A NEW CTE PHOTOMETRIC CORRECTION FORMULA FOR ACS

Marco Chiaberge

TIPS meeting 05/16/2012

CHARGE TRANFER EFFICIENCY (CTE) per pixel

Defined as CTE = 1 - DQ/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) CTE on ACS was not 1.0 at lunch!

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

A significant fraction (all?) of the “lost” flux goes into the “tail”

Timeline of ACS CTE corrections

Time dependent formula based on 3 epochs March 2003 – Feb 2004 Riess & Mack ISR 2004-006

Revised formula Chiaberge et al. ISR 2009-01New approach for data analysis, 4 epochsIncreased accuracy

mag = 10A • SKYB • FLUXC • Y/2000 • (MJD-52333)/365

Linear in log Dmag v log flux and log sky!

Anderson & Bedin 2010 pixel-based CTE correctionIncluded in the new CALACS

WHY DO WE STILL NEED A PHOTOMETRIC CORRECTION FORMULA?

We need to keep monitoring the time dependenceand make sure that photometry is correctly recovered

Some users may prefer to use a correction formula.

The correction formula may be more accurate for someregions of the parameter space

What should we use for extended sources?

Photometric test

WFC

B A

D C

Y1

Y2

Y Transfers=DY=Y1-Y2

For y=1024DY=0 Dmag=0

Post-SM4 ObservationsPrograms: CAL/ACS 11880, 12385, 12730

FILTERS: F606W, F502N EXP TIMES: Between 30s and 400s 5 Background levels between ~0.1 and 40e- Low sky

CR-REJECTION, no dithering1 epoch/cycle

Target: 47 Tuc (7’ off center)

Cycle 19: + pointing 3’ south of 47Tuc center for the lowest sky level 9 external orbits

47 Tuc 7’ off center ~ 2000 stars

47 Tuc 3’ off center ~ 7000 stars

ANALYSIS PROCEDURE (semi-automatic, thanks to Pey Lian!)

• 1 Generate “clean”, deep, drz image using all data

• 2 Identify saturated pixels and mark them on the DQ extension of FLT files

• 3 Mask out area around the saturated stars

• 4 Find stars on the deep mosaic, then measure flux of all stars that are detected on (both of) the single_sci files (aperture phot)

• 5 Fit Dmag vs # of transfers for different bins of flux (rejecting outliers with iterative sigma clipping)

• 6 Find the best fit model parameters to reproduce the dependence of Dmagy=2000 on Sky and Flux levels

A linear fit is performedfor each bin of flux (red lines)

Rms errors on the slope are estimated (yellow lines)

At y = 2000 Mag loss of 0.56 ± 0.07 mag

F502N 30s November 2011

CTE Correction formula assumed dependence on fluxmag = 10A • SKYB • FLUXC • Y/2000 • (MJD-52333)/365

Cycle 17

0.3e-

32e-

Cycle 19

0.6 e-

Y=

2000

2 problems:

CTE improves at low Signal levels???

Large deviations fromthe assumed linear dependence

Photometry with different detection thresholds: 3 and 10s

100e-50e-

Far from amplifiers Close to amplifiers

Detection threshold

N

Flux

Dmag

Average flux I measure in that bin

BUT THIS IS WHAT USERS NORMALLY DO!

CTE Correction formula assumed dependence on fluxmag = 10A • SKYB • FLUXC • Y/2000 • (MJD-52333)/365

Cycle 17

0.3e-

32e-

Cycle 19

0.6 e-

Cycle 19

0.6 e-Dmag = a Log flux + b , = (a b a sky), (b sky)

14e-

Much better represented by a linear relation in Dmag v Log sky

Dmag = [a Log(flux) + b] • y/2000 = a p Log(sky) + q

b = p’ Log(sky) + q’

Dmag = p Log(sky) Log(flux) + q Log(flux) + p’ Log(sky) + q’p, q, p’, q’ = p(t), q(t), p’(t), q’(t)

Dmag = p Log(sky) Log(flux) + q Log(flux) + p’ Log(sky) + q’p, q, p’, q’ = p(t), q(t), p’(t), q’(t)

Linear fit using “R”, for each epoch

The new time dependence does not assume CTE = 1at T = Tlaunch

T – Tlaunch (d)

Comparison with pixel-based CTE correction

CTE formula is more accurate than the pix-CTE correction at the lowest background levels

FUTURE WORK (from TIPS 2009)

• 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?

FUTURE WORK • Better estimate of the errors on the coefficients• Formula for different aperture radii• Extended sources?• Webpage for correction

Thanks to Jay, Linda, Roberto and Pey Lian

RESULTS

• A new accurate CTE photometric correction formula is now available• The accuracy is comparable to (or better than!) the pix-cte correction• Does not depend on the assumption of CTE = 1 at T = Tlaunch

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

D

B

C

A

47 TucF606W40s exp

Long vs short: find stars in F606W 400sMeasure the flux of all stars in F502N 30s

No correction

Corrected

iin F606W 400s