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Precision Imaging Polarimetry
with ACS
Dean C. Hines
Space Telescope Science Institute
Polarimetry
• Polarization can provide vital and otherwise unobtainable
constraints on
– the origin of light from astronomical sources
– the nature of particles involved in emission and scattering
– the geometry of the regions producing polarized light
• Polarization probes asymmetry
• Provides a “scattering mirror” to “see” around obscuration
• Science Examples
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Synchrotron emission in M87 jet
Unified AGN Model
Aspherical supernovae explosions
Detailed structure, composition, shape/size, or orientation
of scattering dust particles
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Scattering & Polarization
Polarization induced by scattering off small particles
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Polarimetry 101
Sk = AkI + ek (BkQ + CkU),
Ak = ½ tk (1+lk),
Bk = Ak cos (2fk),
Ck = sin (2fk)
Hines et al. (2000: see also Sparks & Axon 1999)
I = 2/3(S0 + S60 + S120),
Q = 2/3(2S0 + S60 + S120),
U = 2(S60 – S120)/sqrt(3)
ACS Instrument Handbook (see also Jackson!)
p = 100% (Q2 + U2 )1/2/I
θinstr = 0.5 tan−1 (U/Q)
Every Polarimetry Paper…Ever!
Hines et al. (2000)
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Calibration of a Polarimeter
• Ground-based TV testing with polarized and unpolarized
flat field
– measure throughputs
– polarizing efficiencies
– FOV variations
– instrumental polarization
– fiducial polarization “axis” of the instrument
• On-orbit using polarized and unpolarized ‘standards”
– Multiple “rolls” for separating throughput vs. polarizing
efficiency
– Measure know extended polarized source to assess field
distortions
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Updated Calibration
• ACS internal flat mirrors
• ACS Cal programs
• Previous calibration
– Cracraft, M., & Sparks, W. B. 2007, ISR ACS 2007–10
– Biretta, J., et al. 2004, ISR ACS 2004–09
• New calibrations
IcorPOL∗ V = CPOL∗V x Iobs x POL*V ,
CPOL0V = 1.2960,
CPOL60V = 1.3238,
CPOL120V = 1.2781
– Repeatability ~ 0.003 fractional polarization
– Absolute ~ 0.003 fractional polarization
– Position angle ~ 2-3˚
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Field Distortion
NICMOS
Cryogenic Optical Bench KPNO
Hines et al. (2000)
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Field Distortion
Polarization Position Angles
Perpendiculars
Weintraub et al. (2000)
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Field Distortion
I
p
Theta
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p*I
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Field Distortion
I
p
Theta
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p*I
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Field Distortion
I
p
Theta
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p*I
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HST/ACS Observations of Comet ISON
• Observed with Wide Field Camera (WFC) module of the
Advanced Camera for Surveys (ACS)
• Two Orbits DD time May 8, 2013
– F606W (Broad-V)
• Weak or no emission lines
– rh = 3.81 AU from Sun, D = 4.34 AU from Earth
– Phase angle was α ≈ 12.25º (“bottom” of negative
polarization branch)
• Three Orbits GO time October 26/27, 2013
– F606W (Broad-V) & F775W (SDSSi)
• Strong C2 in F606W
• Minimal CN in F775W
– rh = 1.12 AU, D = 1.37 AU, α ≈ 45.60˚
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ACS/WFC F606W Imaging Polarimetry
To Sun
5″
Hines et al. (2014, ApJ, 780, 32L)
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ACS/WFC F606W Imaging Polarimetry
• Halo p ~ -1.8%, suggest abundance of abundant
absorbing particles
• Central region p ~ + 2.5%, suggests abundant ice-coated
grains. Characteristic of cometary jets.
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ACS/WFC F606W Imaging Polarimetry
• Halo p ~ -1.8%, suggest abundance of abundant
absorbing particles
• Central region p ~ + 2.5%, suggests abundant ice-coated
grains. Characteristic of cometary jets.
Hines et al. (2014, ApJ, 780, 32L)
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Polarimetry Observations of Comet 67/P
• Rosetta will measure the properties of the coma of 67P, including
the inner coma during Philae lander release by 10 km altitude.
• 21 Orbit, Cycle 22 HST/ACS F606W (PI: Hines)
– Observations bracket the period when Rosetta operates closest
to 67P, and will deploy the Philae lander.
– 2014-Aug-19: rh = 3.52 AU, D = 2.76 AU, α ≈ 12.0˚
– 2014-Nov-17: rh = 2.96 AU, D = 3.43 AU, α ≈ 15.7˚
– Planning to propose Post-Perihelion follow-up
• Contemporaneous observations with remote sensing assets from
more distant vantage points will enable linkage of the properties
and distribution of dust observed from Earth to the material
measured in-situ near the nucleus.
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Fin
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