Polariametry with NICMOS Dean C. Hines & Glenn Schneider Space - - PowerPoint PPT Presentation

Polariametry with NICMOS

Dean C. Hines & Glenn Schneider

Space Science Institute & Steward Observatory

Space Science Institute 10/2005 DCH-2

2000 4000 6000 POL0L POL120L POL240L 50 100 150 200 Mean Counts/sec (Background Subtracted)

Position Angle of Incoming 100% Polarized Light (Þ)

2000 4000 POL0S POL120S POL240S Mean Counts/sec (Background Subtracted)

CIRCE Polarized Light Source (p=100%) NICMOS Polarizer

Thermo-Vac Tests

Space Science Institute 10/2005 DCH-3

Thermo-Vac Tests

• Unique polarizing efficiencies

(ε = 100% for ideal polarizers).

– POL0S highest at ε = 97% – POL120S lowest at ε = 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 Institute 10/2005 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 Institute 10/2005 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 kth polarizer oriented at position angle φk is:

Sk = Ak I + εk (Bk Q +Ck U)

Ak = tk /2(1+lk ), Bk = Ak cos(2φk ), Ck = Ak sin(2φk ) εk 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. εk =(1- lk )/(1+ lk ). Hines, Schmidt & Schneider (2000), Generalized treatment of N- polarizers by Sparks & Axon (1999).

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Stokes Uncertainties

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Stokes Uncertainties

• Three Regimes: Read Noise, Photon (C) Noise,

Flat Field Residuals

• σp ≈ 1/<S/N> ≈ sqrt(2/C) per image in photon

regime

• Require p/σp > 4, for accurate polarimetry
• No formal debiasing technique is endorsed or

used in reduction

• Errors in θ standard, but must rectify images

Space Science Institute 10/2005 DCH-8

Ground-Truth

(a) (b)

NICMOS: P(max) = 78% KPNO/COB: P(max) = 50%

Results essentially identical when NICMOS data smoothed (in Flux Weighted Stokes)

Space Science Institute 10/2005 DCH-9

NIC1 - The Egg Nebula

Data courtesy of R. Sahai

Total Image of CRL 2688

Space Science Institute 10/2005 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 Institute 10/2005 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 Institute 10/2005 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 Institute 10/2005 DCH-13

Coronagraphic Polarimetry

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Coronagraphic Polarimetry

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Coronagraphic Polarimetry

5 10 15 20 25 5 10 15 20 25 30 35 40 0.5 1 1.5 2 2.5 3

Polarization (%) Radius (Pixels) Radius (Arcsec)

Space Science Institute 10/2005 DCH-16

Coronagraphic Polarimetry

0.1 1 10 5 10 15 20 25 30 35 40

V30_I V30_P*I V31_I V31_P*I V60_I V60_P*I V61_I V61_P*I I_median P*I_median

0.5 1 1.5 2 2.5 3

Intensity (Arbitraty Units) Radius (Pixels)

Polarized Intensity (P*I) Total Intensity (I)

Radius (arcsec)

Space Science Institute 10/2005 DCH-17

Two Gyro Mode

Two Gyro Mode tests from Schneider et al. 2005 (NICMOS ISR 2005-001)

Direct In Coronagraph Orbit 1 - Orbit 2

Space Science Institute 10/2005 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 2µm pol filters not yet done
• An unpolarized standard star should be
• bserved in addition to the primary target
• bject

Space Science Institute 10/2005 DCH-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)

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NICMOS Polarimetry Science

Ueta et al. 2005

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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 Institute 10/2005 DCH-24

Coronagraphic Polarimetry