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Hubble Space Telescope
ACS+WFPC2 STAN, 04 January 2006


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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 4 January 2006
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CONTENTS:

1.  HST Calibration Workshop

2.  Two-gyro Observations and Scheduling

3.  ACS Instrument Handbook for Cycle 15

4.  Calibration Outsourcing

5.  ACS SBC Flat Fields and Zero Point Checks

6.  Multidrizzle Developments, Expanded Associations

7.  New Version of the aXe Spectral Extraction Package

8.  New Apertures for Ramp Filters

9.  Update on WFPC2 - WF4 Bias Anomaly

10. Recent Instrument Science Reports

11. Recent Publications

12. Recent Press Releases

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1.  HST Calibration Workshop

The 2005 HST Calibration Workshop was held at STScI on
October 26 to 28. Video archives and supporting materials
for oral presentations, as well as electronic versions of
posters have been made available on the Calibration Workshop
website at 
http://www.stsci.edu/institute/conference/cal05
Writeups of talks and posters received by 2005-Dec-02
will be included in the Workshop Proceedings. The planned
distribution date of the Proceedings is 2006-Mar-31.

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2.  Two-gyro Observations and Scheduling.

Since transitioning to two-gyro mode for observations on
August 28, 2005 the data quality for ACS and WFPC2 imaging
has remained unchanged with respect to earlier three gyro
data.  The failure rate for guide star acquisitions is
only slightly higher than in three-gyro mode.  Full
trending of the failure rate will require a longer time
baseline.  Recent adjustments to the two-gyro mode and
parameters related to guide star selection should improve
performance even further.  The Institutes website provides
additional details:

http://www.stsci.edu/hst/HST_overview/TwoGyroMode

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3.  ACS Instrument Handbook for Cycle 15

Version 6.0, October 2005, of the ACS Instrument Handbook
was made available to support observations with HST in Cycle
15. Recent use found that the PDF version on our web page
was out-of-date; the HTML version was correct.  Although the
differences are expected to be minor, we advise anyone
having downloaded the PDF version before December 21 to
download a new version.  When downloading the current
version of the PDF file, please make sure to clear your
memory/disk caches in your browser first so that the
browser retrieves the current version instead of a
previously cached version.

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4.  Calibration Outsourcing

The Call for Proposals for Cycle 15 specifies a category of
"Calibration GO Programs" in Section 3.2.3 defining how
community members may apply for support of activities,
observations, and/or archival research, to advance instrument
calibration.  We urge careful consideration of this prospect.
In addition to general areas mentioned in the CP, candidates
for such work include development of polarimetry calibration
for ACS/WFC, and analysis of ramp-filter flat field data.
Significant polarimetry and ramp filter calibration data
exist or are planned in Cycle 14 that could be analyzed by
an interested GO.

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5.  ACS SBC Flat Fields and Zero Point Checks

Improved flatfields have been derived for the SBC long-pass
imaging filters and were delivered to the calibration pipeline
on November 16. Using dithered observations of a UV-bright
field, low-frequency variations in detector response of 8%
to 20% have been measured. The new pipeline flats are
expected to be accurate to ~1% for F115LP, F125LP, and F140LP
and to ~2% for F150LP, F165LP, and F122M. Small drifts of
quantum efficiency in time (as seen earlier with STIS MAMAs)
were also derived. For more information, see ACS ISR 05-13:

http://www.stsci.edu/hst/acs/documents/isrs/isr0513.pdf

The SBC absolute zeropoints were derived just after launch
using 10 spectroscopic standards in NGC6681. With the
application of the new positionally dependent flatfield
corrections, these zeropoints may have changed significantly.
Using the same set of observations, we have applied the new
flats and measured the mean change in stellar magnitude. The
differences with respect to the original calibration are less
than 3% for all 6 filters, i.e. less than the uncertainty in
the absolute calibration of the UV standards. No changes to
the existing SBC throughput curve will be made at this time.

Improved calibrations for the SBC are in progress, including
an updated geometric distortion solution, aperture
corrections, and photometric zeropoints using new
observations obtained in Cycle 14. Once these data have been
analyzed, new SBC reference files will be delivered for
use in the ACS calibration pipeline.

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6.  Multidrizzle Developments, Expanded Associations

MultiDrizzle improvements since Spring 2005 include improved
geometric distortion calculations, as well as more robust
treatment of associations with mixed apertures. Work is
currently underway to improve the combination of datasets
containing a mixture of very short and long exposures, where
the short exposures can provide additional information on
bright objects that may be saturated in the long exposures.

The initial prototype version of Tweakshifts has been
publicly released in Pyraf/STSDAS version 3.4 (Nov. 2005).
This script can be used to refine the shifts of a set of
exposures, using catalog matching or cross-correlation,
with parameters available to control its behaviour.
The resulting shifts are written to a file that can be
directly given to MultiDrizzle, improving the quality of the
resulting combined image. (Koekemoer et al. 2005,
HST Calibration Workshop, in press).

The original definition of associations was expanded to
include all exposures from a given visit that were obtained
using the same filter and aperture, for a given visit, if
the guidestars remain unchanged. This therefore allows POS
TARG dithers to be associated and combined in the HST archive
pipeline, thereby broadening the number of datasets for which
scientifically useful combined drizzled images can be produced
automatically by the pipeline.

