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?