ACS/WFC
Geometric Distortion and CTE
V. Kozhurina-Platais & ACS team
ACS/WFC geometric distortion is changing linearly with time
Anderson 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
Kiselev A.A, 1989, pub.,”Nauka”, St.Petersburg, Russia
Ouv - standard orthogonal system
MXY - measured system
Y
v

u AB X  X o  uo 
   
  
v  CDY  Yo  vo 
Y
vo
u
uo

M
X
X
B
tg 
D
Yo
O

Xo
90+
A
C
A B C D
tg  
ADC  B
tg(   )  
720 s ,  =-163.2
2002.4
22 s ,  = -122.9
2004.5
22 s ,  =- -23.9
2004.7
22 s ,  = -67.8
2005.0
22 s ,  = -117.9
2002.4
22 s,  = 51.9
2005.9
The skew term as
 A  B  C  D 
  arctan

 A  D  C  B 
  0.095  0.090  (date 2004.5)/2.5
  0.029  0.030  (date 2004.5)/2.5

f x  (Xc  2048) 2048

f y  (Yc  2048) 2048
Xo  fx    fx    fy
Yo  f y    f y    f x

Many variables:
PID - 10368:
 exposure
•
•
•
 pointing
 orientation
 epoch
The same pointing
Exposures - 400s, 30s
Two orientations : +154.8, -29.4
400sec--> 400sec, 154.7-> 154.7
LAMP=OFF/OFF
400sec--> 400sec, 154.7-> -29.4
LAMP=OFF/OFF
30sec--> 400sec, 154.7-> 154.7
LAMP=OFF/OFF
30sec--> 400sec, 154.7-> 154.7
LAMP=MED/OFF
30sec--> 400sec, 154.7-> 154.7
LAMP=HIGH/OFF
30sec--> 400sec, -29.4->-29.4
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