WFC3 SMOV UVIS and IRGeometric Distortion Calibration

and Multidrizzle

Vera Kozhurina-Plataisand

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 todistortion 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 - 0.2 pixels (8 mas) IR - 0.2 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 11444 : 47Tuc and LMC through F606W with dither POSTARGS

IR 11445 : 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 - 2004. 9 precision 1 mas

LMC observations with ACS/WFC (F606W) mean epoch - 2006. 4 precision 1 mas

Standard Astrometric Catalogs

and UVIS/IR observations

Geometric Distortion Model represented by 4th order polynomials:

U = A0 + A1X + A2Y + A3X2 + A4XY + A5Y2 + A6X3 + … + A14Y4

V = B0 + B1X + B2Y + B3X2 + B4XY + B5Y2 + B6X3 + … + B14Y4

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 Aj and Bj are obtained by Least Squares minimization

XY residuals wrt Standard Astrometric Catalogs:

UVIS XY residuals wrt 47Tuc Standard Catalog

47Tuc starsSMC 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

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CX j =−A j

B j

⎡

⎣ ⎢

⎤

⎦ ⎥×

cosθ

sinθ

⎡

⎣ ⎢

⎤

⎦ ⎥

CY j =A j

B j

⎡

⎣ ⎢

⎤

⎦ ⎥×

sinθ

cosθ

⎡

⎣ ⎢

⎤

⎦ ⎥

Aj,Bj are an arrays of coefficients from the polynomials

€

θ

Multidrizzle Results with newly derived IDCTAB

2-D XY residual map: drizzled UVIS frame of 47Tuc wrt Standard Astrometric Catalog

Scaled by factor of 200RMS of linear fit - 0.07 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 2000RMS of linear fit - 0.05 pix - 4 x times better of SMOV requirement

ERO WFC3 Images

Conclusions

Geometric Distortion derived from SMOV calibration is accurate to:

UVIS - 0.05 pixels (2 mas) 4x better than SMOV requirement

IR - 0.07 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-2009-33 by Kozhurina-Platais, et.al.

WFC3-ISR-2009-34 by Kozhurina-Platais, et.al.

New updated UVIS and IR IDCTABs will be released in January 2010

Residual Filter - Dependency Distortion