HWP zero angle: April/May 2007 Run

The polarization zero angle was measured with the wire grid (2 grids? where were they placed...?) at an angle of 2.19° CW from vertical (measured looking towards SHARP [from M3 ?]). The optical path was...? Nylon...?

cold/warm load → aperture → 2 grids perpendicular to path → SHARP → SHARC-II

Harware was leveled only at the beginning of the test with the HWP at an angle of 50°.

The low polarization efficiency and large DC offset is due to the lack of leveling between HWP angles. However, the phase angle is unchanged by this.

Using a value of δ=5.0° ± 0.1°=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 = 90.0°
Subtracting the 2.2° offset of the grid yields HWP zero angle = 90.0 ° - 2.2° = 88°.

Last updated by John Vaillancourt. 2007-May-22.
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