HWP Zero Angle Analysis: April 2011 run

For the March 2011 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 350 μm HWP and filter for these tests. The optical path was

cold/warm load → aperture → 1 grid perpendicular to path → SHARP → SHARC-II (n.b.: no nylon put in front of SHARC)

The differential measurement was undertaken by taking half of a file with a warm load (a piece of eccosorb) in front of the aperture and then removing it so the cold load could be seen for the second half of the file. This procedure was repeated each time the HWP was moved. Cold-Hot leveling was done in software (kilauea:~sharp/bin/IDL/poleff5.pro). An old SHARC2PIXEL.dat file was used to remove bad pixels from the analysis; this file was updated to account for obvious bad pixels not in that "legacy" file. All data files are in kilauea:~sharp/Runs/2011Apr/calibs/poleff/.

The results, shown in the figure below, yield P = 88.6 ± 0.5 %, δ = 5.0° = 95.0° ± 0.5°, and an offset of 5.5%. 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.

The polarization efficiency of P = 88.6 ± 0.4% measured here is consistent with that expected for a single grid (e.g., November and July 2006) since 1.00 - (1.00 - 0.88)^2 = 99%.

We used a starting angle value of 50°. Using a value of δ=95.0° (measured from the vertical) from the above data we find

V_null_encoder = 95.0°.
V_null_rel = 95.0 - 50 = 45.0°
HWP zero angle = 2 × 45.0 = 90°

Last updated by John Vaillancourt. 2011-Apr-25.
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