HWP Zero Angle

For the June 2007 observing run 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

Tests were also taken by placing 1/16-inch of nylon between the cold load and aperture. 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 fit results, shown in the figure below, yield P = 92 ± 1 %, δ = -30.6 = 59.4 ° ± 0.5°, and an offset of ~ 6%. 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 measured polarization efficiency is lower than the 98% that was measured at 450μm in December 2006 using 2 wire grids in parallel. These measurements would be consistent if we were to assume a grid polarization efficiency of ~94%.

Using a value of δ=59.4° (measured from the vertical) from the above data we find

V_null_encoder = 59.4°.
V_null_rel = 59.4 - 50 = 9.4°
HWP zero angle = 2 × 9.4 = 19°

Last updated by John Vaillancourt. 2007-July-1.
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