single grid (e.g., November and July 2006) since 1.00 - (1.00 - 0.80)^2 = 96%.

Using a value of δ=52.3° (measured from the vertical) from the above data we find

V_null_encoder = 52.3°.
V_null_rel = 52.3 - 50 = 2.3°
HWP zero angle = 2 × 2.3 = 5°

Last updated by John Vaillancourt. 2009-Apr-2.
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