ACS/WFC

Geometric Distortion and CTE

V. Kozhurina-Platais & ACS team

ACS/WFC geometric distortion is changing linearly with timeAnderson J., ISR-ACS/WFC , August 2006 a) Standard astrometric reference field (47Tuc) ~ 53,000 stars b) User friendly software - XYM2MAT

Our plan was to: a) Check the time-dependency on FLT images; b) Implement the time dependency in MultiDrizzle c) Verify the implementation

The observations of 47Tuc

• F606W filter • 2002-2006 (PIP-9018-10771)• different orientations• different postargs• different exposures: 22 - 1200s

The reductions:

• effective PSF fitting on FLT - X,Y mag; • standard frame: 2002 March, 690s, F606W,=-163

€

tgθ =B

D

tg(θ + γ ) = −A

C

tgγ = −A ⋅B + C ⋅DA ⋅D − C ⋅B

Ouv - standard orthogonal system MXY - measured system

Kiselev A.A, 1989, pub.,”Nauka”, St.Petersburg, Russia

€

u

v

⎛

⎝ ⎜

⎞

⎠ ⎟=

AB

CD

⎡

⎣ ⎢

⎤

⎦ ⎥X − Xo

Y −Yo

⎛

⎝ ⎜

⎞

⎠ ⎟+

uo

vo

⎛

⎝ ⎜

⎞

⎠ ⎟

X

Y

Xo

YoO

M

90+

u

v

Y

X

uo

vo

720 s , =-163.2 2002.4

22 s , =- -23.9 2004.7

22 s, = 51.9 2005.9

22 s , = -117.9 2002.4

22 s , = -67.82005.0

22 s , = -122.92004.5

The skew term as

€

=arctan −A ⋅B + C ⋅DA ⋅D − C ⋅B

⎛ ⎝ ⎜

⎞ ⎠ ⎟

€

α = 0.095 + 0.090 ⋅(date − 2004.5)/2.5

β = −0.029 − 0.030 ⋅(date − 2004.5)/2.5

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fx = (Xc − 2048) 2048

fy = (Yc − 2048) 2048

Xo = fx + β ⋅ fx + α ⋅ fy

Yo = fy + β ⋅ fy + α ⋅ fx

Many variables: PID - 10368:• The same pointing

• Exposures - 400s, 30s

• Two orientations : +154.8, -29.4

exposure

pointing

orientation

epoch

LAMP=OFF/OFF

400sec--> 400sec, 154.7-> 154.7

400sec--> 400sec, 154.7-> -29.4

30sec--> 400sec, 154.7-> 154.7

30sec--> 400sec, 154.7-> 154.7

30sec--> 400sec, 154.7-> 154.7

30sec--> 400sec, -29.4->-29.4

LAMP=OFF/OFF

LAMP=OFF/OFF

LAMP=MED/OFF

LAMP=HIGH/OFF

LAMP=LOW/OFF

Schematic illustration of the WFC1 and WFC2 plane:- Red arrows show the readout direction- Green color shows the PSF profile affected by CTE

Conclusions The CTE-induced centroid shift depends on: magnitude of a star position on a CCD chip sky background The amplitude of CTE-induced centroid shift is ~ 0.1 pix CTE effect leads to a discontinuity in the reference frame Accounting for CTE effect is critical to reach a 1 mas accuracy requirement for JWST standard astrometric field