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Hubble Space Telescope
ACS+WFPC2 STAN, 25 July 2006

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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 25 July 2006
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CONTENTS:

1.  Failure of the Side-1 Low Voltage Power Supply

2.  Switch to ACS Side-2 Electronics and Science Resumption

3.  Change in WFC temperature Setpoint

4.  Calibration results for Side 2

5.  Cycle 15 calibration plans

6.  MultiDrizzle update

7.  HRC Quantum Yield Correction Implemented in the ETC

8.  Dithering in Cycle 16

9.  Recent Instrument Science Reports

10. Recent Refereed Publications

11. Recent Press Releases

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1.  Failure of the Side-1 Low Voltage Power Supply

A recent hiatus in ACS observing occurred when the ACS suspended
operations on June 19, 2006.  ACS had been observing without
major interruption since installation in HST in March 2002.
ACS accounted for ~70% of the observing time with
HST in Cycle 14.

An Anomaly Review Board consisting of members from the Goddard
Space Flight Center, Ball Aerospace, STScI, and corporate contractors
was convened to determine the cause of the suspension and to provide
a recommendation for restoring ACS to service.  The ARB found that
the most likely explanation for the suspension was a failure within
the +15V circuitry in the Side-1 electronics of the Low Voltage Power
Supply (LVPS) or the Main Electronics Box (MEB).  This problem
affects both the Wide Field Channel (WFC) and High Resolution
Channel (HRC) CCDs.  It does not affect the Solar Blind Channel (SBC)
MAMA detector.  The ARB concluded that it would not be possible to
restore full observing capabilities on Side-1, but that it would be
possible to do so if the CCDs and MAMA were controlled from the
redundant set of electronics on Side-2.

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2.  Switch to ACS Side-2 Electronics and Science Resumption

The Side-2 electronics and the CCD electronics boxes on ACS were
powered up on June 30, 2006.  After passing checkout, ACS was safed
to await normal resumption of activities on July 2.
After an initial series of calibration bias, dark and flat field
exposures, the first science observations on Side-2 were obtained
on the evening of July 4.  ACS is presently conducting a full
complement of GO science programs and routine calibration
observations.
A press release describing one of the first science images obtained
on Side-2 can be found at
http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/36/

A brief moratorium imposed on SBC activities during the Anomaly
Review Board's investigation has been lifted, and SBC orbital
verification activities are currently in progress.  Science
observations with the SBC will resume at the next schedulable
opportunity.

At this time, there are no on-orbit tests of the failed Side-1
electronics planned.  ACS operations will rely upon the Side-2
electronics until further notice.  The ACS Team at STScI is in the
process of providing calibration reference files for new data
obtained on Side-2.  Additional details about these activities can
be found below and in future STANs.

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3.  Change in WFC temperature Setpoint

On July 4, 2006, the temperature setpoint of the WFC CCDs was
lowered from -77 C to -81 C soon after the switch to the Side-2
electronics. This change allows a significant amount of the
re-calibration effort associated with the side-2 switch to be
consolidated with the calibration needed for a temperature change.
Such a temperature change would likely have been needed within the
next 12-24 months to mitigate the impact of radiation damage to
the CCDs.  Setting the temperature of the ACS WFC to -81 C
has several beneficial effects:
      - The dark current rate and the hot pixel contamination are
        reduced by more than 50%.
      - The Charge Transfer Efficiency (CTE) tails are fainter.
      - The fainter CTE tails and the lower hot pixel contamination
        reduce the image noise. We estimate a gain in sensitivity
        for deep exposures of ~ 0.1 mag ( or equivalently, ~ 20% in
        observing time).
The decrease in the temperature setpoint has slightly decreased the
quantum efficiency (by 1-2%) and may have modified the flat field
structure by a smaller amount. Recalibration is in progress. Results
will be posted on the ACS web site.

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4.  Calibration results for Side 2.

New calibrations taken after the Side-2 switch and
lowering of the WFC temperature will be used to
assess a full range of calibration issues:  biases, darks,
flats, gains, and sensitivity.  New bias and dark
reference files are now in the pipeline. The biases
reflect only small changes. The darks show the (expected)
factor of 2 reduction in hot pixel numbers
for the WFC.  On the WFC the quantum efficiency is
slightly lower (as expected) by ~2.5% at F435W
declining to about a 1% drop for F555W and redder
filters.  The gains (WFC) do not show any
change with Side-2 operations. Read noise levels
differ only in detail, being comparable on the two
sides.  An initial QE measurement for the HRC based
on repeated GO observations using the G800L grating showed
no change (at 0.1% level) across the side switch
(none was expected since the operating temperature
should be nearly the same).  The pointing has been
verified to be the same for HRC and WFC as it was on Side-1.
PSFs and geometric distortion have shown no detectable change
as a result of the side switch or temperature change.

Confirmation calibrations will be obtained for the
SBC during the first week of August -- no changes
are anticipated.

