2010-02-18 TIPS/JIM 1

The “Happy Bunny:” Characterizing Long-Wavelength Fringing in WFC3/UVIS

Mike WongINS/WFC3 Visiting Scientist

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Outline

• What is fringing?

• The fringe model– thanks: Eliot Malumuth

• The ground test data sets– thanks: DCL staff, Howard Bond, Elizabeth Barker, S. Rinehart,

Bob Hill, Bryan Hilbert, Howard Bushouse, Jen Mack, Ray Lucas, Megan Sosey, André Martel, Linda Dressel

• Fitting data with the model

• Correcting on-orbit data

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What is fringing?

• Silicon grows transparent at long wavelengths

• Multiple internal reflections

• Interference effects (constructive/destructive)– strong sensitivity to wavelength

– strong sensitivity to detector layer thickness

• The curse becomes the cure:– measure fringe patterns at multiple wavelengths

– determine thickness of detector layer

– use model to predict fringe patterns for any wavelength or SED

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Filters affected by fringing

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Monochromatic fringe flat

TV3 data

977 nm

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Histogram

TV3 data

977 nm

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The fringe model

• Model described in Malumuth et al. (2003 Proc. SPIE 4854, 567-576)– used to correct STIS slitless spectroscopic data

• Solves the Fresnel equations: – continuity of electromagnetic field components across layer

boundaries

– multi-layer model

• Model inputs:– light wavelength and incidence angle

– layer thicknesses and roughnesses

– layer indices of refraction (n + ik), based on composition

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Model schematic

Table: Malumuth et al. (2003)

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Test data

• DCL data– 2001-12-06 to 2001-12-12

– detector chips tested separately, not integrated

– incident light angle 0°

– 146-151 (monochromatic) wavelengths/chip, nominally 700–1060 nm

• TV3 data– 2008-03-28 to 2008-04-12

– detectors integrated into the instrument

– flight-like incidence angle of 21°

– 78 (2-nm FWHM) wavelengths/chip, 845–990 nm

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Test data

• Basic processing– DCL chip 2, commanded

wavelength = 760.26 nm

– overscan/bias

– flatfield

– CR/hot pixels

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Test data

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Test data

Data for 1 pixel in Quad A

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Test data

Data for 1 pixel in Quad A,

Bandpasses of UVIS filters affected by fringing

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Deriving thicknesses

• For 1 pixel, best thickness minimizes residuals between model and data at all wavelengths

• Problem: DCL and TV3 data sets give different answer !!

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Thickness maps

TV3 data only (other maps being developed)

Black: 13.5 µm thick, White: 17.5 µm thick

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Reconciling TV3/DCL data sets

• Order errors? No.

• Basic processing, or normalization methods? No.

• Errors in DCL and TV3 incident angles? No.

• Anti-reflective coating index of refraction? No.

• Wavelength error in DCL data?– Malumuth: DCL wavelengths could be off by 2–3 nm (But, no.)

– comprehensive test of wavelength error provided surprising result...

– actual wavelengths shorter than commanded wavelengths by about 20 nm

– scale factor of 0.972 ± 0.003 gives best result

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Optimal determination

• For this frame, com-manded = 997.35 nm (black point)

• Calculate whole-chip residuals between: – this DCL data frame at

0° incidence– 0° model with TV3-

derived parameters

• Minimum residual yields chip-averaged optimal wavelength, in this case 969.4 nm (red point)

• Procedure repeated for each frame to get full spectrum of optimal vs. commanded wavelengths

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Optimal spectrum

• Strong systematic relationship between commanded and optimum wavelengths

• Best parameterization:– constant scale factor at

all wavelengths

– higher-order fits not justified

• scatter in data

• lack of physical explanation for wavelength errors

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Constant scale factor...

• Order errors cycle periodically through mean scale factor "", so more confidence in this parameterization

• Fun note:If opt / cmd = , then:opt / cmd = ncmd / nopt

• So finding a constant scale factor is like finding an error in the index of refraction for the DCL experiments...

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(or n error)

• Data could be construed as indicating an experimental index of refraction of1.029 ± 0.003

• This is close to the index of refraction of aerogel of 1.024–1.026 (Poelz & Riethmüller, 1982, Nuc. Instr. Meth. Phys. Res. 195, 491-503)

On-orbit test data

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On-orbit test data

• Cycle 17 calibration data to be collected in 2 (for sure) or all filters affected by fringing

• Photometry in Omega Cen

• Data will allow comparison of models

• On-orbit test data is best way to verify fringe corrections extrapolated beyond ground test data range

• Another correction approach: create fringe models based on restricted wavelength ranges of test data (Kalirai)– may compensate for uncertainty in silicon index of refraction as a function

of wavelength