SPACE TELESCOPE SCIENCE INSTITUTE. ACS Polarization Calibration: Introduction and Progress Report

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1 1 ACS Polarization Calibration: Introduction and Progress Report J. Biretta, V. Platais, F. Boffi, W. Sparks, J. Walsh Introduction: Theory / ACS Polarizers / Supported Modes Potential Issues for ACS Polarization Calibration Calibration Programs / Results Preliminary Calibration for GO Data Future

2 2 Introduction: Theory Polarization of target usually expressed as a Stokes Vector -- (I, Q, U, V) I = total intensity Q = linear polarized intensity with E-vector along principle axes U = linear pol intensity with E-vector along 45 degrees or both axes V = circular pol intensity (usually ignored) Alternate expression -- (I, P, θ ) I = total intensity Q 2 + U 2 P = fraction of I in linear polarization P = I θ = angle of linear pol. E-vector Three unknowns requiring three independent observations of target -- observer needs three independent observations of target to solve. θ = Tan Q U

3 3 Introduction: ACS Polarizer Filter Sets Visible Polarizer set (wheel 2) -- POL0V, POL60V, POL120V UV Polarizer set (wheel 1) -- POL0UV, POL60UV, POL120UV Polarization E-vectors set at nominal 60 degree angles Use with either HRC or WFC detectors Small filters -- illuminate full HRC or ~ quadrant of WFC Designed to be used with spectral filter (include weak lens -- distortion!)

4 4 Introduction: Supported / Unsupported Modes Supported (already in use by GOs; 39 combinations): WFC x POLV(0,60,120) x F475W, F606W, F775W HRC x POLV(0,60,120) x F475W, F606W, F625W, F658W, F775W HRC x POLUV(0,60,120) x F220W, F250W, F330W, F435W, F814W Unsupported but Available: WFC x POLV(0,60,120) x F625W, F658W WFC x POLUV(0,60,120) x any either detector x POLV(0,60,120) x F555W, F550M, F502N, G800L either detector x POLUV(0,60,120) x F660N, FR388N, FR656N, PR200L, F344N, FR459M, FR914M, FR505N

5 5 Potential Issues for ACS Polarization Calibration Polarizer Filters Perpendicular transmissions are high for UV polarizers. Polarization angles of the filters on the sky not known. Non-uniformities in polarization properties across filters. Spurious distortion due to extra lens in the pol filters, polarizing films. ACS Optics Tilted components modify pol. properties of wavefront... Mirrors (especially IM3, M3) -- reflectance varies with position angle of wavefront -- phase retardance converts linear pol to elliptical pol CCD detectors have effects similar to mirrors Spectral filter anomalies (birefringence, etc.)

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7 6 Calibration Programs Lab measurements on polarizer filters. Lab measurements on M3 and IM3 mirrors. ACS RAS/HOMS test at Ball (2 March 2001) -- instrumental pol. ACS RAS/Cal test at Ball (15-22 August 2001) -- polarizer angles. On-orbit programs 9586, 9661, unpolarized and polarized standard stars, star cluster 47 Tuc, reflection nebula.

8 7 Results: Polarizer Filters (Leviton) Lab measurements with unpolarized light source -- throughputs of single polarizer filters and crossed pairs (parallel and perpendicular axes). Throughputs appear identical for 0, 60, and 120 degree filters in each set. POLV - excellent rejection of cross-polarized light (low leakage). POLUV - 5% leakage in UV, 20% leakage in far-red.

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10 8 Results: Polarizer angles on the sky (WFC) Filter POL0V + F625W POL60V + F625W POL120V + F625W POL0UV + F814W POL60UV + F814W POL120UV + F814W E-vector angle on sky (PA_V3 +...) Derived from design RAS/Cal test 38.2± ± ± ± ± ± ± ± ± ± ± ± 0.4 Nice agreement for POLUV+F814W... but... Poor agreement for POLV+F625W -- problem with test or F625W filter(?).

11 9 Results: Instrumental Polarization Define as fractional polarization P seen when observing unpolarized target. Provides a measure of spurious polarization within the instrument. Ideally should be zero. Design goal 5% HRC, 1% WFC. Data: RAS/HOMS test at Ball using flatfields. Program 9586 using unpolarized star GD319 (turns out to be a double star). Lab data and models for M3 and IM3 mirrors.

