How Does WFC3 Geometric
Distortion Vary with Time ?
Kozhurina-Platais
Observations
 Ω Cen observed with the WFC3/UVIS and IR F606W & F160W filters
 Wide Range PA_V3 and Large POSTARG ±40”
L_Flat and Distortion Cal Programs:
POSTARG ±40” & wide range PA_V3
Special Cal Program (12094)
Δ(PA_V3) = ±10°
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
μα = -4.9mas/yr , μδ = -3.5mas/yr
internal velocity dispersion ~1 mas/yr
 Defined X,Y measured system from positions on all other UVIS/IR drizzled
images
Definition of Skew
Y
v
Ouv - standard orthogonal system
MXY - measured system

Y
u = X0 + A×X + B×Y
v = Y0 + C×X + D×Y
vo
u
Yo
O
uo
X0 Y0
- offset between Ouv and MXY
A,B,C,D - linear terms
M
Xo

X
90° + 
X
TAN(  ) = B / D - rotation between Ouv and MXY
TAN(  ) = - (A×B + C×D) / (A×D − C×B) - skew term
Kiselev A.A., 1989, “Nayka”, St.Petersburg, Russia
UVIS Skew () Variation
UVIS skew variation over 2 years :
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UVIS skew variation over 3 consecutive orbits
Δ(PA_V3) = ±10°
Yellow – target occultation
Green – orbital target visibility
Blue – orbital night
All compared w.r.t first epoch (2009)
Purple - linear fit f(t) = ao+ a1
Red - constant f(t) = ao
Skew () = 1.7“ +/- 0.4”
1.7” scaled by 2048 ~0.02pix or 0.8mas
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 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
ACS/WFC & WFC3/UVIS Skew
 Anderson, J., “Variation of the Distortion Solution of the WFC”, 2007, ACS-ISR-07-08
u = X0 + A×X + B×Y
v = Y0 + C×X + D×Y
(A − D) / 2
(B + C) / 2
- on - axis skew
- off - axis skew
ACS/WFC skew linearly changed vs time
Off-axis skew ~0.3 WFC pixels (15mas) over 5-6 yr
 Kozhurina-Platais, V., and Petro, L., 2012 , WFC3-ISR-12-03
On-axis skew ~0
Off-axis skew < 0.02 pix or (0.4mas)
WFC3/IR Skew () Variation
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)=ao+a1
Red - constant f(t)=ao
 Skew () = -3.6“ +/- 1.2”
 -3.6” scaled by 512 ~0.02pix (~2mas)
On-axis skew < 0.02 pix (2mas)
Off-axis skew ~0
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
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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
UVIS XY residuals wrt 47Tuc and LMC Standard Catalog
IR XY residuals wrt 47Tuc
Standard Catalog
UVIS XY residuals wrt Omega Cen Standard Catalog