How Does WFC3 Geometric Distortion Vary with Time ? Kozhurina-Platais.
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Transcript of How Does WFC3 Geometric Distortion Vary with Time ? Kozhurina-Platais.
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°
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
Ouv - standard orthogonal systemMXY - measured system
X
Y
Xo
YoO
M
90° +
u
v
Y
X
uo
vo
Definition of Skew
TAN( ) = B / D - rotation between Ouv and MXYTAN( ) = - (A×B + C×D) / (A×D − C×B) - skew term
u = X0 + A×X + B×Y
v = Y0 + C×X + D×Y
X0 Y
0 - offset between Ouv and MXY
A,B,C,D - linear terms
Kiselev A.A., 1989, “Nayka”, St.Petersburg, Russia
UVIS Skew (g) Variation
UVIS skew variation over 3 consecutive orbits Δ(PA_V3) = ±10° Yellow – target occultationGreen – orbital target visibilityBlue – orbital night
UVIS skew variation over 3 consecutive orbits PA_V3 no changed, POSTARG ±40”Yellow – target occultationGreen – orbital target visibilityBlue – orbital night
UVIS skew variation over 2 years : All compared w.r.t first epoch (2009) Purple - linear fit f(t) = ao+ a1g Red - constant f(t) = ao
Skew (g) = 1.7“ +/- 0.4” 1.7” scaled by 2048 ~0.02pix or 0.8mas
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
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 = X0 + A×X + B×Y
v = Y0 + 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 ~0Off-axis skew < 0.02 pix or (0.4mas)
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)=ao+a1 Red - constant f(t)=ao
Skew () = -3.6“ +/- 1.2” -3.6” scaled by 512 ~0.02pix (~2mas)
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