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13 10 Results: Instrumental Polarization (HRC) RAS/HOMS test bad -- modeling of RAS/HOMS optics indicates ~6% internal polarization data on GD319 questionable -- double star, saturated images. Model for M3 mirror cannot account for observations in UV... other sources of instrumental polarization... CCD?

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17 11 Results: Instrumental Polarization (HRC&WFC) New on-orbit data & 9661 for GD319 (double star) & G191B2B (single star, used for WFPC2) are all in good agreement. Including CCD effects (Si / SiO 2 model) improves model predictions... exact CCD details are proprietary however... F625W sticks out from general trend. Bottom line: HRC ~ 5% instrumental pol. in red; 8-14% instrumental pol. UV WFC ~ 2% instrumental pol. F475W, F606W, F775W Design goal met only for HRC in far-red

18 12 Results: Geometric Distortion (Platais) Compare observations of 47 Tuc with / without polarizers Large scale distortion due to filter power well-corrected (HRC F606W) Unexpected small-scale distortion caused by ripples in polaroid material (+/- 0.3 pixel)

19 13 Preliminary Polarization Calibration for GO Data Method: Calibrate polarization zero point using results for standard star G191B2B. Assume POL filter angles derived from ACS design specs. Correct for cross-polarization leakage (T perp in POLUV filters) Ignore all complex effects in mirrors, detectors (phase retardance, etc.) Test: compare known properties of polarized standard stars Vela I and BD+64D106 with those measured on-orbit (programs 9586, 9661)

20 14 Preliminary Polarization Calibration for GO Data Math: Apply correction C to observed count rate r obs for each polarizer filter (n=0, 60, 120). Example: r(n) = C(n, spectral filter, detector) r obs (n) Compute Stokes vector I, Q, U 2 I = -- 3 [ r ( 0 ) + r( 60) + r( 120) ] 2 Q = -- 3 [ 2r ( 0 ) r( 60) r( 120) ] 2 U = [ r( 120) r( 60) ] 3

21 15 Compute fractional polarization of target: Q 2 + U 2 P = Tpar + Tperp I Tpar Tperp Correct angles for rotation of POL0 filter on sky (PA_V3 and camera specs); target polarization E-vector is at PA: PA = Tan Q + ( PAV3 ) 69.4 U (HRC) PA = Tan 1 U --- Q + ( PAV3 ) 38.2 (WFC)

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24 16 Preliminary Polarization Calibration for GO Data Results of test on polarized standard stars: Good accuracy from 300nm to 700nm: Fractional pol +/-1% (i.e. 5% +/- 1% pol) and PA +/- 2 degrees Larger errors for F220W, F250W, F775W, and F814W. Remaining uncalibrated systematics errors (phase retardance, etc.): Detailed modeling of HRC optics and calibration process... Fractional pol has systematics of ~1 part in 10 (i.e. 40% +/- 4% pol) PA have systematics +/- 3 degrees

25 17 Advice for Observers Most accurate modes are likely to be HRC + POLV + visible filter (e.g. F606W). Poor calibration for some modes: F220W, F250W, F330W (no lab data, effects in CCD), F625W (anomalies), F775W, F814W (larger systematics, no lab data for IM3 mirror). WFC is somewhat risky until more calibration data (IM3 mirror phase retardance is unknown). Impacts on non-polarization data: if the target is significantly polarized the high instrumental polarization for HRC (especially in UV) will decrease photometric accuracy.

26 18 Future Better modeling of mirrors and detectors (diattenuation, phase retardance, etc.) -- proprietary coatings on IM3 and CCDs are an issue. Calibrate higher-order terms that depend on HST roll angle (10% effects) -- polarized std target at many ORIENTs (program in progress, etc.). Generate model-based calibration with full mirror & detector effects included (similar to WFPC2 calibration). Full calibration planned for only F330W and F606W... but filter anomalies (e.g. F625W) are a concern... what about other filters? Distortion: corrections for small-scale ripples in polarizer filters. Field dependence: polarimetric cal. as function of field position (improved flats from 47 Tuc data, dither standard star (10055, etc.).