Summary [current goals: Resolve the ~2° offset. Improve ]

Summary of all polarization orientation calibrations

Polarization efficiencies

[2006 2/5 4/27 6/28] Calibration w/ 0.72(±0.03) mil PP

Uncertainty >8% (due to ~4% thickness uncertainty)

[2005/12/30 SP] Calibrator tests

2-mil PETP @ 196 Hz AC bias (10mV-70mV).

0.7-mil PP @ 70mV bias.

[2004/8/1 JPL] PET film (0.002") calibrator test

Sheet holder 28"x22", @ 45°

T_amb ~ 300 K (Eccosorb 22" x 22"), T_sky ~ 80 K (JPL sky)

Requirements:

* Volts => μK (<10% accuracy)

* Orientation (±3.5° for E-B leakage < r=0.1)

* Faraday modulation angle / efficiency

* Cross-polarization / polarized sidelobes

Q ~ (T_amb - T_sky) (R_TE - R_TM) sin(2a)

(R_TE - R_TM) ∝ f² d² n⁴

Temp: σ(T)<10%, Thickness: σ(d)<5%, Index: σ(n)<2%

Procedure:

1. Remove the Upper Baffle (Release 8 clamps and lift 17 kg).
2. Mount the Calibrator (14 kg, place the arms into the slots attached to the Boot frame and hold the bearing w/ clamps) so that the film is nearly horizontal when the elevation is lowered.
3. Take data while rotating the telescope about boresite at constant speed (recording the θ position), at least 2 revolutions (calibrator fixed).
4. Determine the temperatures of the sky and the ambient load.
5. Lower in EL to ~50° and measure the tilt of the film holder in both directions.
6. Replace the Calibrator with the Upper Baffle.
7. For each pixel, compare the signal to the known polarization (=>) to calibrate the amplitude & orientation.

Dielectric sheet

Fill 18° FoV + 12cm where beam response <1% (<=check window scattering): ellipse 31"x21.5"

Thickness d < 2 mil; uncertainty σ(d) <5% [CP beam ~ 1 cm, pixel5 ~ 5 cm]

Refractive index uncertainty σ(n) <2%

Dielectric	n (97.7/151.8 GHz)	thermal contraction	strength, emissivity, UV?
PP (polypropylene)	1.50168 / 1.50165	6x10-5/K	hard to find thin films!
PETP (Mylar)	~1.74 / 1.83	5x10-5/K	tanδ~0.01 (emission not negligible if < ~2 mil!)

[2004/3/26] Reflectance v. incidence angle (45° ± 9°)

(R_TE - R_TM) ∝ f² d² n⁴ = 0.1~0.7 % => ~1 K polarization

[2004/6/8] Mylar reflection measurements @ 100 GHz

Sheet holder (flat <- tension)

33.5" x 24" elliptical, 2.7 kg

Aluminum: 3x10^-5/K contraction (851 => 849 mm @ -60 °C)

( StainlessSteel: 1~2x10^-5/K, Phenolic: 2x10^-5/K )

T_amb load (inner surface)

Uniform T filling the reflected beam

Eccosorb	TE reflectivity (metal backing)	transmission
HR-10	<3 % @ 100/150 GHz	<5 % @ 100 GHz
AN-72	1-3% @ 100 GHz, 5-15 % @ 150 GHz	~0.2 % @ 100 GHz

Thermister (<10% accuracy)

Cylinder (Al): 1/16" thick, 24" tall, 24.5" diameter, 7.3 kg

Interface (replaceable w/ upper ground shield)

Fixed to the boot frame, while cryostat rotates

Bearing: ~26" (3.7 kg) ~20" (3.2 kg) (Kaydon, Rotek, RBC, Avon )

Molykote 33 extreme low temp bearing grease, light

.

References:

* POLAR (PP film 3 mil < λ)

Q ~ (T_amb - T_sky) (R_TE - R_TM) sin(2a) ~ (260K)(0.002)

Partial polarization (0.5K / 10K)

~10% accuracy (d², n⁴ uncertainties)

Beam-spillover to spurious polarization from lucite calibrator frame

15 minutes / day

2001 O'Dell thesis, 2002 O'Dell et al, 2003 Keating et al, 1991 absorber reflectivity

* Wire grid ~100% polarized: Q ~ (T_hot - T_sky) sin(2a)

Measure cross-polarization when a=0

* Flat metal plate (Al)

Q ∝ δ a (T_plate - T_sky) ~ (0.0001)(250K) ~ 30 mK

δ = skin depth, T_plate (thermometer), T_sky (total power measurements)

~10% accuracy (~20% uncertainty of Al conductivity <- alloy composition/temperature)

2002/3 Staggs et al Calibrating CMB Polarization Telescopes, 2002/6 Hedman PIQUE, 2004/2 Herzog, 2004/6 Barkats CAPMAP

* 1999/2 Mather et al COBE FIRAS

* Miscellaneous data on materials for millimeter and submillimeter optics (Lamb 1996)