ACS/WFC
Astrometry and Photometry-
Flat-fielded vs. MultiDrizzled.
V.Kozhurina-Platais
A.Koekemoer
R.Gilliland
The agenda for this next TIPS/JIM Meeting is as follows:
TIPS/JIM 18 May 2006, 10:00 am, Auditorium:
1. ACS/WFC astrometry and photometry: Flat-fielded versus
Vera Kozhurina-Platais
MultiDrizzled
The ACS calibration pipeline consists of two steps:
flt
drz
Koekemoer et al (2002)
Fruchter and Hook (2002)
Astrometric and Photometric Standard in 47 Tuc
Observations:
-GO -9028 (April 2002) - 47 Tuc
- F475W filter
- same pointing and orientation (-83˚)
- dithered 0 ± 8˝ (≤170 pix)
Reductions:
- ePSF (library)  x,y,magnitude
- geometry distortion and filter dependency
Xc=x + X(x,y) + Fx(x,y)
Yc=y + Y(x,y) + Fy(x,y)
X(x,y) and Y(x,y) - 4th order polynomial
Fx(x,y) and Fy(x,y) - filter dependency table,
Anderson & King (2002, 2006)
Comparison X,Y (ePSF) in
X o  A1  A2 X f  A3Y f
overlapping FLT images
Yo  B1  B2 X f  B3Y f

Comparison ePSF photometry in
overlapping FLT images
RMS ≈ 0.01 mag in X,Y
Astrometric and Photometric Standard in 47 Tuc
~85,000 stars
ePSF - X,Y positions accurate to 0.01 pix
ePSF photometry accurate to 0.01mag
PyRaf/MultiDrizzle
-input 5 dithered images 0±5pix
-Lanczos3 kernel function
F x  
sin( x)sin 2x 
2 2 x
Analytical PSF: IRAF/PHOT/PSF
≈50,000 stars
X,Y positions accurate to ≤0.05 pixels
≈2% photometry
Xdrz,Ydrz vs. XePSF,YePSF
X ePSF  A1  A2 X drz  A3Ydrz
YePSF  B1  B2 X drz  B3Ydrz

RMS =0.014
RMS =0.014
Photometry: analytical PSF vs. ePSF
RMS ≈ 0.03 mag in X,Y
On drizzled images analytical PSF
does not replicate well
the re-sampled PSF
Good on small scale distortion



Good on large scale - non-linear terms
Good on all scale distortion
First set :
GO-9028 47 Tuc (April, 2002)
Orientation -83˚
1
1
1
ePSF  X ePSF ,YePSF ,magePSF
1
1
1
Analytical IRAF/PSF  X drz,Ydrz,magdrz

Second 
set:
GO-9443 47 Tuc (July 2002)
Orientation -172˚
2
2
2
ePSF  X ePSF ,YePSF ,magePSF
2
2
2
Analytical IRAF/PSF  X drz,Ydrz,magdrz
2
1
X drz
 A1  A2 X 1drz  A3Ydrz
Xdrz,Ydrz vs. Xdrz,Ydrz
2
1
Ydrz
 B1  B2 X 1drz  B3Ydrz

RMS=0.03
RMS=0.03
Photometry: analytical PSF vs. analytical PSF
from drizzled images
Accuracy: 0.03 mag
-image is crowded
-PSF varies across image
XePSF,YePSF
vs. XePSF,YePSF
RMS= 0.013
RMS= 0.015
Photometry: ePSF vs. ePSF
RMS ≈ 0.01 mag in X,Y
Conclusions
•Astrometric accuracy
FLT - 0.01 pix (ePSF)
DRZ - 0.03 pix (aPSF)
•Photometric accuracy
FLT - 0.01 mag (ePSF)
DRZ - 0.03 mag (aPSF)
• Geometry distortion in MultiDrizzle is incomplete (?);
there is an indication of a scale error across the chips.
• Re-sampled PSF varies across the drizzled image