Putting the electrons back where they belong

THE NEW CALACS
Pixel-based CTE Correction of ACS/WFC:
CTE Time And Temperature Dependency
Leonardo UBEDA, Jay ANDERSON, and the ACS Team
Space Telescope Science Institute, Baltimore MD
In the presence of a high electric field, the dark current of a single pixel can be
greatly enhanced. These hot pixels accumulate as a function of time on orbit;
however, the reduction of the operating temperature of the WFC CCDs has
dramatically reduced the dark current of the hot pixels. ACS devices undergo a
monthly annealing process which greatly reduces the population of hot pixels and
does not affect the normal pixels.
Putting the electrons back where they belong
INTRODUCTION
The new Advanced Camera for Surveys (ACS) calibration
pipeline (CALACS) contains the pixel-based charge transfer
efficiency (CTE) correction developed according to the Anderson &
Bedin (2010) code.
We developed a comprehensive analysis based on the profiles
of hot pixels in most of the dark frames produced by ACS/WFC
between 2002 and 2011.
RESULTS
We identified most of the hot pixels in each anneal cycle by
selecting only those pixels which comply with the following
conditions:
We studied the evolution of the pixel-based CTE correction by
plotting the estimated average number of electrons in the first,
second, and third upstream pixels of the hot pixels CTE trails
found in dark frames obtained using the ACS/WFC.
(1) pixel should have a dark current
value greater than 0.08 electrons/
second.
(2) pixel should appear consistently in
at least 80% of the dark _FLT frames
in each anneal cycle.
(3) pixel should be isolated within a 4
pixel radius.
(4) pixel must be at least 1500 pixels
away from amplifier.
This procedure allowed us to create a
single hot pixel catalog for each
anneal cycle by combining the results
from individual dark frames. See
Figure 3.
−77°C
−81°C
Figure 6 presents the results :
% of hot pixels
We perform a comprehensive and detailed study of the evolution of the effect of
charge transfer efficiency (CTE) of the Wide-Field Channel of the Advanced Camera
for Surveys (ACS). The study is based on the profiles of hot pixels in most of the
dark frames ever produced by ACS between 2002 and 2011. We apply the pixelbased empirical approach by Anderson & Bedin (2010, PASP, 122, 1035) which
restores flux, position and shape of sources in the original images. We demonstrate
that this image-restoration process properly accounts for the time and temperature
dependence for CTE in ACS, and that it works for all epochs: the original setting
when the camera was operated at −77°C and also on the post-SM4 data obtained
with the current temperature set at −81°C. We also demonstrate that the code has
been successfully integrated in the ACS calibration and reduction pipeline CALACS.
THE IDENTIFICATION OF HOT PIXELS
103 hot pixels
ABSTRACT
red symbols: _FLT data obtained when the detector was at temperature −77°C
blue symbols: _FLT data obtained when the detector was at temperature −81°C
green symbols: _FLC data
Figure 3: Number of ACS/WFC hot
pixels as a function of time.
THE HOT PIXEL TRAIL STRUCTURE
THE DARK CURRENT DATA SET
For our analysis we used the _RAW dark-current observations
obtained from 2002 to 2011. See Figure1.
Figure 1: Number of dark frames used in each anneal cycle. During an anneal
cycle, the CCDs and the thermal electric coolers are turned off and the heaters are
turned on to warm the CCDs to 19°C.
THE CALIBRATION PROCESS
We calibrated all _RAW dark frames using the latest version of
CALACS. The automated pipeline performs: a bias correction, the
removal of bias stripes, the pixel-based CTE correction, and a
transformation from counts (DN) to units of electrons using the
appropriate gain for each of the four amplifiers. This process
generates two calibrated FITS images with extensions _FLT and
_FLC (Figure 2).
We studied the hot pixel CTE trail structure as depicted in
Figure 4.
Figure 4: A detail of the WFC2 chip of an _FLT image from December 2010. The
vertical trails extending upward from the hot pixels are indicative of imperfect CTE.
The readout amplifiers are located towards the bottom of the figure. The color
schematic zooms into one hot pixel to show the CTE trail structure and our notation.
The distributions of the number of electrons in the first, second,
and third upstream pixels show different averages for different
anneal cycles. Figure 5 shows histograms of the P1 distributions
for three anneal cycles.
January 2004
October 2010
August 2011
Figure 6: Evolution of number of electrons in the first, second, and third upstream
pixels in the CTE trail as a function of time. The black lines represent linear fits to
the empirical data. We have scaled the _FLT data obtained at the operating
temperature of −81°C to the data obtained at −77°C.
We observe similar results for flux intensities in the following
ranges: 100 ± 10 e−, 1000 ± 50 e−, and 10000 ± 1000 e−.
We find that the expected flux restoration is achieved
regardless of the operating temperature, electronics used and
camera GAIN setting.
The flux from the pixels in the trail (P1, P2, and P3) is being
transferred back to the pixels where it originated and restoring the
original values measured in late 2002 to within the expected 10%
accuracy.
As a bonus of our analysis, we have shown that the integration
of the pixel-based CTE correction has been successfully
implemented onto CALACS.
REFERENCES
Figure 2: (Left) A 1000 × 1000 pixel region of the top of the extension 1 chip in a
processed _FLT image. The CTE vertical trails are clearly visible. (Right) The
reconstructed _FLC" image after the execution of CALACS.
Figure 5: Histogram distribution of the number of electrons in the first upstream
pixel (P1) provided for three anneal cycles: January 2004 (left), October 2010
(centre), and August 2011 (right). The hot pixels considered for this example are
those with 1000±50 electrons. The solid green line represents the calculated
average and the dotted lines are the adopted errors around the average.
Anderson, J. & Bedin, L. R. 2010, PASP, 122, 1035–1064