WFC3 SMOV UVIS and IR Geometric Distortion Calibration and Multidrizzle Vera Kozhurina-Platais and WFC3 team

Astrometry is the base….

The UVIS and IR channels have ~7% linear distortion, caused by inclination of the camera focal plane to the principal telescope axis Displacement of UVIS detector position due to distortion is ~140 pixels or 6 arcseconds Displacement of IR detector position due to distortion is ~35 pixels or 4 arcseconds WFC3 Focal Plane & Geometric Distortion

 Required Accuracy: UVIS pixels (8 mas) IR pixels (26 mas) WFC3 Geometric Distortion  Deliver IDCTAB for Multidrizzle Goals of the SMOV Geometric Distortion Calibration:

 Observations: SMOV calibration programs (PI-L.Dressel): UVIS : 47Tuc and LMC through F606W with dither POSTARGS IR : 47Tuc and LMC through F160W with dither POSTARGS  Standard Astrometric Fields (Anderson, ACS-ISR-2006 & private communication) 47Tuc observations with ACS/WFC (F606W) mean epoch precision 1 mas LMC observations with ACS/WFC (F606W) mean epoch precision 1 mas Standard Astrometric Catalogs and UVIS/IR observations

 Geometric Distortion Model represented by 4th order polynomials: U = A 0 + A 1 X + A 2 Y + A 3 X 2 + A 4 XY + A 5 Y 2 + A 6 X 3 + … + A 14 Y 4 V = B 0 + B 1 X + B 2 Y + B 3 X 2 + B 4 XY + B 5 Y 2 + B 6 X 3 + … + B 14 Y 4 where U,V are tangential-plane positions in the astrometric catalog X,Y are measured positions in UVIS chips or IR frames, normalized to the center of chip/frame A j and B j are obtained by Least Squares minimization  XY residuals wrt Standard Astrometric Catalogs: UVIS XY residuals wrt 47Tuc Standard Catalog 47Tuc stars SMC stars IR XY residuals wrt 47Tuc Standard Catalog IR H (F160W) magnitudes wrt 47Tuc Standard Catalog V magnitudes (F606W) CMD of globular cluster 47Tuc UVIS and IR Geometric Distortion Model

 XY residuals for WFC3/UVIS chip (LMC image) UVIS High - Frequency Distortion High-frequency, low amplitude XY residuals from the polynomial fit for UVIS chip.  XY residuals across WFC3/UVIS chip in vertical and horizontal slices X (pix) Y (pix) Structure of high - frequency distortion depends on location on the CCD chip. This is fine-structure in the F606W filter most likely due to manufacturing process

From XY detector coordinates to IDCTAB Schematic illustration of the WFC3 UVIS (IR) detector coordinate system and V2V3 system IDCTAB - Instrument Distortion Correction Table ( Hack & Cox, 2001 ACS-ISR-0108) = ORIENAT - PA_V3 A j,B j are an arrays of coefficients from the polynomials

Multidrizzle Results with newly derived IDCTAB  2-D XY residual map: drizzled UVIS frame of 47Tuc wrt Standard Astrometric Catalog Scaled by factor of 200 RMS of linear fit pix - 3 x times better of SMOV requirement  2-D XY residual map: drizzled IR frame of 47Tuc wrt Standard Astrometric Catalog Scaled by factor of 2000 RMS of linear fit pix - 4 x times better of SMOV requirement

ERO WFC3 Images

Conclusions  Geometric Distortion derived from SMOV calibration is accurate to: UVIS pixels (2 mas) 4x better than SMOV requirement IR pixels (8 mas) 3x better than SMOV requirement  Geometric Distortion will be further improved in Cycle 17: UVIS and IR multi-wavelength geometric distortion dependency applying better centering technique using ePSF library  Detailed description of the Geometric Distortion calibration is in: WFC3-ISR by Kozhurina-Platais, et.al. WFC3-ISR by Kozhurina-Platais, et.al.  New updated UVIS and IR IDCTABs will be released in January 2010

Residual Filter - Dependency Distortion