Polariametry with NICMOS Dean C. Hines & Glenn Schneider Space Science Institute & Steward Observatory

Space Science Institute10/2005 DCH-2 CIRCE Polarized Light Source (p=100%) NICMOS Polarizer Thermo-Vac Tests

Space Science Institute10/2005 DCH-3 Thermo-Vac Tests Unique polarizing efficiencies (  = 100% for ideal polarizers). –POL0S highest at  = 97% –POL120S lowest at  = 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%).

Space Science Institute10/2005 DCH-4 Thermo-Vac Tests Grisms act as partial linear polarizers, with G206 producing the largest intensity variations (  I ≈ 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.

Space Science Institute10/2005 DCH-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 k th polarizer oriented at position angle  k is: S k = A k I +  k (B k Q +C k U) A k = t k /2(1+l k ), B k = A k cos(2  k ), C k = A k sin(2  k )  k is the polarizing efficiency, t k is the fraction of light transmitted through the polarizer for a 100% polarized input aligned with the polarizer axis l k is the "leak" term — the fraction of light transmitted through the polarizer (exclusive of that involved in t k ) when the incident beam is polarized perpendicular to the axis of the polarizer.  k =(1- l k )/(1+ l k ). Hines, Schmidt & Schneider (2000), Generalized treatment of N- polarizers by Sparks & Axon (1999).

Space Science Institute10/2005 DCH-6 Stokes Uncertainties

Space Science Institute10/2005 DCH-7 Stokes Uncertainties Three Regimes: Read Noise, Photon (C) Noise, Flat Field Residuals  p ≈ 1/ ≈ sqrt(2/C) per image in photon regime Require p/  p > 4, for accurate polarimetry No formal debiasing technique is endorsed or used in reduction Errors in  standard, but must rectify images

Space Science Institute10/2005 DCH-8 Ground-Truth NICMOS: P(max) = 78%KPNO/COB: P(max) = 50% Results essentially identical when NICMOS data smoothed (in Flux Weighted Stokes)

Space Science Institute10/2005 DCH-9 NIC1 - The Egg Nebula Data courtesy of R. Sahai Total Image of CRL 2688

Space Science Institute10/2005 DCH-10 NIC1 Ghosts Images Prominent Ghost Images Differ in each polarizer Causes p = 100% regions Remedy – –multiple spacecraft roll angles –Multiple epochs possible

Space Science Institute10/2005 DCH-11 NCS Cycles vs. Cycles 7 & 7N Some change evident in null observations Adjusted “A” coefficient (  A ≈ 1%) Change appears to mimic a ~1-2% (in p%) effect New coefficient tables published in Handbook No obvious problem for objects polarized p ≥ 10% Two Gyro Mode should have minimal effect on direct imaging polarimetry

Space Science Institute10/2005 DCH-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”

Space Science Institute10/2005 DCH-13 Coronagraphic Polarimetry

Space Science Institute10/2005 DCH-14 Coronagraphic Polarimetry

Space Science Institute10/2005 DCH-15 Coronagraphic Polarimetry

Space Science Institute10/2005 DCH-16 Coronagraphic Polarimetry

Space Science Institute10/2005 DCH-17 Two Gyro Mode Two Gyro Mode tests from Schneider et al (NICMOS ISR ) Direct In Coronagraph Orbit 1 - Orbit 2

Space Science Institute10/2005 DCH-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

Space Science Institute10/2005 DCH-19 NICMOS Polarimetry Science Active Galaxies –Centaurus A – Capetti et al –Cygnus A – Tadhunter et al –NGC 1068 – Simpson et al Proto-Planetary Nebulae –CRL2688 (Egg Nebula) – Sahai et al. 1998, Weintraub et al –CIT 6 – Schmidt, Hines & Swift 2002 –Eta Car – King et al –Polarization of PPN – Su et al. 2003, Ueta et al Young Stellar Objects –GG Tau – Silber et al –AFGL437 – Meakin, Hines & Thompson 2005 Solar System (Io & Titon)

Space Science Institute10/2005 DCH-20 NICMOS Polarimetry Science Ueta et al. 2005

Space Science Institute10/2005 DCH-21 NICMOS Polarimetry Science Silber et al. 2000

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

Backup Slides

Space Science Institute10/2005 DCH-24 Coronagraphic Polarimetry