fringing in the wfc3/uvis detector
DESCRIPTION
Fringing in the WFC3/UVIS detector. Mike Wong UC Berkeley. Outline. Intro to fringing Magnitude of fringing in the WFC3/UVIS filters The fringe model thanks: Eliot Malumuth The ground test data sets - PowerPoint PPT PresentationTRANSCRIPT
2010-07-22 STScI Calibration Workshop 1
Fringing in the WFC3/UVIS detector
Mike WongUC Berkeley
2010-07-22 STScI Calibration Workshop 2
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
2010-07-22 STScI Calibration Workshop 3
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”
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Monochromatic fringe flat
TV3 data
977 nm
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Filters affected by fringing
WFC3 ISR-2010-04
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Assessing fringe amplitude
WFC3 ISR-2010-04
2010-07-22 STScI Calibration Workshop 7
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° ± 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
2010-07-22 STScI Calibration Workshop 10
Test data
• Basic processing– DCL chip 2, commanded
wavelength = 760.26 nm
– overscan/bias
– flatfield
– CR/hot pixels
WFC3 TIR-2010-01
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Test data
Data for 1 pixel in Quad A,
Bandpasses of UVIS filters affected by fringing
WFC3 ISR-2010-05
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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
2010-07-22 STScI Calibration Workshop 13
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
2010-07-22 STScI Calibration Workshop 14
Thickness maps
• Thickness map based on TV3 data
WFC3 ISR-2010-05
2010-07-22 STScI Calibration Workshop 15
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
2010-07-22 STScI Calibration Workshop 16
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
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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
WFC3 ISR-2010-05
2010-07-22 STScI Calibration Workshop 18
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
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Thickness maps
• Thickness map based on DCL data
WFC3 ISR-2010-05
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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
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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
2010-07-22 STScI Calibration Workshop 23
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