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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 11 April 2005
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CONTENTS:

1.  ACS Photometric Calibration and Performance Paper

2.  Calibration Workshop later this year

3.  ACS Data Handbook Version 4.0

4.  HST Cycle 14 Phase II notes: Deadline: May 13, 2005

5.  New ACS Aperture Definitions

6.  Results From 2-gyro Testing

7.  Pipeline MultiDrizzle Image Orientation

8.  Whirlpool Galaxy (M51) Project Observations and Data Release

9.  Paper Products for ACS

10. WFPC2 Bug in Nov 2004 MultiDrizzle Release

11. Recent Instrument Science Reports

12. Recent Publications

13. Recent Press Releases

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1.  ACS Photometric Calibration and Performance Paper

A calibration paper that discusses the ACS photometric
performance and photometric transformations is in the
final stages of the refereeing process for PASP publication.
This paper, "The Photometric Performance and Calibration
of the HST Advanced Camera for Surveys"
by Sirianni, M., et al. may be found in current form at:

http://www.stsci.edu/hst/acs/analysis/transformations
or http://acs.pha.jhu.edu/instrument/calibration/photometry/

This paper provides a thorough discussion of data reduction
steps, performance characterizations, and analysis approaches
relevant to obtaining accurate photometry in the ACS photometric
system. Transformations based on observations of star clusters,
and separately with synthetic photometry, are provided to the
WFPC2 and UBVRI photometric systems.

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2.  Calibration Workshop later this year

We plan to host an HST Calibration Workshop at STScI in late
2005. The goal of the workshop is to foster the sharing of
information and techniques between observers, instrument
developers, and instrument support teams.

We invite the ACS/WFPC2 community to provide suggestions for 
topics to discuss at the calibration workshop. If you have 
developed a specific calibration technique, you have suggestions 
for improving the scientific return of ACS data or you would 
like to discuss the challenges of pushing ACS/WFPC2 instruments 
to the limit please send a email to cal05@stsci.edu


Also, please be reminded that if you have questions about the
ACS instrument, issues relating to your ACS data or suggestions
to improve the ACS calibration you can always
contact help@stsci.edu.

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3.  ACS Data Handbook Version 4.0

Version 4.0 of the ACS Data Handbook has been released. It is
available in various formats from:

http://www.stsci.edu/hst/acs/documents/documents/handbooks/DataHandbookv4/
       ACS_longdhbcover.html

This version has been completely revised to include discussions
on how to run MultiDrizzle on ACS data, as well as describing
the MultiDrizzle processing of ACS associations that takes
place automatically when the data are requested from the
archive. A number of other updates are also described, including
bad pixels and data quality flags.

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4.  HST Cycle 14 Phase II notes: Deadline: May 13, 2005

See the HST page providing information on the Phase II
preparation of accepted programs for Cycle 14 at:

http://www.stsci.edu/hst/programs

For ACS a change in the default GAIN with the WFC has been
made from the old value of 1 electron/DN, to 2 e-/DN.
The tradeoffs between these two gain values are discussed
in section 8.4.1 -- WFC Gain -- in the current ACS Instrument
Handbook. Observers wishing to stay with GAIN = 1 may easily
do so by specifying such in their Phase II.

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5.  New ACS Aperture Definitions

In recent months, several ACS aperture reference positions
have been redefined. First, the prism apertures used with HRC
and SBC were repositioned. Both apertures show some vignetting
in the x-direction when the prism is in place and so the center
of the exposed area is not at the full aperture center. This
was known and allowed for before launch, but examination of
in-flight images showed that positions could be better chosen.
The HRC reference point was moved from [x,y] = [741,512] to
[671,512] while the SBC was moved from [432, 512] to [425,400].
The additional shift in the y-direction for the SBC was
undertaken because there is a broken anode which causes rows
599 to 605 to be disabled. We have moved the reference point
for the prism observations away from this area. These changes
took effect on November 8th 2004.

Three reference points for the Wide Field Camera have also
been moved. The former position of nearly all reference points
lay on the half-way mark in the x-direction, that is at x=2048
on the 4096 pixel wide effective detector area. This is right
on the boundary of the amplifier readout; the range x=1 to 2048
is read out through A and C amplifiers while 2049 to 4096 is
read by the B and D amplifiers. Because of this there is a
small discontinuity in the bias image at the boundary. This
might slightly complicate the analysis of a point source
landing at this position and so we have shifted the normal
reference points for WFC, WFC1 and WFC2 to the position x=2124,
a change of 74 pixels or about 2.5 arcseconds. This still keeps
the point near the center of the 200 arcsecond square aperture
and was chosen to avoid known bad columns.

For observers who need to repeat observations at the same
position as before, the apertures WFC-FIX, WFC1-FIX and
WFC2-FIX remain available and, as their names suggest, will
never be moved. This change was announced by direct e-mail
to all observers who might have been affected to provide an
opportunity to change to the FIX apertures where necessary.
This change took effect on March 14th, 2005.

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6.  Results From 2-gyro Testing

For many months work has been underway at STScI and GSFC to
develop a two-gyro observing capability for HST. In this
mode, information from one of the optical Fine Guidance Sensors
is substituted for pointing information which is lost when
observing with two gyros instead of the normal three gyros,
such as might occur after additional gyro failures.

On Feb 21-23 this work culminated in an on-orbit imaging
test of the new Two-Gyro Mode. Approximately 400 images were
taken with ACS and WFPC2 spanning exposure times from 10s - 500s,
guidestars from 11th to 14th magnitude, different HST roll
angles, and different targets spread around the sky. In all
cases the performance was superb. No significant differences
were seen in the PSF size or shape, pointing accuracy or
stability, or coronagraphic performance between three-gyro
and two-gyro modes.

