ACS/WFC CTE correction for point source photometry Marco Chiaberge ACS Team STScI

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 and time

The effect of CTE on stellar photometry is to reduce the measured flux A significant fraction (all?) of the “lost” flux goes into the “trail”

Timeline of ACS CTE photometric 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 Anderson & Bedin 2010 pixel-based CTE correction Included in the new CALACS New model derived in 2013 to provide more accurate correction Chiaberge ISR

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

Post-SM4 Observations Programs: CAL/ACS 11880, 12385, 12730, 13155, 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 : + 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

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

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

ANALYSIS PROCEDURE 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 (ap=3 pix) that are detected on 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 0.69 ± 0.03 mag F502N 30s Cycle 20

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: Incompleteness at low fluxes Large deviations from the assumed linear dependence

100e - 50e - Far from amplifiersClose to amplifiers Detection threshold N Flux  mag Average flux I measure in that bin

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

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

GLOBAL FIT WITH ALL POST-SM4 EPOCHS Δmag (Y, t, SKY, FLUX) = [p1 Log(SKY) Log(FLUX) t + p2 Log(SKY) Log(FLUX) + p’1 Log(SKY) t + +q1Log(Flux) t + p’2 Log(SKY) + q2Log(FLUX) + q’1 t + q’2] * Y / 2000 Accuracy

WFC#

Before and after correction test Before Global loss ~16% After Accuracy ~1% F606W 150s sky ~16 e-

Before and after correction test (pixelCTE) Before Global loss at Y=2000 ~16% Pixel-based CTE Global accuracy <6% (overcorrection) F606W 150s sky ~16 e-

Before and after correction test Before Global loss at Y=2000 ~0.65mag After Accuracy <7% F502N 30s sky ~0.6 e-

Comparison with the CALACS pixel-based CTE correction CTE formula is more accurate than the pix-CTE correction CY20

CONCLUSIONS Cycle 20 data allowed to derive improved coefficients Photometric correction is now more accurate than ever Global accuracy is <3% for most background levels Largest deviations for very low levels (~7%)