1. 2010-02-18 TIPS/JIM1 The “Happy Bunny:” Characterizing Long- Wavelength Fringing in WFC3/UVIS Mike Wong INS/WFC3 Visiting Scientist

2. 2010-02-18 TIPS/JIM2 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, Andr é Martel, Linda Dressel Fitting data with the model Correcting on-orbit data

3. 2010-02-18 TIPS/JIM3 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

4. 2010-02-18 TIPS/JIM4 Filters affected by fringing

5. 2010-02-18 TIPS/JIM5 Monochromatic fringe flat TV3 data 977 nm

6. 2010-02-18 TIPS/JIM6 Histogram TV3 data 977 nm

7. 2010-02-18 TIPS/JIM7 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

8. 2010-02-18 TIPS/JIM8 Model schematic Table: Malumuth et al. (2003)

9. 2010-02-18 TIPS/JIM9 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

10. 2010-02-18 TIPS/JIM10 Test data Basic processing –DCL chip 2, commanded wavelength = 760.26 nm –overscan/bias –flatfield –CR/hot pixels

11. 2010-02-18 TIPS/JIM11 Test data

12. 2010-02-18 TIPS/JIM12 Test data Data for 1 pixel in Quad A

13. 2010-02-18 TIPS/JIM13 Test data Data for 1 pixel in Quad A, Bandpasses of UVIS filters affected by fringing

14. 2010-02-18 TIPS/JIM14 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 !!

15. 2010-02-18 TIPS/JIM15 Thickness maps TV3 data only (other maps being developed) Black: 13.5 µm thick, White: 17.5 µm thick

16. 2010-02-18 TIPS/JIM16 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

17. 2010-02-18 TIPS/JIM17 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

18. 2010-02-18 TIPS/JIM18 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

19. 2010-02-18 TIPS/JIM19 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 = n cmd / n opt So finding a constant scale factor is like finding an error in the index of refraction for the DCL experiments...

20. 2010-02-18 TIPS/JIM20 (or n error) Data could be construed as indicating an experimental index of refraction of 1.029 ± 0.003 This is close to the index of refraction of aerogel of 1.024–1.026 (Poelz & Riethm ü ller, 1982, Nuc. Instr. Meth. Phys. Res. 195, 491- 503)

21. On-orbit test data

22. 2010-02-18 TIPS/JIM22 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