Putting the electrons back where they belong

Pixel-based CTE Correction of ACS/WFC:
Modifications to the ACS Calibration
Pipeline (CALACS)
THE NEW CALACS
Linda Smith1, J. Anderson1, A. Armstrong1, R. Avila1, L. Bedin1, M.
1, S.
presence
of a high electric field,
the dark current of a single pixel can be
Chiaberge1, M. Davis1, B. Ferguson1, A. FruchterIn 1the, D.
Golimowski
greatly enhanced. These hot pixels accumulate as a function of time on orbit;
1, M.
however, 1the
reduction
of the operating
temperature of the WFC CCDs
has
Gonzaga1, N. Grogin1, W. Hack1, P. L. Lim1, R. Lucas
, A.
Maybhate
Putting
the
electrons
back
where
they
belong
dramatically reduced the dark current of the hot pixels. ACS devices undergo a
McMaster1, S. Ogaz1, A. Suchkov2, L. Ubeda1 monthly annealing process which greatly reduces the population of hot pixels and
1Space
does not affect the normal pixels.
Telescope Science Institute, Baltimore MD, 2Johns Hopkins University
ABSTRACT
NEW CALACS DATA PRODUCTS
The Advanced Camera for Surveys (ACS) was installed on the Hubble Space Telescope (HST) nearly ten years ago. Over the last
decade, continuous exposure to the harsh radiation environment has degraded the charge transfer efficiency (CTE) of the CCDs. The
worsening CTE impacts the science that can be obtained. To ameliorate this, Anderson & Bedin (2010) developed a pixel-based
empirical approach to correcting ACS data by characterizing the CTE profiles of trails behind warm pixels in dark exposures. The
success of this technique means that it is now possible to correct full-frame ACS/WFC images for CTE degradation in the standard
data calibration and reduction pipeline CALACS. Over the past year, the ACS team has developed, refined and tested the new
software. The new CALACS will include the automatic removal of the low-level bias shift and bias stripes (produced by the post-repair
ACS electronics) and pixel-based CTE correction. In addition to the standard cosmic ray corrected, flat-fielded and drizzled data
products (CRJ, FLT and DRZ files) there are three new equivalent files (CRC, FLC and DRC) which contain the CTE-corrected data
products. The user community will be able to choose whether to use the standard or CTE-corrected products.
INTRODUCTION
The Advanced Camera for Surveys (ACS) was installed on the
Hubble Space Telescope (HST) nearly ten years ago. Over the last
decade, continuous exposure to the harsh radiation environment of
space has degraded the charge transfer efficiency (CTE) of the
CCDs. There are now many defects in the silicon lattice, which can
trap electrons as the charge is being read out. The result is that
images contain vertical, deferred-charge trails extending away from
bright objects, as shown in the left-hand side of Fig. 1.
Raw Image
Bias shi4 correc1on
Bias stripe correc1on
CTE correc1on
Figure 1: Left: A portion of a short exposure of the cluster 47 Tuc showing the
distinctive CTE trails extending away from the stars in the opposite direction to the
readout direction. Right: The same image after correction using the AB10 code
This CTE degradation impacts the science that can be obtained
with ACS because it alters the photometric, astrometric, and
morphological characteristics of both faint and bright sources,
particularly those farthest from the readout amplifiers, and data
obtained with low backgrounds.
Recently, significant advances have been made in correcting
ACS/WFC images for CTE degradation at the pixel level. Massey
et al. (2010) showed that CTE trails could successfully be removed
by fitting exponential decay parameters to hot pixel trails in science
images. Subsequently, Anderson & Bedin (2010; AB10) extended
this work to lower background levels using an empirical model to
describe the trails of hot pixels in stacked dark exposures. The
right-hand side of Fig. 1 shows that the AB10 code can
successfully remove the CTE trails in science images.
The ACS team has made many refinements and additions to the
AB10 code over the last two years with the aim of incorporating it
into the CALACS data reduction pipeline. We describe below the
resulting new version of CALACS and the many changes that
users will find. We expect the public release to occur at the end of
February.
CTE-corrected data products will be available in the MAST
archive for new data after the public release. Older data can be reprocessed “on-the-fly” via a standard MAST retrieval request and
CTE-corrected products will be included.
★_CRJ = Cosmic ray corrected FITS image
★_CRC = Cosmic ray and CTE-corrected FITS image
★_FLT = Flat-field corrected FITS image
★_FLC = Flat-field and CTE-corrected FITS image
★_DRZ = Drizzled FITS image
★_DRC = Drizzled CTE-corrected FITS image
Any data file with a type ending in “C” will be the CTEcorrected equivalent of a standard CALACS file
THE NEW CALACS
Dark correc1on using standard DRK files
3 pairs of products:
Dark correc1on using new DKC files
Data Products:
CRJ, FLT, DRZ
Data Products:
CRC, FLC, DRC
Figure 3: Details of the new naming convention for CALACS data products
THE CTE CORRECTION
The AB10 code has undergone many enhancements during the
course of its implementation into CALACS and our extensive
testing. It now includes:
Time and temperature dependence of CTE losses (see poster of
Ubeda et al. #241.03)
Improved CTE correction at low signal and background levels
(see poster of Anderson et al. #241.04)
Allowance for the column dependency of CTE losses (see
poster of Ogaz et al. #241.02)
Parallel processing to reduce the run time to ~ 5 mins for a
single image
Figure 2: Schematic flow diagram showing the new structure of CALACS. In
addition to the standard data products (_CRJ, _FLT and _DRZ.fits files), there will
be three new CTE-corrected data products (_CRC, _FLC and _DRC.fits files)
In parallel with our development work, we have tested the new
code for its ability to:
The new CALACS will include additional steps, as shown
schematically in Fig. 2:
Correctly reproduce the photometry and astrometry of point
sources and the morphology of extended sources (see the poster
by Lucas et al. #241.06 for details of extended source tests)
Correction for the signal-dependent “bias shift”, a post-SM4
electronics artefact (see poster by Norman Grogin #241.08)
Correction for the bias striping (another post-SM4 artefact),
which uses the pre-scan region of all 4 amplifiers.
CALACS will then have two branches:
Recover the characteristics of the signal-to-noise ratio and point
source photometry for simulated images (see poster by Avila et al.
#241.07)
For more information, visit http://www.stsci.edu/hst/acs
or contact the help desk at help@stsci.edu
BRANCH 1:
Traditional CALACS processing using standard darks (_DRK) to
produce standard data products: _CRJ, _FLT and _DRZ.fits files
BRANCH 2:
Correction for CTE losses using a refined version of the AB10
code (see poster by Anderson et al. #241.04 and below), dark
correction using CTE-corrected darks (_DKC files), and standard
processing to produce new data products: _CRC, _FLC and
_DRC.fits files
The user will be able to choose whether to use the CTE-corrected
or standard data products. The naming conventions are explained
in more detail in Fig. 3.
REFERENCES
Anderson, J. & Bedin, L. R. 2010, PASP, 122, 1035
Massey, R. et al. 2010, MNRAS, 401, 371