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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 24 Novemberr 2010
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CONTENTS:
1.
ACS/WFC Bias Stripe and CTE Correction Software
2.
ACS/WFC Bias and Dark Reference Files
3.
Status of ACS/SBC Operations
4.
Recent Instrument Science Reports
5.
Recent Refereed Publications
6.
Recent Press Releases
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1.
ACS/WFC Bias Stripe and CTE Correction Software
ACS Wide Field Channel images obtained after SM4 exhibit instrumental artifacts
caused by the CCD Electronics Box Replacement (CEB-R) or by prolonged exposure
to HST's trapped radiation environment. The most conspicuous of these artifacts
are faint stripes that extend along the CCD rows and pronounced charge trails
that extend from each imaged source mostly along the CCD columns. The
horizontal stripes are caused by low-frequency 1/f-noise on the bias reference
voltage generated by the CEB-R. (For more information, see the ACS Instrument
Handbook or the 9 Sep 2009 issue of the ACS+WFPC2 STAN.) The vertical charge
trails are the result of the CCDs' severely degraded charge transfer efficiency
(CTE) after more than eight years of radiation exposure. Such long-term
radiation damage is a problem experienced by all space-based CCD cameras,
including those in WFPC2, STIS, ACS, and WFC3.
The scientific impact of the faint horizontal stripes is negligible in most
cases, but the correlated nature of the noise can adversely affect the detection
of very faint sources and compromise the aesthetic appeal of deep ACS exposures.
The impact of degraded CTE is much more serious, as it significantly alters the
photometric, astrometric, and morphological characteristics of both faint and
bright sources, especially those imaged far from the output node.
Since SM4, STScI has been developing corrective algorithms that remove or
mitigate the horizontal stripes and vertical charge trails in ways that not only
improve the qualitative appearance of WFC images, but also restore with high
fidelity the intrinsic image pixel signals.
The pixel-based CTE correction was
initially developed by Anderson & Bedin (2010, PASP, 122, 1035) for images
obtained shortly after SM4, but STScI is now refining the algorithm for use with
pre-SM4 images and very faint sources (i.e., pixel signals < 10 e-). STScI
plans to incorporate these and other corrective algorithms into the CALACS image
calibration software by September 2011, but the bias stripe and CTE correction
algorithms are now publicly available as standalone Python tasks in the PyRAF
acstools package. These standalone tasks should be treated as beta versions of
the future CALACS pipeline modules.
The bias stripe correction task (acs_destripe) and the pixel-based CTE
correction task (PixCteCorr) are available for public use via the ACS software
website, http://www.stsci.edu/hst/acs/software/. The tasks operate in the PyRAF
environment, which is a Python-based command language for running IRAF tasks.
The latest version of PyRAF can be downloaded from the website
http://www.stsci.edu/resources/software_hardware/pyraf/.
Instructions and
tutorials for running acs_destripe and PixCteCorr are also available on the
website.
The following caveats apply to the use of acs_destripe:
* The task is intended for use only with post-SM4 full-frame ACS/WFC images
that have been processed by CALACS through the flat-fielding stage (i.e.,
"flt" images).
* The task should only be used on dark-subtracted images.
* The task performs poorly in the presence of high backgrounds and/or
complicated scenes.
* The task also performs an approximate correction for WFC intra-CCD amplifier
crosstalk assuming GAIN=2 crosstalk properties.
The following caveats apply to the use of PixCteCorr:
* The task was developed for full-frame ACS/WFC flt images recorded with A/D
gain = 2 e-/DN.
It has not been fully tested for other image formats and
operational settings. Consult the tutorial webpage for instructions about
processed other image types.
* The task may be applied to pre- and post-SM4 images, but the time dependence
of the CTE degradation is defined to be linear based on actual CTE
measurements obtained in Autumn 2009 and an assumption of perfect CTE after
ACS's installation in March 2002.
* Because the task is a deconvolution algorithm, the noise floor is enhanced
after image processing.
PixCteCorr contains an option for mitigating this
noise enhancement, but the resulting noise characteristics with or without
this option have not yet been properly quantified or modeled. Consequently,
the error images generated by PixCteCorr should be used with caution.
