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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 6 October 2004
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CONTENTS:
1. Addition of MultiDrizzle to the ACS Pipeline
2. ACS Data Handbook Version 3.0
3. HST Cycle 14 Phase I Proposal Deadline: Friday January 21, 2004
4. ACS and WFPC2 Instrument Handbooks for Cycle 14
5. ACS Geometric Distortion Corrections
6. ACS Zeropoints
7. ACS Bias and Dark Reference Files
8. Discontinuation of WFPC2 Supplemental Darks
9. Recent Instrument Science Reports
10. Recent Publications
11. Recent Press Releases
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1. Addition of MultiDrizzle to the ACS Pipeline
As of September 22, 2004, the MultiDrizzle software has been
incorporated into the ACS data reduction pipeline. This means
that when data are requested from the HST Archive, a
"*_drz.fits" image product will be delivered which consists
of a cosmic-ray cleaned, combined, and geometrically corrected
image. This image product will be created for all ACS
associations, which are defined as groups of exposures within
a single visit, obtained using the recommended dither patterns
or CR-SPLITs as specified in the Phase II proposal instructions.
The pipeline previously constructed drz.fits image products
using PyDrizzle. The advantages of MultiDrizzle over PyDrizzle
are described in Section 4.5 of the ACS Data Handbook. Section
4.6 describes how observers can run the MultiDrizzle software
outside of the HST Archive to recalibrate the data if necessary.
A future update to the ACS Data Handbook (scheduled to appear
by the end of 2004) will include a full discussion of how the
software was implemented into the ACS pipeline. In the meantime,
users should refer to information at
http://www.stsci.edu/hst/acs/analysis/multidrizzle
for details about the implementation and the data products
that are produced.
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2. ACS Data Handbook version 3.0
Version 3.0 of the ACS Data Handbook was released. It is
available in various formats from
http://www.stsci.edu/hst/acs/documents/documents/handbooks/DataHandbookv3/
ACS_longdhbcover.html
This new version has two entirely new Chapters. Chapter 5
covers ACS Error Sources, and discusses issues such as Readout
Noise, A-to-D Conversion, Bias Subtraction, Dark Current, Hot
Pixels, Cosmic Rays, Flat Fields, Image Anomalies, and Generic
Detector and Camera Properties. Chapter 6 covers ACS Data
Analysis and discusses issues such as Photometry, Astrometry,
Polarimetry, Coronagraphy, Ramp Filters, and Grism/Prism
Spectroscopy.
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3. HST Cycle 14 Phase I Proposal Deadline: Friday January 21, 2004
See the HST Call for Proposals and related information that
will soon be made available at:
http://www.stsci.edu/hst/proposing/docs/cycle14announce
Proposers should note the new requirement for Cycle 14 to
provide information on how the scheduling and science of a
proposal would be affected by a potential transition of Hubble
Space Telescope to a pointing mode with only two functional
gyroscopes. A detailed description of this mode is contained
in the Two-Gyro Handbook, which will be available from the
aforementioned Cycle 14 announcement web page. Chapters 7
and 8 contain discussions of impacts unique to observations
with ACS and WFPC2, respectively.
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4. ACS and WFPC2 Instrument Handbooks for Cycle 14
The ACS and WFPC2 Instrument Handbooks both have undergone
revisions to update for Cycle 14. The new versions will soon
be made available respectively at:
http://www.stsci.edu/hst/acs/documents/
http://www.stsci.edu/instruments/wfpc2/
These new Handbooks reflect our current understanding of the
instruments, and include calibrations and insights obtained
during the past year.
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5. ACS Geometric Distortion Corrections
ACS images have considerable geometric distortion. Until
recently, the geometric distortion corrections applied in the
ACS pipeline to create drizzled "*_drz.fits" files were based
on fourth-order polynomial solutions. However, it was found
that more accurate results can be obtained when corrections
are applied that: (a) are dependent on the filter; and (b)
contain higher frequency information than can be easily
captured with a polynomial. The version of Multidrizzle that
was installed in the ACS pipeline (see item 1 above) now
applies such corrections for all of the most common filters
on WFC and HRC. The corrections are based on the work of
Anderson & King (2004; ACS ISR 04-15, hyperlink given under
Item 9 below). For the corrections the pipeline uses new
calibration reference files "*_dxy.fits".
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6. ACS Zeropoints
Photometric zeropoints for ACS are available at the web page
http://www.stsci.edu/hst/acs/analysis/zeropoints
On May 10, 2004, we made minor updates to the VEGAmag
zeropoints for ACS. The new zeropoints are based on the
latest spectrophotometric calibration of Vega (Bohlin &
Gilliland 2004, AJ 127, 3508). The new composite flux
standard alpha_lyr_stis_002.fits (available at
http://www.stsci.edu/hst/observatory/cdbs/calspec.html)
consists of IUE data from 1256-1675A, STIS CCD fluxes from
1675-4200A, and a specially tailored Kurucz model longward
of 4200A (Kurucz 2003) and from 900-1256A. The STmag and
ABmag zeropoints have not been affected by this change.
