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

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- Slide 1
- How Does WFC3 Geometric Distortion Vary with Time ? Kozhurina-Platais
- Slide 2
- 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) (PA_V3) = 10
- Slide 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 = -4.9mas/yr, = -3.5mas/yr internal velocity dispersion ~1 mas/yr Defined X,Y measured system from positions on all other UVIS/IR drizzled images
- Slide 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( ) = - (AB + CD) / (AD CB) - skew term u = X 0 + AX + BY v = Y 0 + CX + DY X 0 Y 0 - offset between Ouv and MXY A,B,C,D - linear terms Kiselev A.A., 1989, Nayka, St.Petersburg, Russia
- Slide 5
- UVIS Skew ( ) Variation UVIS skew variation over 3 consecutive orbits (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
- Slide 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
- Slide 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 + AX + BY v = Y 0 + CX + DY (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)
- Slide 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)
- Slide 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
- Slide 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