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 -  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) CTE on ACS was not 1.0 at lunch! 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 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 Revised formula Chiaberge et al. ISR New approach for data analysis, 4 epochs Increased accuracy  mag = 10 A SKY B FLUX C Y/2000 (MJD-52333)/365 Linear in log  mag v log flux and log sky! Anderson & Bedin 2010 pixel-based CTE correction Included in the new CALACS

WHY DO WE STILL NEED A PHOTOMETRIC CORRECTION FORMULA? We need to keep monitoring the time dependence and make sure that photometry is correctly recovered Some users may prefer to use a correction formula. The correction formula may be more accurate for some regions of the parameter space What should we use for extended sources?

Photometric test WFC B A D C Y1 Y2 Y Transfers=  Y=Y1-Y2 For y=1024  Y=0  mag=0

Post-SM4 Observations Programs: CAL/ACS 11880, 12385, FILTERS: F606W, F502N EXP TIMES: Between 30s and 400s 5 Background levels between ~0.1 and 40e - Low sky CR-REJECTION, no dithering 1 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  mag 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  mag y=2000 on Sky and Flux levels

A linear fit is performed for 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 flux  mag = 10 A SKY B FLUX C Y/2000 (MJD-52333)/365 Cycle e - 32e - Cycle e - Y= problems: CTE improves at low Signal levels??? Large deviations from the assumed linear dependence

Photometry with different detection thresholds: 3 and 10 

100e - 50e - Far from amplifiersClose to amplifiers Detection threshold N Flux  mag Average flux I measure in that bin BUT THIS IS WHAT USERS NORMALLY DO!

CTE Correction formula assumed dependence on flux  mag = 10 A SKY B FLUX C Y/2000 (MJD-52333)/365 Cycle e - 32e - Cycle e -

Cycle e -  mag =  Log flux +   sky),  sky) 14e - Much better represented by a linear relation in  mag v Log sky

 mag = [  Log(flux) +  y/2000  p Log(sky) + q  = p’ Log(sky) + q’  mag = p Log(sky) Log(flux) + q Log(flux) + p’ Log(sky) + q’ p, q, p’, q’ = p(t), q(t), p’(t), q’(t)

 mag = 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 = 1 at T = T launch T – T launch (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 = T launch

Photometric test 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. WFC D B C A

47 Tuc F606W 40s exp

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

No correction Corrected i in F606W 400s