NOTE: From May 10, 2004 until August 23, 2004, the VEGAMAG
zeropoints for filters F660N and F775W on ACS/HRC were
erroneously listed on the aforementioned web page as 25.827
and 25.551. The correct values are 20.827 and 24.551,
respectively.
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7. ACS Bias and Dark Reference Files
ACS bias and dark reference files (which are made from many
contemporaneous bias and dark frames) are typically available
in the HST Data Archive within a few weeks after any WFC or
HRC observation. However, due to transitions in our reference
file production processes and infrastructure over the summer,
this turnaround time has been longer for most of the WFC and
HRC data obtained between June 17 and August 25. We have now
completed the process of creating and delivering the reference
files for this period. WFC and HRC users who obtained data in
this period are advised to re-retrieve their data from the HST
Data Archive to obtain the optimal bias and dark calibration.
More generally, users are advised to monitor our reference
file webpages at
http://www.stsci.edu/hst/acs/analysis/reference_files
These list when the best reference files for any particular
date of observation have become available. Once dark and bias
frames for a particular date have become available, users
should (re-)retrieve their data from the HST Archive.
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8. Discontinuation of WFPC2 Supplemental Darks
We are attempting to reduce the number of low-priority
exposures taken on HST in an effort to prolong the life of
the radio transmitters which are used to down-link science
data. Hence, we are discontinuing the WFPC2 supplemental dark
calibration program, where typically three dark frames were
taken per day. In principle these could be used to optimize
removal of hot pixels from WFPC2 science images, since dark
frames could be selected very close in time to the science
data. However, in practice they were rarely used. We will
of course be continuing the weekly dark calibration program,
which provides reference files for the WFPC2 data reduction
pipeline.
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9. Recent Instrument Science Reports
ACS:
ACS coronographic flat fields (ISR04-16)
http://www.stsci.edu/hst/acs/documents/isrs/isr0416.pdf
Multi-filter PSFs and Distortion Corrections for the HRC
(ISR 04-15)
http://www.stsci.edu/hst/acs/documents/isrs/isr0415.pdf
SBC Dark and Cumulative Images (ISR 04-14)
http://www.stsci.edu/hst/acs/documents/isrs/isr0414.pdf
Cross-Talk in the ACS WFC Detectors. II: Using GAIN=2 to
Minimize the Effect (ISR 04-13)
http://www.stsci.edu/hst/acs/documents/isrs/isr0413.pdf
Cross-Talk in the ACS WFC Detectors. I: Description of the
Effect (ISR 04-12)
http://www.stsci.edu/hst/acs/documents/isrs/isr0412.pdf
ACS/HRC Polarimetry Calibration III: Astrometry of Polarized
Stars (ISR 04-11)
http://www.stsci.edu/hst/acs/documents/isrs/isr0411.pdf
ACS Polarization Calibration - II. The POLV Filter Angles
(ISR 04-10)
http://www.stsci.edu/hst/acs/documents/isrs/isr0410.pdf
ACS Polarization Calibration I. Introduction and Status
Report (ISR 04-09)
http://www.stsci.edu/hst/acs/documents/isrs/isr0409.pdf
Detector Quantum Efficiency and Photometric Zero Points of
the ACS (ISR 04-08)
http://www.stsci.edu/hst/acs/documents/isrs/isr0408.pdf
Bias and dark calibration of ACS data (ISR 04-07)
http://www.stsci.edu/hst/acs/documents/isrs/isr0407.pdf
Time Dependence of ACS WFC CTE Corrections for Photometry
and Future Predictions (ISR 04-06)
http://www.stsci.edu/hst/acs/documents/isrs/isr0406.pdf
For a list of all ACS ISRs please see:
http://www.stsci.edu/hst/acs/documents/isrs/
WFPC2:
The Accuracy of WFPC2 Photometric Zeropoints (ISR 04-01)
http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0401.html
For a list of all WFPC2 ISRs please see:
http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html
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10. Recent Refereed Publications
ACS:
http://www.stsci.edu/hst/acs/documents/pub_acs_2004_10.pdf
WFPC2:
http://www.stsci.edu/hst/acs/documents/pub_wfpc2_2004_10.pdf
WF/PC:
http://www.stsci.edu/hst/acs/documents/pub_wfpc1_2004_10.pdf
The papers listed above are based on data from ACS, WFPC2,
and WF/PC. The lists include all refereed papers received
and identified by the STScI Library during the current year.
Please include our Library in your preprint distribution
list. The listings were generated with STEPsheet:
http://stepsheet.stsci.edu/
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11. Recent Press Releases
ACS:
http://www.stsci.edu/hst/acs/documents/press_acs_2004_10.html
WFPC2:
http://www.stsci.edu/hst/acs/documents/press_wfpc2_2004_10.html
The press releases listed above are based on data from WFPC2
and/or ACS. The listing was compiled from the collection of
STScI Press Releases available at
http://hubblesite.org/newscenter/newsdesk/archive/ .
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