ACS/WFC CTE correction for point source photometry

Marco ChiabergeACS Team

STScI

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 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 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

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

New model derived in 2013 to provide more accurate correctionChiaberge ISR 2012-05

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

Post-SM4 ObservationsPrograms: CAL/ACS 11880, 12385, 12730, 13155, 13592

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 19-20-21: + 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 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

Photometric test

WFC

B A

D C

Y1

Y2

Y Transfers=DY=Y1-Y2

For y=1024DY=0 Dmag=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 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 0.69 ± 0.03 mag

F502N 30s Cycle 20

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:

Incompleteness at low fluxes

Large deviations fromthe assumed linear dependence

Cycle 19

0.6 e-

Dmag = a Log flux + b

, = (a b a sky), (b sky)

14e-

Well represented by a linear relation in Dmag v Log sky

Dmag = [a Log(flux) + b] x Y/2000

= a p Log(sky) + qb = p’ Log(sky) + q’

Dmag = [p Log(sky) Log(flux) + q Log(flux) + p’ Log(sky) + q’] x Y/2000p, 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

Dmag = [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

http://www.stsci.edu/hst/acs/performance/cte/ctecorr.py

WFC#

Before and after correction test

BeforeGlobal loss ~16%

AfterAccuracy ~1%

F606W 150s sky ~16 e-

Before and after correction test (pixelCTE)

BeforeGlobal loss at Y=2000~16%

Pixel-based CTEGlobal accuracy <6%(overcorrection)

F606W 150s sky ~16 e-

Before and after correction test

BeforeGlobal loss at Y=2000 ~0.65mag

AfterAccuracy <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%)