fringing in the wfc3/uvis detector

2010-07-22 STScI Calibration Workshop 1

Fringing in the WFC3/UVIS detector

Mike WongUC Berkeley


Outline

• Intro to fringing

• Magnitude of fringing in the WFC3/UVIS filters

• 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

• Using data and model to solve for detector thickness

• Future work: improvement and verification of fringing model solutions


Intro to 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:– Data: measure fringe patterns at multiple wavelengths

– Model: determine thickness of detector layer

– Model: predict fringe patterns for any wavelength or SED, create “fringe flats”


Monochromatic fringe flat

TV3 data

977 nm


Filters affected by fringing

WFC3 ISR-2010-04


Assessing fringe amplitude

WFC3 ISR-2010-04


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


Model schematic

Table: Malumuth et al. (2003)


Test data

• DCL data– 2001-12-06 to 2001-12-12

– detector chips tested separately, not integrated

– incident light angle 0° ± 1°

– 146-151 (<0.5 nm FWHM) 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° ± 1°

– 77 (2-nm FWHM) wavelengths/chip, 845–990 nm

WFC3 TIR-2010-01, ISR-2010-05


Test data

• Basic processing– DCL chip 2, commanded

wavelength = 760.26 nm

– overscan/bias

– flatfield

– CR/hot pixels

WFC3 TIR-2010-01


Test data

Data for 1 pixel in Quad A,

Bandpasses of UVIS filters affected by fringing

WFC3 ISR-2010-05


Modeling the data

• Single-pixel test data and best-fit model

• Model tests 8000 thickness values and 6 sets of auxiliary parameters

• Best fit minimized residuals between data and model

WFC3 ISR-2010-05


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 !!

WFC3 ISR-2010-05


Thickness maps

• Thickness map based on TV3 data

WFC3 ISR-2010-05


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


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 wavelengthsWFC3 ISR-2010-05


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

WFC3 ISR-2010-05


Constant scale factor...

• Order errors cycle periodically through mean scale factor

• This behavior is expected

• 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...aerogel ?!?!

WFC3 ISR-2010-05


Thickness maps

• Thickness map based on DCL data

WFC3 ISR-2010-05


Thickness maps

• Map of TV3 thickness – DCL thickness

• Difference is reduced by about a factor of 4 by wavelength correction to DCL data

• Difference gradient indicates potential problem in our understanding of TV3 tests

WFC3 ISR-2010-05


Test data format

• In DCL tests, Chip 1 was rotated 180° (amplifiers at “bottom”) with respect to TV3 and flight configuration (amplifiers at “top”)

• Something fishy in DCL tests would produce inverted thickness gradients between Chips 1 and 2

On-orbit test data


Future work

• Cycle 17 calibration data to be collected in all filters affected by fringing

• Photometry in Omega Cen

• Data will allow comparison of TV3 and DCL models

• On-orbit test data is best way to verify fringe corrections extrapolated beyond ground test data range (11922 Sabbi, 12091 Wong)

• Ideas for new model solutions:– Combine TV3 and DCL data together

• explored, but unlikely to be successful

– Create fringe models based on subsets of test data (Kalirai)• may compensate for uncertainty in silicon index of refraction as a function of

wavelength

– Incorporate ground flats in fringe-affected filters as additional test data• wavelength-targeted approach

fringing in the wfc3/uvis detector

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