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5.  Cycle 15 calibration plans

  The calibration program to support the annual science
cycle is always developed within the few months following
ingest of the Phase II science programs in order to allow
a detailed match to support science.  We are now in advanced
phases of discussing the Cycle 15 calibration plans.
Further details may be found on the ACS web site at:
http://www.stsci.edu/hst/acs/analysis/calib_plan

Comments on our calibration plans are always welcome,
though the next month would be a particularly opportune
time for these since the normal planning is still under way.
Comments should be sent to:  help@stsci.edu
The electronics side switch and lowering of the WFC operating
temperature may require additional calibrations in some areas
(analogous to those needed just after launch of ACS) to reach
standard levels of calibration for ACS (in particular
for flat fields, and sensitivity in the far red with WFC).

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6.  MultiDrizzle update

During recent months we have been rigorously verifying the
astrometric precision of MultiDrizzle, aiming to quantify
time-dependent changes in the ACS distortion and improve the accuracy
of the code. To date, discrepancies up to 0.1 - 0.2 pixels have been
reported, occasionally larger when applying external shifts by means
of input shiftfiles. Subsequently, significant improvements have been
made to the software to ensure that the fourth-order polynomial
distortion solutions are now handled to a precision better than
0.001 - 0.01 pixels internal to the code itself, reaching the limits
of the distortion model. The improved code will be in the next
Pyraf/STSDAS release, around late summer 2006.

However, inherent astrometric uncertainties up to ~0.1 pixel may
still exist in the observations and can be introduced by differences
between commanded offsets versus those that are obtained, as well as
long-term thermal changes and other effects that result in evolution
of the distortion solutions over time. All ACS data currently make
use of a single distortion solution obtained soon after the
installation of ACS, and discussions are underway to produce
time-dependent solutions that would be more appropriate for more
recent data. In the meantime, it is possible to refine shifts
iteratively, if necessary, as described in section 4.6 of the ACS
Data Handbook. Observers will be notified as soon as more updated
time-dependent distortion solutions are available.

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7.  HRC Quantum Yield Correction Implemented in the ETC

In the ACS/HRC CCD a single UV photon (below ~ 3350 A)
may generate more than one photo-electron (Quantum Yield > 1).
In order to accurately calculate the S/N ratio of an HRC observation
in the UV, the shot noise characteristics of the incident photons
cannot be directly inferred from the number of electrons
generated. The source signal and the sky level need to be corrected
for the quantum yield.  Without such a correction, the S/N can be
overestimated up to ~18% in the F220W and ~26% at the bluest edge of
PR200L.  In late July 2006 a revised version of the Exposure Time
Calculator (ETC) that includes such correction will be released.

http://www.stsci.edu/hst/acs/software/etcs/ETC_page.html

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8.  Dithering in Cycle 16

The ACS Team has long recommended that observers dither (or offset)
their observations to remove hot pixels, cosmetic defects, and cosmic
rays in the combined data.  Dithering allows for optimal sampling
of the point spread function and provides substantive improvements
over the use of CR-SPLITs, which do not remove hot pixels or other
permanent cosmetic defects (e.g., bad columns or the WFC interchip
gap). Dithering can be achieved in two ways: 1) By using exposure-to-
exposure position offsets specified explicitly by POS-TARGs, or
2) By using flexible predefined dither patterns.  Both POS-TARGs and
dither patterns now result in associations, and dither patterns can
also be nested to achieve various pixel sampling strategies for a
wide variety of science goals. Currently available dither patterns
within APT and their recommended uses are described on the ACS Dither
Web page:
http://www.stsci.edu/hst/acs/proposing/dither

We strongly encourage observers to dither their ACS observations.
Beginning in Cycle 16, observers who choose not to dither their
ACS WFC and HRC imaging exposures should provide sound scientific
justification for not doing so in the Phase I Proposal Description
of Observations. The choice of dither pattern is left to the
discretion of the observer since it may depend upon the science
goals of the program.  The ACS Team is ready to assist observers who
need help in selecting the appropriate pattern for their
observations.

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9. Recent Instrument Science Reports

ACS:
Policy and Procedure for MAMA Targets Subject to
Unpredictable Outbursts(ACS ISR 06-04)
http://www.stsci.edu/hst/acs/documents/isrs/isr0604.pdf

Wavelength and Flux Calibration of the ACS/HRC PR200L
prism (ISR 06-03)
http://www.stsci.edu/hst/acs/documents/isrs/isr0603.pdf

Wavelength and Flux Calibration of the ACS/SBC PR110L and
PR130L prisms(ISR 06-02)
http://www.stsci.edu/hst/acs/documents/isrs/isr0602.pdf

PSFs, Photometry, and Astrometry for the ACS/WFC (ISR 06-01)
http://www.stsci.edu/hst/acs/documents/isrs/isr0601.pdf


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

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_july_2006.pdf

WFPC2:
http://www.stsci.edu/hst/acs/documents/pub_wfpc2_july_2006.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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11. Recent Press Releases

ACS:
http://www.stsci.edu/hst/acs/documents/press_acs_july_06.html

WFPC2:
http://www.stsci.edu/hst/acs/documents/press_wfpc2_july_06.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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| 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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| Inc., under NASA contract NAS5-26555.
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