TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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
2
2
Q +U
• P = fraction of I in linear polarization P = ------------------------I
• θ = angle of linear pol. E-vector
1
–1 U
θ =  --- Tan  ----
 Q
 2
Three unknowns requiring three independent observations of target -observer needs three independent observations of target to solve.
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Introduction: ACS Polarizer Filter Sets
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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!)
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Calibration Programs
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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, 10055 -- unpolarized and polarized standard
stars, star cluster 47 Tuc, reflection nebula.
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Polarizer angles on the sky (WFC)
Filter
E-vector angle on sky (PA_V3 + ...)
Derived from design
RAS/Cal test
POL0V + F625W
– 38.2 ± 1.0
– 39.5 ± 0.2
POL60V + F625W
21.8 ± 1.0
28.3 ± 0.4
POL120V + F625W
81.8 ± 1.0
78.1 ± 0.3
POL0UV + F814W
– 38.2 ± 1.0
– 38.4 ± 0.4
POL60UV + F814W
21.8 ± 1.0
22.6 ± 0.4
POL120UV + F814W
81.8 ± 1.0
81.8 ± 0.4
Nice agreement for POLUV+F814W.... but ....
Poor agreement for POLV+F625W -- problem with test or F625W filter(?).
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Instrumental Polarization
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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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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Instrumental Polarization (HRC)
• RAS/HOMS test bad -- modeling of RAS/HOMS optics indicates ~6% internal polarization.
• 9586 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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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Results: Instrumental Polarization (HRC&WFC)
• New on-orbit data -- 9586 & 9661 for GD319 (double star) & G191B2B (single star, used for WFPC2) are all in good agreement.
• Including CCD effects (Si / SiO2 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
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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)
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Preliminary Polarization Calibration for GO Data
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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 (Tperp 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)
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Preliminary Polarization Calibration for GO Data
Math:
Apply correction “C” to observed count rate robs for each polarizer filter
(n=0, 60, 120). Example:
r(n) = C(n, spectral filter, detector) robs(n)
Compute Stokes vector I, Q, U
2
I =  --- [ r ( 0 ) + r ( 60 ) + r ( 120 ) ]
 3
2
Q =  --- [ 2r ( 0 ) – r ( 60 ) – r ( 120 ) ]
 3
2
U =  ------- [ r ( 120 ) – r ( 60 ) ]
 3
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
Compute fractional polarization of target:
2
2
Q +U
T par + T perpP = ------------------------- × --------------------------------------------I
T par – T perp
Correct angles for rotation of POL0 filter on sky (PA_V3 and camera specs);
target polarization E-vector is at PA:
1
–1 U
PA =  --- Tan  ---- + ( PAV 3 ) – 69.4°
 Q
 2
(HRC)
1
–1 U
PA =  --- Tan  ---- + ( PAV 3 ) – 38.2°
 2
 Q
(WFC)
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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.
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TIPS / ACS Polarization
SPACE TELESCOPE SCIENCE INSTITUTE
20 May 2004
John Biretta
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 10055 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.).
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