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Hubble Space Telescope
ACS+WFPC2 STAN, 30 November 2006

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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 30 November 2006
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CONTENTS:

1.  New ACS Bulletin Board Online

2.  HRC Relay Anomaly

3.  SBC Operations

4.  ACS Calibration Status

5.  Calibration Outsourcing

6.  Optical Redleak in the ACS SBC Detector

7.  New MultiDrizzle Release and Improved Distortion Solutions

8.  WFPC2 WF4 CCD Anomaly

9.  WFPC2 Closeout Plans

10. Recent Instrument Science Reports

11. Recent Refereed Publications

12. Recent Press Releases

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1.  New ACS Bulletin Board Online

STScI has established an online ACS bulletin board to allow
staff and HST observers to correspond with each other on
ACS-related topics. Feel free to peruse ongoing discussions,
post a reply to an existing thread, or start a new message
of your own. Your participation is needed to establish a useful
dialogue, and the postings will be moderated to ensure maximum
utility for users and to prevent misuse. To be able to post
messages, you must first register with the bulletin board
moderators. This is a quick one-time process - simply click
on the "Register" link on the bulletin board webpage and follow
the instructions. No registration is required to view the ongoing
discussions. The bulletin board gives the STScI ACS Team a more
efficient method to disseminate late-breaking news, and
to respond in a more widely visible way to common questions
often handled via the e-mail Help Desk.  We look forward
to communicating with you.

http://forums.stsci.edu/
(The forum is no longer available.)

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2.  HRC Relay Anomaly

The ACS suspended on September 23, 2006 because of a problem
with one of the relays on a signal processing chain board in
the High Resolution Channel [HRC] CCD detector electronics.
Analysis of the condition by an ACS Anomaly Review Board
concluded that the most likely cause of the problem was a bit
of non-conductive debris between two relay contacts that must
meet to make a proper electrical connection.  The missing
voltage resulting from this problem did not affect the ACS Wide
Field Channel [WFC], which was brought back into service on
October 2, or the Solar Blind Channel [SBC], which will be
brought back into service on November 27.

After a thorough assessment of the risk to the HRC and other
ACS hardware, the Anomaly Review Board recommended that the
problematic relay be toggled open to clear the debris and then
closed to reestablish the electrical connection.  The relay was
toggled open and closed on October 9.  This procedure evidently
cleared the debris because the missing connection was restored
when the low voltage power supply for the the CCDs was turned
on again.  The HRC was configured for science operations and
resumed service on October 16.

The Anomaly Review Board recommended that the problematic HRC
relay be left in its present (working) position.  The WFC and
HRC will be left powered during routine operations, except when
monthly anneals are performed.  Presently, all three ACS channels
are operating nominally and may be requested for use in Cycle 16
observing proposals.

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3.  SBC Operations

The Anomaly Review Board's recommendation to leave the HRC
relay in its present position required changes to the instrument
commanding used to operate the SBC.  Previously, SBC use has been
tied to a set of commands used to toggle the HRC relay so that the
HRC and SBC were not on simultaneously.  After studying the
electrical current, power, and thermal implications of having all
three ACS channels powered simultaneously during SBC operations,
approval was given to implement SBC operations with both CCDs
powered.  The SBC will be powered only when it is being used for
science or calibration observations.  As before, the SBC cannot be
used in parallel with the other ACS channels. For all practical
purposes, the changes to the commanding of the SBC are transparent
to observers.  The SBC observing capabilities now are the same as
those that have been available in the past.

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4.  ACS Calibration Status

Details for the Cycle 15 calibration plans may be found at:

http://www.stsci.edu/hst/acs/analysis/calib_plan/index_html#Section7
These programs support the annual science cycle.  Emphasis
this cycle includes maintaining up-to-date standard reference
files, responding to any changes after the switch to Side 2
electronics and 4 degree C cooldown of WFC last July, and
providing additional SBC specific calibrations.

The WFC temperature decrease, and Side 2 switch in July led
to minor calibration changes.  New biases and darks were
put in place in July with standard releases of these since.
The WFC flat fields have changes of ~0.6% in the blue to
~0.15% at F814W, new flat fields for filters with wavelengths
between those corresponding to F435W and F814W were delivered
November 2, 2006 -- see ACS/ISR 2006-06.  The WFC detector
sensitivity table will be updated to account for the
-2.5% at F435W to about 1% drop for redder filters as a
result of the lower operating temperature. We will provide
documentation quantifying limits to existing performance
changes for the HRC and SBC for which updates are not expected
to be necessary.

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5.  Calibration Outsourcing

The Call for Proposals for Cycle 16 specifies categories of
"Calibration GO and AR Programs" in Sections 3.2.4 and
3.4.3 respectively 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, a candidate for such work
includes polarimetry calibration for ACS/WFC for which
significant calibration data exists that could be analyzed
by an interested GO.

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6.  Optical Redleak in the ACS SBC Detector

Recent observations of cool stars using the ACS SBC PR110L prism
have revealed that the sensitivity of the ACS Solar Blind Channel
to optical and near-UV light is apparently much larger than was
measured during ground testing. The throughput given in the standard
reference files for the SBC detector at 3500 angstroms is about
9.5e-9. Preliminary estimates suggest that the real SBC detector
throughput at this wavelength is approximately 3.4e-6, or about 2.5
orders of magnitude larger than the existing estimate.

