WFC3 SMOV UVIS and IR
Geometric Distortion Calibration
and
Multidrizzle
Vera Kozhurina-Platais
and
WFC3 team
Astrometry is the base….
WFC3 Focal Plane & Geometric Distortion
Displacement of UVIS
detector position due to
distortion is ~140 pixels
or 6 arcseconds
The UVIS and IR
channels have ~7%
linear distortion, caused
by inclination of the
camera focal plane to the
principal telescope axis
Displacement of IR
detector position due to
distortion is ~35 pixels
or 4 arcseconds
WFC3 Geometric Distortion
Goals of the SMOV Geometric Distortion Calibration:
 Required Accuracy:
UVIS - 0.2 pixels (8 mas)
IR - 0.2 pixels (26 mas)
 Deliver IDCTAB for Multidrizzle
Standard Astrometric Catalogs
and UVIS/IR observations
 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
 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
UVIS and IR Geometric Distortion
Model

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
SMC stars
47Tuc 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 High - Frequency Distortion
 XY residuals for WFC3/UVIS chip (LMC image)
High-frequency, low amplitude XY
residuals from the polynomial fit for
UVIS chip.
 XY residuals across WFC3/UVIS chip in vertical and horizontal slices
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
X (pix)
From XY detector coordinates to
IDCTAB
Schematic illustration of the WFC3 UVIS (IR) detector coordinate system and V2V3 system


= ORIENAT - PA_V3
A j  cos  
CX j    

B j  sin  
A j  sin  
CY j    

B
cos



 j 
Aj,Bj are an arrays of coefficients from the polynomials

IDCTAB - Instrument Distortion Correction Table ( Hack & Cox, 2001 ACS-ISR-0108)
Multidrizzle Results with newly derived
IDCTAB
 2-D XY residual map: drizzled IR frame of 47Tuc wrt Standard Astrometric Catalog
Scaled by factor of 200
RMS of linear fit - 0.07 pix - 3 x times
better of SMOV requirement
 2-D XY residual map: drizzled UVIS frame of 47Tuc wrt Standard Astrometric Catalog
Scaled by factor of 2000
RMS 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
3x better than SMOV requirement
- 0.07 pixels (8 mas)
New updated UVIS and IR IDCTABs will be released in January 2010
 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.
Residual Filter - Dependency
Distortion