acs/wfc cte correction for point source photometry marco chiaberge acs team stsci
TRANSCRIPT
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ACS/WFC CTE correction for point source photometry
Marco ChiabergeACS Team
STScI
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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
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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”
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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
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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
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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
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47 Tuc 7’ off center ~ 2000 stars
47 Tuc 3’ off center ~ 7000 stars
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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
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Photometric test
WFC
B A
D C
Y1
Y2
Y Transfers=DY=Y1-Y2
For y=1024DY=0 Dmag=0
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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
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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
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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
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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
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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)
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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)
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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
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http://www.stsci.edu/hst/acs/performance/cte/ctecorr.py
WFC#
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Before and after correction test
BeforeGlobal loss ~16%
AfterAccuracy ~1%
F606W 150s sky ~16 e-
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Before and after correction test (pixelCTE)
BeforeGlobal loss at Y=2000~16%
Pixel-based CTEGlobal accuracy <6%(overcorrection)
F606W 150s sky ~16 e-
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Before and after correction test
BeforeGlobal loss at Y=2000 ~0.65mag
AfterAccuracy <7%
F502N 30s sky ~0.6 e-
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Comparison with the CALACS pixel-based CTE correction
CTE formula is more accurate than the pix-CTE correction
CY20
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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%)