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7.  New Version of the aXe Spectral Extraction Package

In November a new version of the spectroscopic data
extraction software aXe was released as part of STSDAS 3.4.
The aXe package is designed to handle the ACS data from all
slitless modes (WFC and HRC with the G800L grism, HRC with
the PR200L prism and SBC with the PR110L and PR130L grisms).

The new version 1.5 of aXe features substantial improvements
in several areas: (a) a new task helps to create the object
lists for the spectral extraction; (b) the possibility to
specify a fixed extraction width (in pixels) was added;
(c) the "quantitative contamination" method estimates, for
every spectral element extracted from the data, the
contribution of all neighbouring objects to the extracted
flux in a quantitative way; (d) optimal extraction using
the weighting scheme developed by Horne (1986, PASP 98, 609)
was implemented.

Details on aXe and the new aXe-1.5 are available on the aXe
webpages:  
http://www.stecf.org/software/aXe

Further details on quantitative contamination and optimal
extraction are given in Kuemmel, Larsen & Walsh (2005,
ST-ECF Newsletter 38, 8)and Walsh, Kuemmel & Larsen (2005,
ST-ECF Newsletter 39, 8), respectively.

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8.  New Apertures for Ramp Filters

Starting in Cycle 15 we will provide users of the ramp
filters on ACS with an easy means of obtaining either single
quadrant readouts only to allow efficiency gains (when the
wavelength of interest is contained with a single quadrant),
or full frame images (with serendipitous coverage at other
wavelengths).  Included will be a capability of using the
new aperture specifications with broadband filters as well
to enable observations with ramps and standard filters to be
obtained at the same detector positions.  Full details will
be available in the new Phase II instructions next spring.

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9.  Update on WFPC2 - WF4 Bias Anomaly

The anomaly in the WF4 CCD in WFPC2 continues to worsen.
The bias level is unstable, with more and more images either
being blank or showing faint streaks in the background.
We have also confirmed photometric errors associated with
the anomaly -- otherwise normal looking images of faint
targets in WF4 can be up to ~70% too faint depending on the
bias level and gain setting.  Details of the anaomaly
as well as techniques for repairing affected images are
described in a new report available at
http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0502.html
We may be able to mitigate the anomaly
by modifying temperature set points inside WFPC2;
tests of this strategy will be conducted in January.

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10. Recent Instrument Science Reports

ACS:

SBC L-flat Corrections and Time-Dependent Sensitivity (ISR 05-13)
http://www.stsci.edu/hst/acs/documents/isrs/isr0513.pdf

Earth Flats (ISR 05-12)
http://www.stsci.edu/hst/acs/documents/isrs/isr0512.pdf

ACS/HRC Polarimetry Calibration IV. Low-Frequency Flat-Fields 
for Polarized Filters (ISR 05-10)
http://www.stsci.edu/hst/acs/documents/isrs/isr0510.pdf

The Internal CCD Flat Fields (ISR 05-09)
http://www.stsci.edu/hst/acs/documents/isrs/isr0509.pdf

Updated Wavelength Calibration for the WFC/G800L Grism (ISR 05-08)
http://www.stsci.edu/hst/acs/documents/isrs/isr0508.pdf

Two-Gyro Pointing Stability of HST Measured with ACS (ISR 05-07)
http://www.stsci.edu/hst/acs/documents/isrs/isr0507.pdf

Demonstration of a Significant Improvement in the Astrometric 
Accuracy of HST Data (ISR 05-06)
http://www.stsci.edu/hst/acs/documents/isrs/isr0506.pdf

ACS Coronograph Performance in Two-Gyro Mode (ISR 05-05)
http://www.stsci.edu/hst/acs/documents/isrs/isr0505.pdf

Flats: SBC Internal Lamp P-Flat (ISR 05-04)
http://www.stsci.edu/hst/acs/documents/isrs/isr0504.pdf

For a list of all ACS ISRs please see:
http://www.stsci.edu/hst/acs/documents/isrs/

WFPC2:

Early Assessment of the WF4 Anaomaly (ISR 05-02)
http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0502.pdf

Hot Pixels as a Probe of WFPC2 CTE Effects (ISR 05-01)
http://www.stsci.edu/instruments/wfpc2/Wfpc2_isr/wfpc2_isr0501.pdf

For a list of all WFPC2 ISRs please see:
http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html

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11. Recent Refereed Publications

ACS:   
http://www.stsci.edu/hst/acs/documents/pub_acs_2006_01.pdf

WFPC2:
http://www.stsci.edu/hst/acs/documents/pub_wfpc2_2006_01.pdf

The papers listed above are based on data from ACS and WFPC2. 
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/

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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12. Recent Press Releases

ACS:   
http://www.stsci.edu/hst/acs/documents/press_acs_2006_01.html

WFPC2:
http://www.stsci.edu/hst/acs/documents/press_wfpc2_2006_01.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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| Visit the ACS and WFPC2 websites for more news:
| ACS:   http://www.stsci.edu/hst/acs
| WFPC2: http://www.stsci.edu/instruments/wfpc2
| Need help? http://www.stsci.edu/hst/acs/help.html
| or send your questions to the Help Desk: help@stsci.edu
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| The Space Telescope Science Institute is operated by the
| Association of Universities for Research in Astronomy,
| Inc., under NASA contract NAS5-26555.
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