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2010-02-18 TIPS/JIM 1
The “Happy Bunny:” Characterizing Long-Wavelength Fringing in WFC3/UVIS
Mike WongINS/WFC3 Visiting Scientist
2010-02-18 TIPS/JIM 2
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
2010-02-18 TIPS/JIM 3
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
2010-02-18 TIPS/JIM 4
Filters affected by fringing
2010-02-18 TIPS/JIM 5
Monochromatic fringe flat
TV3 data
977 nm
2010-02-18 TIPS/JIM 6
Histogram
TV3 data
977 nm
2010-02-18 TIPS/JIM 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
2010-02-18 TIPS/JIM 8
Model schematic
Table: Malumuth et al. (2003)
2010-02-18 TIPS/JIM 9
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
2010-02-18 TIPS/JIM 10
Test data
• Basic processing– DCL chip 2, commanded
wavelength = 760.26 nm
– overscan/bias
– flatfield
– CR/hot pixels
2010-02-18 TIPS/JIM 11
Test data
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Test data
Data for 1 pixel in Quad A
2010-02-18 TIPS/JIM 13
Test data
Data for 1 pixel in Quad A,
Bandpasses of UVIS filters affected by fringing
2010-02-18 TIPS/JIM 14
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 !!
2010-02-18 TIPS/JIM 15
Thickness maps
TV3 data only (other maps being developed)
Black: 13.5 µm thick, White: 17.5 µm thick
2010-02-18 TIPS/JIM 16
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
2010-02-18 TIPS/JIM 17
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
2010-02-18 TIPS/JIM 18
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
2010-02-18 TIPS/JIM 19
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...
2010-02-18 TIPS/JIM 20
(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
2010-02-18 TIPS/JIM 22
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