For a solar-type spectrum, this means that about 1/2 of the total
counts detected are due to optical and near-UV photons rather than
from the expected FUV photons. There is also some evidence that
the amplitude of this 'red leak' changes as the SBC detector warms
up, increasing by as much as 30% over the course of 5 orbits. It is
not yet clear whether this red leak has also been increasing
secularly over time. Imaging data  taken with the STIS FUV-MAMA
detector (for which the ACS SBC was the backup detector) seem to
show a similar, although perhaps somewhat smaller effect.

For dispersed PR110L and PR130L observations, it is straightforward
to identify this extra red light; however, it clearly also affects
SBC imaging observations done with the long-pass filters. The ACS
team is undertaking efforts to obtain a better calibration of the
red leak effect. Until  this better characterization is in place,
extreme caution should be exercised when interpreting  FUV imaging
observations of red targets. Observers conducting FUV imaging of red
targets with the SBC may wish to consider interleaving observations
with two different  long-pass filters (e.g., F140LP and F165LP), so
that the difference in the count rates can be used to isolate the
true FUV flux.

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7.  New MultiDrizzle Release and Improved Distortion Solutions

We announce the release of updated MultiDrizzle/PyDrizzle software,
together with improved polynomial distortion solutions ("IDCTABs")
as well as improved distortion residual images ("DXY" files). The
new software (MultiDrizzle v.2.7.2, PyDrizzle v5.7.0) is available
at

http://www.stsci.edu/resources/software_hardware/pyraf
and has also been incorporated into the HST archive pipeline.
The new distortion solutions were provided by J. Anderson and will
be available from the archive, and from the reference file webpage:

http://www.stsci.edu/hst/observatory/crds/SIfileInfo/ACS/reftablequeryindex
Together, the distortion files and updated software are capable of
providing an astrometric precision of about 0.01 pixels for data
obtained during the first few years of ACS operation, thereby
correcting pixel shift problems that had been previously reported.

We caution that inherent uncertainties up to ~0.1 pixel or more
may still exist in the observations. These can be introduced by
guide star problems which cause errors in the commanded offsets,
as well as long-term thermal changes and other effects resulting
in evolution of the distortion solutions over time. The released
versions of the software and distortion solutions do not yet
contain time-dependent information, which will be available in
a subsequent release. Meanwhile, it is possible to iteratively
refine shifts, if necessary, as described in section 4.6 of the
current ACS Data Handbook. Observers will be notified as soon as
more updated time-dependent distortion solutions are available.

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8.  WFPC2 WF4 CCD Anomaly

The bias level in the WF4 CCD has begun to trend downwards again,
mimicking its behavior in late 2004 and early 2005.  We have not
yet seen any further blank images in WF4, but a few blank images
could occur if the trend accelerates during the next two months.
We are tentatively planning another WFPC2 temperature adjustment
in January or February 2007 to counteract the trend. Observers
are reminded that WF4 images with low bias values (below about
300 DN, and above about 5 DN) will have low photometric counts,
but the photometry should be correctable.  This information will
be updated as needed at
http://www.stsci.edu/hst/wfpc2.

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9.  WFPC2 Closeout Plans

A summary of the WFPC2 calibration observations for its
remaining time onboard HST has been posted at

http://www.stsci.edu/hst/wfpc2/analysis/WFPC2_cyc1516_cal_plan_final.pdf
As always, observers are welcomed to suggest additional
calibrations, or propose additional calibration observations via
Calibration Outsourcing proposals.  These Outsource proposals
can also be of an archival nature, and utilize existing or planned
WFPC2 calibration data.  We would be happy to discuss current
calibration plans and potential outsource projects with interested
parties.

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10. Recent Instrument Science Reports

ACS:

ACS Post Flash Measurements (ACS ISR 06-07)

http://www.stsci.edu/hst/acs/documents/isrs/isr0607.pdf

WFC L-flats Post Cooldown (ACS ISR 06-06)

http://www.stsci.edu/hst/acs/documents/isrs/isr0606.pdf

Relative Astrometry Within ACS Visits (ACS ISR 06-05)

http://www.stsci.edu/hst/acs/documents/isrs/isr0605.pdf

Related ISR from the Telescopes branch:
Modeling Aberrations in the Advanced Camera for Surveys(ISR TEL 06-01)

http://www.stsci.edu/institute/org/telescopes/Reports/ISR-TEL-2006-01.pdf

WFPC2:

A list of the recent WFPC2 ISRs can be found at:

http://www.stsci.edu/hst/wfpc2/documents/isrs

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11. Recent Refereed Publications

ACS:
http://www.stsci.edu/hst/acs/documents/pub_acs_nov_06.ps
(This document is no longer available.)

WFPC2:
http://www.stsci.edu/hst/acs/documents/pub_wfpc2_nov_06.ps
(This document is no longer available.)

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_nov_06.html
(This link is no longer available.)

WFPC2:
http://www.stsci.edu/hst/acs/documents/press_wfpc2_nov_06.html
(This link is no longer available.)

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/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/hst/wfpc2
| Need help? Please contact us at the helpdesk.
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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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