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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 - D Q/Q = 1 - CTI For an ideal CCD CTE = 1.0 For real CCDs CTE < 1 Manufacturing imperfections in the crystalline lattice

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slide1

A NEW CTE PHOTOMETRIC

CORRECTION FORMULA FOR ACS

Marco Chiaberge

TIPS meeting 05/16/2012

slide2

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

slide3

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”

slide4

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 2009-01

New approach for data analysis, 4 epochs

Increased 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 correction

Included in the new CALACS

slide5

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
Photometric test

D

C

Y1

Y2

B

A

Y Transfers=DY=Y1-Y2

For y=1024

DY=0

Dmag=0

WFC

slide7

Post-SM4 Observations

Programs: 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 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

slide8

47 Tuc 7’ off center ~ 2000 stars

47 Tuc 3’ off center ~ 7000 stars

slide9

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_scifiles (aperture phot)
  • 5 Fit Dmagvs # 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
slide10

F502N 30s November 2011

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

slide11

CTE Correction formula assumed dependence on flux

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

2 problems:

CTE improves at low

Signal levels???

Large deviations from

the assumed linear dependence

Cycle 17

Cycle 19

Y=2000

0.6 e-

0.3e-

32e-

slide13

BUT THIS IS WHAT USERS NORMALLY DO!

Average flux I measure in that bin

Dmag

N

50e-

100e-

Flux

Far from amplifiers

Close to amplifiers

Detection threshold

slide14

CTE Correction formula assumed dependence on flux

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

Cycle 19

Cycle 17

0.6 e-

0.3e-

32e-

slide15

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

Cycle 19

Dmag = aLog flux + b

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

0.6 e-

14e-

slide16

Dmag = [aLog(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)

slide17

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 = 1

at T = Tlaunch

T – Tlaunch (d)

slide18

Comparison with pixel-based CTE correction

CTE formula is more accurate than the pix-CTE correction

at the lowest background levels

slide19

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

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 PeyLian

photometric test1
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.

D

C

B

A

WFC

slide21

47 Tuc

F606W

40s exp

slide22

Long vs short: find stars in F606W 400s

Measure the flux of all stars in F502N 30s

slide23

i

in F606W 400s

No correction

Corrected