* The algorithm cannot distinguish between cosmic ray events (CRs) that occur
during image exposure and image readout. Consequently, the CTE trails from
CRs occurring during readout will likely be oversubtracted by the algorithm.
The ACS team at STScI solicits feedback from users regarding the performance,
applicability, and possible improvement of these tasks. Please submit this
feedback to the STScI Help Desk and it will be forwarded to the ACS Team.
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2.
ACS/WFC Bias and Dark Reference Files
Production of daily dark and biweekly bias calibration reference files for
full-frame ACS/WFC images is now routine, so users should wait no longer than
one month after the observation date to retrieve images from MAST that are
calibrated with contemporary reference files. As of this STAN, contemporary
bias and dark reference files are available for observations conducted before
November 2010.
Production of bias reference files for ACS/WFC subarray readout modes is also
routine but presently lags the production of full-frame reference files. As of
this STAN, contemporary subarray bias reference images exist for observations
conducted before mid-July 2010.
Because of the number of subarray images
planned for Cycle 18 is small, STScI has halted weekly production of subarray
bias reference images and will instead produce these images within one month
of the scheduled subarray science images. Cycle 18 subarray users should
submit to the STScI Help Desk any questions regarding the status of calibrated
subarray images retrieved through MAST.
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3.
Status of ACS/SBC Operations
After an electronic upset in HST's Science Instrument Control and Data Handling
Unit (SI C&DH) in September 2010, it was unexpectedly learned that the high
voltage power supply of the ACS Solar Blind Channel cannot be turned off during
such an event. Under certain circumstances, such a failure could damage the
SBC's MAMA detector. Consequently, all SBC observations have been suspended
until corrective updates to the flight software can be installed. STScI
currently expects SBC science operations to resume in March 2011.
In August 2010, STScI eliminated the default subtraction of a scaled dark
reference image from science images in the SBC image calibration pipeline.
Because the SBC's dark current is so low (1e-05 electrons/pixel/sec), dark
frame subtraction has negligible scientific benefit and unnecessarily enhances
the noise of the calibrated science images.
Consequently, the default header
keyword setting for SBC image calibration is now DARKCORR=OMIT.
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4.
Recent Instrument Science Reports
ACS:
An Empirical Pixel-Based Correction for Imperfect CTE. I. HST's Advanced Camera
for Surveys (ACS ISR 10-03)
http://www.stsci.edu/hst/acs/documents/isrs/isr1003.pdf
ACS/WFC Crosstalk after Servicing Mission 4 (ACS ISR 10-02)
http://www.stsci.edu/hst/acs/documents/isrs/isr1002.pdf
Pixel-based correction for Charge Transfer Inefficiency in the Hubble Space
Telescope Advanced Camera for Surveys (ACS ISR 10-01)
http://www.stsci.edu/hst/acs/documents/isrs/isr1001.pdf
ACS after SM4: Relative Gain Values Among the Four WFC Amplifiers
(ACS ISR 09-03)
http://www.stsci.edu/hst/acs/documents/isrs/isr0903.pdf
WFPC2:
WFPC2 CTE for Extended Sources: I. Photometric Correction (WFPC2 ISR 10-02)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr1003.html
WFPC2 Standard Star CTE (WFPC2 ISR 10-01)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr1001.html
Red Leak Characterization for the WFPC2 UV Filters (WFPC2 ISR 09-07)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0907.html
On-Orbit Photometric Calibration of the WFPC2 Linear Ramp Filters
(WFPC2 ISR 09-06)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0906.html
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5.
Recent Refereed Publications
A list of refereed publications for ACS or WFPC2 may be obtained using the
HST Bibliography Search:
http://archive.stsci.edu/hst/bibliography
A more complete list can be found through the ADS abstract service by using the
standard search parameters:
http://adsabs.harvard.edu/abstract_service.html
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6.
Recent Press Releases
Press releases, many of which are based on data from WFPC2 and/or ACS may be
identified from the collection of STScI Press Releases available at:
http://hubblesite.org/newscenter/newsdesk/archive/
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| or http://www.stsci.edu/hst/wfpc2/help.html
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