1. How Does WFC3 Geometric Distortion Vary with Time ? Kozhurina-Platais

2.  Ω Cen observed with the WFC3/UVIS and IR F606W & F160W filters  Wide Range PA_V3 and Large POSTARG ±40” Observations L_Flat and Distortion Cal Programs: POSTARG ±40” & wide range PA_V3 Special Cal Program (12094) Δ(PA_V3) = ±10°

3. Reduction  Multidrizzled UVIS and IR images:  correcting for distortion   correcting for velocity aberration (VAFACTOR)  Measured X,Y positions on all drizzled UVIS/IR images  Defined u,v reference system from X,Y on first UVIS/IR drizzled image (2009)   reducing the epoch difference ~ 4-5 years   reducing scatter in measured positions due to μ α = -4.9mas/yr, μ δ = -3.5mas/yr internal velocity dispersion ~1 mas/yr  Defined X,Y measured system from positions on all other UVIS/IR drizzled images

4. Ouv - standard orthogonal system MXY - measured system X  Y XoXo YoYo O M  90° +  u v Y X uouo vovo Definition of Skew TAN(  ) = B / D - rotation between Ouv and MXY TAN(  ) = - (A×B + C×D) / (A×D − C×B) - skew term u = X 0 + A×X + B×Y v = Y 0 + C×X + D×Y X 0 Y 0 - offset between Ouv and MXY A,B,C,D - linear terms Kiselev A.A., 1989, “Nayka”, St.Petersburg, Russia

5. UVIS Skew (  ) Variation UVIS skew variation over 3 consecutive orbits Δ(PA_V3) = ±10° Yellow – target occultation Green – orbital target visibility Blue – orbital night UVIS skew variation over 3 consecutive orbits PA_V3 no changed, POSTARG ±40” Yellow – target occultation Green – orbital target visibility Blue – orbital night UVIS skew variation over 2 years :  All compared w.r.t first epoch (2009)  Purple - linear fit f(t) = a o + a 1   Red - constant f(t) = a o  Skew (  ) = 1.7“ +/- 0.4”  1.7” scaled by 2048 ~0.02pix or 0.8mas

6. UVIS Skew (  ) Variation and HST Focus? UVIS skew variation over 3 consecutive orbits (~7.5 h), July, 2010  Blue – temperature-based model of the UVIS focus  Red – calculated skew UVIS measured focus and UVIS temperature based focus model  Blue – temperature-based focus model  Red – measured UVIS focus  Difference between measured and its model due to e.g: - combination of Sun angle; - off-nominal HST roll-angle; - difference in temperature on different part of HST Variation of Skew on a orbital time scale is related to focus variations (e.g. orbital breathing)  Focus variation effects PSF centroid  changes skew (astrometric scale)  Deviation of skew ±8” scaled by 2048 pixel ≈ 0.05 pixels or 2mas  Effect is too complex to model astrometric scale change on HST orbital time scale

7.  Anderson, J., “Variation of the Distortion Solution of the WFC”, 2007, ACS-ISR-07-08 ACS/WFC & WFC3/UVIS Skew  Kozhurina-Platais, V., and Petro, L., 2012, WFC3-ISR-12-03 u = X 0 + A×X + B×Y v = Y 0 + C×X + D×Y (A − D) / 2 - on - axis skew (B + C) / 2 - off - axis skew ACS/WFC skew linearly changed vs time Off-axis skew ~0.3 WFC pixels (15mas) over 5-6 yr On-axis skew ~0 Off-axis skew < 0.02 pix or (0.4mas)

8. WFC3/IR Skew (  ) Variation On-axis skew < 0.02 pix (2mas) Off-axis skew ~0 IR skew (  ) variation over 2 years time scale :  All compared w.r.t first epoch i.e.2009 (top)  Average skew per epoch (bottom) Purple - linear fit f(t)=a o +a 1  Red - constant f(t)=a o  Skew (  ) = -3.6“ +/- 1.2”  -3.6” scaled by 512 ~0.02pix (~2mas)

9. Conclusions  WFC3/UVIS and IR geometric distortion is stable on the 2-year time scale  WFC3/UVIS and IR skew linearly changes over short time interval of orbital target visibility Deviation of UVIS skew ±8” scaled by 2048 pixel ~ 2mas Deviation of IR skew ±20” scaled by 512 pixel ~7mas  Short term variation of UVIS and IR skew are likely due to the orbital breathing Acknowledgments  Larry Petro and Knox Long - suggestions and comments on ISR  Bill Januszewski - help on design two special calibration programs CAL-12094 & CAL-12353  George Chapman – discussion on orbital target occultation and target visibility  Matt Lallo – discussion on HST focus model  Matt Bourque – comments on slides

10. UVIS XY residuals wrt 47Tuc and LMC Standard Catalog UVIS XY residuals wrt Omega Cen Standard Catalog IR XY residuals wrt 47Tuc Standard Catalog