Marco Chiaberge
Pey Lian Lim, Vera Kozhurina-Platais, Marco Sirianni
Ron Gilliland, Jennifer Mack

CHARGE TRANFER EFFICIENCY (CTE) per pixel
Defined as CTE = 1 -
D
Q/Q = 1 - CTI
For an ideal CCD CTE = 1.0
For real CCDs CTE < 1 manufacturing imperfections in the crystalline lattice radiation damage (increasing with time)
The total CTE is CTE N significant effect for large CCDs
CTE depends on flux, sky level, # of transfers

The effect of CTE on stellar photometry is to reduce the measured flux
(up to ~20% or more)
A fraction of the “lost” flux goes into the “tail”
But significant flux is just lost and cannot be recovered

Allows to measure the total flux lost and provides correction formulae for photometry.
Stars are positioned at different distance from the readout amplifier thus changing the number of transfers and therefore the impact of CTE.
D C
A
C
B A
HRC
D
WFC

Programs: CAL/ACS 9648, 10043, 10368 (PI:A. Riess),
10730 (PI: Chiab)
FILTERS: F606W, F775W, F502N
EXP TIMES: 30s, 360s for HRC; 30s and 400s for WFC
(for HRC/F502N, 360s only)
Post-flash used to achieve higher background levels
F606W, LOW-FLASH, MED-FLASH, HIGH-FLASH
Exp times: 30s, 360s
No CR-REJECTION, no dithering
2 Observations/year (Cycle 11-13)
Cycle 14 only 1 epoch (March 2006), no post-flash
Cycle 15 1 test visit (Jul- Sept 2006) to test CR-SPLIT + dithering

POTENTIAL PROBLEMS
Background levels
Cycle 13, August 2005, HRC
FLASH=0.5s
FLASH=1s
FLASH=3s

HOT PIXELS HRC, F606W, 30s
25”x25”

COSMIC RAYS WFC F606W, 400s
30”x30”
CR are not uniformly distributed
CRs and hot pixels may affect CTE estimates
More important for low CTI, they increase the error

Published results before our new analysis
WFC time dependent formula based on 3 epochs
March 2003 – Feb 2004
Riess & Mack ISR 2004-006
HRC time dependent formula based on 1 epoch
March 2003
Riess ISR 2003 – 009

ANALYSIS PROCEDURE
• IRAF, SM, some IDL
• Generate “clean”, deep, drz image using all data
• Identify cosmic rays, hot pixels and saturated pixels and mark them on DQ extension of FLT files
• Mask out area around the saturated stars
• Measure flux of “good” stars only on the single_sci files
• Reject outliers (sigma clipping, 3 itearations)
• Fit delta mag vs # of transfers for different bins of flux

HRC – F606W 360sec

WFC F775W – 400s

Aperture photometry with “phot” (iraf. noao)
R = 3 pixels
Larger aperture radii return too few stars
Background is measured in an annulus around each star (r = 15 d = 3)

WFC F775W - 30s March 2006
A linear fit is performed for each bin of flux
(blue lines)
Errors on the slope are estimated
(yellow lines)
D mag = ax + b a = (7.9 ± 0.6) e-5
At y = 2000 this means a loss of 0.158 ± 0.015 mag

WFC F606W – 30s March 2003

HRC F502N 360s Mar 2006

Results (
D mag y=2000
) are collected for all bins of stellar flux and sky background at each epoch
We assume a linear dependence with flux, sky, and # of transfers in agreement with other instruments and with internal CTE tests
D mag y=2000
= 10
A’ x SKY B x FLUX C
For each epoch, the coefficients A’, B, C of the formula are determined by performing a multi-linear regression fit
A’ IS TIME DEPENDENT
We assume linear dependence on time and
D mag ~ 0 at t = 0 (launch)
We use
D mag y=0
= 0
D mag = 10 A x SKY B x FLUX C x Y/2000 x (MJD-52333)/365

The coefficients A, B, C are not time dependent and can be averaged between epochs
Weighted means are calculated using 4 epochs
(March 2003, March 2005, Aug 2005, March 2006)
Typical errors on the coefficients (single epoch) ≤ 0.1
B is the coefficient with the largest spread among epochs
( s =
0.06 WFC, s = 0.04 for HRC)
A
B
C
WFC
-0.15 ± 0.04
(0.14 ± 0.14)
-0.25 ± 0.01
(-0.31 ± 0.02)
-0.44 ± 0.01
(-0.64 ± 0.05)
HRC
-0.44 ± 0.05
(-0.89 ± 0.26)
-0.15 ± 0.02
(-0.24 ± 0.13)
-0.36 ± 0.01
(-0.21 ± 0.07)

TESTING THE FORMULAE
Apply the correction to data from different epochs, for different sky levels using the same data that were used to derive the formula
Apply the correction to photometry performed with ePSF
(both aperture photometry and PSF fitting)
Compare prediction of the formula with measured mag losses at different epochs

F502N - 30s March 2005 F775W – 30s March 2005

HRC F606W 30s - March 2003 March 2006

ePSF photometry
WFC F606W 30s vs 400s - March 2006
Aperture photometry PSF fitting

COMPARISON WITH PREVIOUS FORMULA
ACS fail.
SM4
Sky = 2e
Flux = 650
Sky = 2e
Flux = 2500e
Sky = 2e
Flux = 650

RESULTS and FUTURE WORK
• Data from 4 epochs (March 2003 through March 2006) were analyzed using a new data analysis strategy aimed at obtaining “cleaner” results
• We derived time-dependent correction formulae for both
HRC and WFC that are accurate at the level of a few percent
•
New observations after SM4 using CR-REJ and possibly dithering
•
Procedures should be made automatic (or semi-automatic)
•
Formula for different aperture radii
• Better data might lead to a better characterization
Different form of the formula?