UPDATED CTE CORRECTIONFORMULAE FOR ACS
Marco Chiaberge
Pey Lian Lim, Vera Kozhurina-Platais, Marco SirianniRon Gilliland, Jennifer Mack
CHARGE TRANFER EFFICIENCY (CTE) per pixel
Defined as CTE = 1 - 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 CTEN 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
Photometric testAllows to measure the total flux lost andprovides correction formulae for photometry. Stars are positioned at different distance from the readoutamplifier thus changing the number of transfers and thereforethe impact of CTE.
WFC
HRC
D
B
C
A
A
C
D
Observations
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 levelsF606W, LOW-FLASH, MED-FLASH, HIGH-FLASHExp 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 PROBLEMSBackground levels
Cycle 13, August 2005, HRC
FLASH=0.5s
FLASH=1s
FLASH=3s
25”x25”
HOT PIXELS HRC, F606W, 30s
COSMIC RAYS WFC F606W, 400s
30”x30”
CR are not uniformly distributedCRs 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
Photometry
Aperture photometry with “phot” (iraf. noao)
R = 3 pixelsLarger aperture radii return too few stars
Background is measured in an annulus aroundeach star (r = 15 d = 3)
WFC F775W - 30s March 2006
A linear fit is performedfor each bin of flux (blue lines)
Errors on the slope are estimated (yellow lines)
mag = ax + ba = (7.9 ± 0.6) e-5
At y = 2000 this meansa loss of 0.158 ± 0.015 mag
WFC F606W – 30s March 2003
HRC F502N 360s Mar 2006
Results (magy=2000) are collected for all bins of stellar flux and sky background at each epoch
For each epoch, the coefficients A’, B, C of the formula are determined by performing a multi-linear regression fit
A’ IS TIME DEPENDENT
magy=2000 = 10A’ x SKY B x FLUX C
We assume a linear dependence with flux, sky, and # of transfersin agreement with other instruments and with internal CTE tests
We assume linear dependence on time and mag ~ 0 at t = 0 (launch)
We use magy=0 = 0
mag = 10A x SKYB x FLUXC x Y/2000 x (MJD-52333)/365
The coefficients A, B, C are not time dependent and canbe 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 (0.06 WFC, for HRC)
WFC HRC
A-0.15 ± 0.04 (0.14 ± 0.14)
-0.44 ± 0.05(-0.89 ± 0.26)
B-0.25 ± 0.01
(-0.31 ± 0.02) -0.15 ± 0.02(-0.24 ± 0.13)
C-0.44 ± 0.01(-0.64 ± 0.05)
-0.36 ± 0.01(-0.21 ± 0.07)
TESTING THE FORMULAE
Apply the correction to data from different epochs, for differentsky 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 lossesat different epochs
F502N - 30s March 2005 F775W – 30s March 2005
HRC F606W 30s - March 2003 March 2006
ePSF photometry
Aperture photometry PSF fitting
WFC F606W 30s vs 400s - March 2006
COMPARISON WITH PREVIOUS FORMULA
Sky = 2e Flux = 650
Sky = 2e Flux = 2500e
Sky = 2e Flux = 650
ACS fail. SM4
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?