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Hubble Space Telescope
ACS+WFPC2 STAN, 24 November 2010

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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 24 November 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, /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) 
/hst/acs/documents/isrs/isr1003.pdf

ACS/WFC Crosstalk after Servicing Mission 4 (ACS ISR 10-02) 
/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) 
/hst/acs/documents/isrs/isr1001.pdf

ACS after SM4: Relative Gain Values Among the Four WFC Amplifiers 
(ACS ISR 09-03) 
/hst/acs/documents/isrs/isr0903.pdf


WFPC2:

WFPC2 CTE for Extended Sources: I. Photometric Correction (WFPC2 ISR 10-02)
/hst/wfpc2/documents/isr/wfpc2_isr1003.html

WFPC2 Standard Star CTE (WFPC2 ISR 10-01)
/hst/wfpc2/documents/isr/wfpc2_isr1001.html

Red Leak Characterization for the WFPC2 UV Filters (WFPC2 ISR 09-07)
/hst/wfpc2/documents/isr/wfpc2_isr0907.html

On-Orbit Photometric Calibration of the WFPC2 Linear Ramp Filters 
(WFPC2 ISR 09-06)
/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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