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 2009-01
New approach for data analysis, 4 epochs
Increased accuracy
Dmag = 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
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
D
C
Y1
B
Y2
A
Y Transfers=DY=Y1-Y2
For y=1024
DY=0
Dmag=0
WFC
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 40eLow 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 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
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
CTE Correction formula assumed dependence on flux
Dmag = 10A • SKYB • FLUXC • Y/2000 • (MJD-52333)/365
2 problems:
Cycle 17
Cycle 19
Y=2000
CTE improves at low
Signal levels???
0.6 e0.3e-
32e-
Large deviations from
the assumed linear dependence
Photometry with different detection thresholds: 3 and 10s
BUT THIS IS WHAT USERS NORMALLY DO!
Average flux I measure in that bin
Dmag
N
50e-
Far from amplifiers
100e-
Flux
Close to amplifiers
Detection threshold
CTE Correction formula assumed dependence on flux
Dmag = 10A • SKYB • FLUXC • Y/2000 • (MJD-52333)/365
Cycle17
19
Cycle
0.6 e0.3e-
32e-
Much better represented by a linear relation in Dmag v Log sky
Cycle 19
Dmag = a Log flux + b
a,b = a(sky), b(sky)
0.6 e14e-
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 = 1
at 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
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 Pey Lian
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
B
WFC
C
A
47 Tuc
F606W
40s exp
Long vs short: find stars in F606W 400s
Measure the flux of all stars in F502N 30s
in F606W 400s
i
No correction
Corrected