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Polariametry with NICMOS

This article discusses the use of polarimetry with the NICMOS instrument, including the unique polarizing efficiencies, thermo-vac tests, polarimetry algorithm, Stokes uncertainties, and ground-truth results. It also explores the use of polarimetry in coronagraphic observations and highlights the success of NICMOS in delivering high-fidelity polarimetry in imaging and coronagraphic modes.

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Polariametry with NICMOS

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  1. Polariametry withNICMOS Dean C. Hines & Glenn Schneider Space Science Institute & Steward Observatory

  2. Thermo-Vac Tests CIRCE PolarizedLight Source (p=100%) NICMOSPolarizer

  3. Thermo-Vac Tests • Unique polarizing efficiencies(e = 100% for ideal polarizers). • POL0S highest at e = 97% • POL120S lowest at e = 48% • Polarization efficiency constant over FOV. • Angular offsets differ from nominal (120˚) by arbitrary (but accurately measured) amounts. • Instrumental polarization caused by reflections off the mirrors in the optical train within NICMOS is small (≤ 1%).

  4. Thermo-Vac Tests • Grisms act as partial linear polarizers, with G206 producing the largest intensity variations (DI ≈ 5%) for completely polarized light. • Effect proportional to incoming polarized signal and position angle. • Only important for highly polarized objects. • Grisms cannot be used with either the NIC1 or NIC2 polarizers and are thus unsuitable for spectropolarimetry.

  5. Polarimetry Algorithm (POLARIZE) At any pixel in an image, the observed signal from a polarized source of total intensity I and linear Stokes parameters Q & U measured through the kth polarizer oriented at position anglefk is: Sk = Ak I + ek (Bk Q +Ck U) Ak = tk /2(1+lk ), Bk = Ak cos(2fk ), Ck = Ak sin(2fk ) ek is the polarizing efficiency, tk is the fraction of light transmitted through the polarizer for a 100% polarized input aligned with the polarizer axis lk is the "leak" term — the fraction of light transmitted through the polarizer (exclusive of that involved in tk) when the incident beam is polarized perpendicular to the axis of the polarizer. ek =(1- lk)/(1+ lk). Hines, Schmidt & Schneider (2000), Generalized treatment of N-polarizers by Sparks & Axon (1999).

  6. Stokes Uncertainties

  7. Stokes Uncertainties • Three Regimes: Read Noise, Photon (C) Noise, Flat Field Residuals • sp ≈ 1/<S/N> ≈ sqrt(2/C) per image in photon regime • Require p/sp > 4, for accurate polarimetry • No formal debiasing technique is endorsed or used in reduction • Errors in q standard, but must rectify images

  8. Ground-Truth Results essentially identical when NICMOS data smoothed(in Flux Weighted Stokes) NICMOS: P(max) = 78% KPNO/COB: P(max) = 50%

  9. NIC1 - The Egg Nebula Total Image ofCRL 2688 Data courtesy of R. Sahai

  10. NIC1 Ghosts Images • Prominent Ghost Images • Differ in each polarizer • Causes p = 100% regions • Remedy – • multiple spacecraft roll angles • Multiple epochs possible

  11. NCS Cycles vs. Cycles 7 & 7N • Some change evident in null observations • Adjusted “A” coefficient (DA ≈ 1%) • Change appears to mimic a ~1-2% (in p%) effect • New coefficient tables published in Handbook • No obvious problem for objects polarizedp ≥ 10% • Two Gyro Mode should have minimal effect on direct imaging polarimetry

  12. Coronagraphic Polarimetry • New mode tested in Cycle 12 • Offered in Cycle 14 • Successful combination of NIC2 coronagraph with polarizers • Advantage over direct coronagraphy • Detect polarized emission near bright point-source • Circumstellar Disks • Scattering regions around AGNs • PSF is obtained simultaneously • PSF-subtraction “built-in”

  13. Coronagraphic Polarimetry

  14. Coronagraphic Polarimetry

  15. Coronagraphic Polarimetry

  16. Coronagraphic Polarimetry

  17. Two Gyro Mode Two Gyro Mode tests from Schneider et al. 2005 (NICMOS ISR 2005-001) Direct In Coronagraph Orbit 1 - Orbit 2

  18. Two Gyro Mode • Jitter <~ 7-8 mas implying performance comparable to three gyro mode • Single orbit roll not available, but doesn’t affect polarimetry • Calibration of 2m pol filters not yet done • An unpolarized standard star should be observed in addition to the primary target object

  19. NICMOS Polarimetry Science • Active Galaxies • Centaurus A – Capetti et al. 2000 • Cygnus A – Tadhunter et al. 2000 • NGC 1068 – Simpson et al. 2002 • Proto-Planetary Nebulae • CRL2688 (Egg Nebula) – Sahai et al. 1998, Weintraub et al. 2000 • CIT 6 – Schmidt, Hines & Swift 2002 • Eta Car – King et al. 2002 • Polarization of PPN – Su et al. 2003, Ueta et al. 2005 • Young Stellar Objects • GG Tau – Silber et al. 2000 • AFGL437 – Meakin, Hines & Thompson 2005 • Solar System (Io & Titon)

  20. NICMOS Polarimetry Science Ueta et al. 2005

  21. NICMOS Polarimetry Science Silber et al. 2000

  22. NICMOS Continues to Deliver High Fidelity(Imaging & Coronagraphic) Polarimetry Combined with ACS, HST Offers ImagingPolarimetry Capability from UV - NIR

  23. Backup Slides

  24. Coronagraphic Polarimetry

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