Results are summarized at the Two-Gyro link on the ACS web page:

http://www.stsci.edu/hst/acs/performance/twogyro

Further details will appear in forthcoming ACS instrument
reports. We plan to make additional tests if/when HST does
enter two gyro mode, but no changes in data quality are
expected.

This is good news for Cycle 14 and future observers. We still
expect to see some change in the HST scheduling rules which
observers will need to consider; these will be incorporated
in the APT Phase II software for Cycle 14. It is possible that
a decision might be made to intentionally enter two-gyro mode
early in Cycle 14, so as to conserve the remaining gyros
onboard HST.

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7.  Pipeline MultiDrizzle Image Orientation

The default orientation for pipeline MultiDrizzle ACS images
("*_drz.fits") has been changed to the unrotated image frame,
instead of having North toward the top. This leads to smaller
images which are easier to transfer and display, and can still
readily be oriented with a North-up orientation within most
image display programs such as "ds9". More importantly, images
delivered in the unrotated frame allow for instrument-specific
effects to be much more easily identified and accounted for,
including bad columns, stellar diffraction spikes, CTE trails,
and PSF effects, and are also more useful for polarimetric,
coronagraphic and grism data.

The ACS CTE correction formulae can also be more readily
applied to unrotated images. If images with north up are still
required, these can be readily generated by appropriately
setting the "final_rot" parameter when running MultiDrizzle
off-line on the calibrated FLT files delivered from the pipeline.

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8.  Whirlpool Galaxy (M51) Project Observations and Data Release

An ACS mosaic of the Whirlpool Galaxy to be released April 25, see:

http://archive.stsci.edu/prepds/m51/

In January 2005, a large 4-color (B, V, I, and H-alpha) ACS
mosaic image of the Whirlpool Galaxy (M51), and its companion
NGC 5195, was obtained. Cycle 14 HST proposers were encouraged
to submit General Observer (GO) and Archival Research (AR)
proposals to complement and/or analyze this unique dataset.

On April 25, Hubble's 15th anniversary, the data will be
released to the community, in conjunction with a press release.
In addition to the standard archival products, carefully
drizzle-combined mosaic FITS images will also be made available
for scientific analysis, as "High-Level Science Products" via a
MAST webpage (URL above). Since we anticipate heavy network
traffic on April 25, researchers who wish to avoid downloading
these large mosaic images (~2 GB total) can sign up to have the
data mailed to them (in time to arrive on April 25) on DVD. See
the webpage above for further details.

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9.  Paper Products for ACS

In the early days of HST, observers were provided with printed
material which gave an overview of their observations in a
convenient printed and graphical form. Subsequently this feature
was turned into a tool within STSDAS (hst_calib.paperprod.pp_dads)
which could display similar information in the form of a
Postscript file. This tool is now very infrequently used and
has been found to not be fully functional for ACS observations.
In view of the very low demand we no longer intend to support
this facility.

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10. WFPC2 Bug in Nov 2004 MultiDrizzle Release

WFPC2 observers who have used, or are contemplating using
MultiDrizzle to process their data should be aware of a problem
discovered earlier this year.  It affected all WFPC2 data
processed with the version of MultiDrizzle released with
STScI_Python V2.0 and STSDAS 3.3 in Nov 2004.  The bug was
fixed in the release that went out with STScI_Python V2.1
on 1 April 2005.

The bug caused the shifts used in drizzling to be mis-applied
to the 'blot' step due to not taking into account any scale
changes.  As a result, the median image was not blotted back
to match the inputs and all pixels were subsequently flagged
as bad.  However, this was covered up due to the default value
of 'fillval' being 'INDEF', leaving the values from the last
image as the sole source of flux for the final output.  This
gave the appearance of having worked, when in fact, anyone
looking at the WHT extension could tell everything was masked
out.

This affected all WFPC2 data, all the time, and any other data
when the single-drizzling was done at a pixel scale other than
the default for the detector.  As stated in the MultiDrizzle
release notes, use on WFPC2 data is not fully supported; but we
do apologize for any inconvenience which this caused.  Please
contact help@stsci.edu for questions or additional information.

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11. Recent Instrument Science Reports

ACS:

Internal monitoring of ACS charge transfer efficiency
(ISR 05-03)
http://www.stsci.edu/hst/acs/documents/isrs/isr0503.pdf

Flat-field and Sensitivity Calibration for ACS G800L Slitless
Spectroscopy Modes (ISR05-02)
http://www.stsci.edu/hst/acs/documents/isrs/isr0502.pdf

Switching ACS from Side 1 to Side 2 electronics (ISR05-01)
http://www.stsci.edu/hst/acs/documents/isrs/isr0501.pdf

The Photometric Stability of ACS: Revisiting the Hubble
Deep Field (ISR04-17)
http://www.stsci.edu/hst/acs/documents/isrs/isr0417.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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12. Recent Refereed Publications

ACS:
http://www.stsci.edu/hst/acs/documents/pub_acs_2005_04.pdf

WFPC2:
http://www.stsci.edu/hst/acs/documents/pub_wfpc2_2005_04.pdf

WF/PC:
http://www.stsci.edu/hst/acs/documents/pub_wfpc1_2005_04.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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13. Recent Press Releases

ACS:
http://www.stsci.edu/hst/acs/documents/press_acs_2005_04.html

WFPC2:
http://www.stsci.edu/hst/acs/documents/press_wfpc2_2005_04.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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