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Pixel-based CTE Correction of ACS/WFC:! CTE Losses for the Smallest Pixel Packets!
Jay ANDERSON and the ACS Team! ABSTRACT!
Space Telescope Science Institute, Baltimore MD!
In our recent PASP article (Anderson & Bedin 2010, AB10), we show that it is possible to use warm pixels (WPs) in dark exposures as delta-function experiments to probe how electrons are trapped and released as charge is transferred down the detector. The model has shown considerable success in correcting science images for CTE (charge-transfer efficiency) blurring. Unfortunately, the standard 1000s dark exposures have so many bright WPs that the trails of the fainter WPs (those with less than about 50 e-) overlap, and it is not possible to calibrate the trail profiles. Here we illustrate how shorter darks can be used to fill in this missing information.!

THE NEW CALACS!

Putting the electrons back where they belong!

INTRODUCTION!
Massey et al. (2010) developed a pixel-based correction for CTE from a study of WP trails in COSMOS science images. Unfortunately the high background of the images (65 e-) did not allow charge-transfer losses of small electron packets to be explored.! Anderson & Bedin (2010, AB10) pushed this technique further, studying the WPs in the standard 1000s dark exposures; this enabled them to study much smaller electron packets on much lower backgrounds.! The AB10 model is based on a purely empirical! parametrization of how charge is trapped and released as it is shuffled down the detector.! The delta-function WPs allow us to construct a forward model of the charge-transfer process.!
lost
from
WP
survive
intact
to readout
register

THE NEED FOR DARKER DARKS!
Unfortunately, the standard dark exposures are not completely dark. The image on the left below shows that the standard ~1000s darks are crowded with WPs of more than 50 e-. All five images show a region far from the readout register and have a grayscale from -5 e- to 50 e-.! By taking a variety of shorter darks, we can isolate the fainter WPs and directly probe the interplay between charge-packet size and CTE losses.!

traps
within
traversed
pixels




gained by
downstream
pixels





some electrons are a WP generated
delayed by traps that are the resulting
during 1040s
distribution
encountered
dark exposure
of charge
during charge transfer through the pixel grid


net
change


1040s dark
(corrected)


1040s dark
(uncorrected)


339s dark
(uncorrected)

WP 1 2 3 4 5 6 7 1040s (cor) 187 559 605 643 1281 4971 37241

100s dark
(uncorrected)

Rate 1040s (obs/exp) 339s (obs/exp) /s) (e 0.18 0.54 0.58 0.62 1.27 4.78 35.81 133±12 / 187 380 / 559 416 / 605 443 / 643 1005 / 1281 4349 / 4971 35369 / 37241 33±6 / 61 88 / 182 99±11 / 197 141 / 210 260 / 430 1240 / 1620 12165 / 12140

33s dark
(uncorrected)

100s (obs/ exp) 8±5 / 18 14±6 / 56 16±6 / 58 27 / 64 41 / 128 278 / 497 3408 / 3724 33s (obs/exp) 5±4 / 6 2±4 / 18 7±5 / 19 6±4 / 20 5±4 / 42 62±9 / 158 1088 / 1182

THE STRATEGY!
The CTE-corrected long darks give us a robust estimate of the dark rate in each WP. This allows us to estimate, for darks with various exposure times, the number of electrons contained in each WP before the charge began its shuffle down the detector. We can then compare the observed number of electrons against the expected number to evaluate absolute CTE losses for extremely small charge packets.! The table to the right documents the surviving charge for the specific WPs identified above.! The plot below the table shows the results graphically, along with the overall trend seen for many tens of thousands of WPs.!

With a forward model in hand, we can then construct a reverse model to predict the original distribution of pixel values from an observed distribution. This involves an iterative deconvolution.! The figures below show a portion of a 1040s dark exposure far from the readout register. The grayscale is linear from 0 to 500 e-.!

Observed dark


CTE-corrected dark


CTE losses are truly pathological for small e- packets on low backgrounds. WPs that start their journey at the top of an empty chip with less than 20 e- are often blurred completely out of existence during read out. ! Such devastating losses could never be reconstructed with a pixelbased scheme.!

The same correction can be applied to sci We find that photometry and astrometry are restored, although some noise amplification avoided. The grayscale below is linear from

ence images. generally cannot be -10 to 50 e-.!

THE GOOD NEWS!

Surviving Electrons


THE BAD NEWS!

Virtually no ACS science observations have such low backgrounds. ! ACS observes most efficiently in the visible bands with wide-band ! 10
filters and long exposure times, all of which combine to give backgrounds ! greater than 65 e- per pixel in all but a handful of prime science exposures. ! These high typical backgrounds shield even faint sources from extreme ! CTE losses: most sources retain more than 60% of their original counts.! This study has allowed us to develop an exquisite model of how the traps in the pixels impact charge packets of various sizes, from very small to very big. With such a wellcalibrated model, we can then infer what fraction of electrons should survive the trip to the readout register for sources of different size on different backgrounds. With a better model, we can do a better job reconstructing the original pixel distribution.!

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%


Observed trend
for all WPs


20
50
100
200
500
1000
2000
5000
10,000
20,000


WP pixel value at top of chip (e-)


REFERENCES!
Anderson, J. & Bedin, L. R. 2010 PASP 122 1035­1064! Massey, R. et al. 2010 MNRAS 401 371 !

Observed science image

CTE-Corrected science image