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Hubble Space Telescope
ACS+WFPC2 STAN, 07 February, 2008

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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 7 February 2008
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CONTENTS:

1.  ACS-Repair Status

2.  ACS SBC Bright Object Limit Revisions

3.  Calibration Outsourcing

4.  Plans for WFPC2 Data Reprocessing

5.  Update on WFPC2 - WF4 Anomaly

6.  Recent Instrument Science Reports

7.  Recent Refereed Publications

8.  Recent Press Releases

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1.  ACS-Repair Status

In January 2007 ACS lost its Wide Field Channel and High
Resolution Channel due to a failure in the low voltage power
supply of the Side-2 Electronics.  ACS was operating on
side-2 because of the previous failure of the side-1
electronics in 2006.

Soon after the failure the HST Project rapidly assembled
a team to determine if the ACS could be repaired as part of
HST Servicing Mission 4 (SM4). This team includes contributors
from NASA's Goddard Space Flight Center, Ball Aerospace,
Teledyne Imaging Sensors and STScI.  It determined that all
of the capabilities of the ACS could be restored and created
a concept for the ACS-Repair (ACS-R) component of SM4.

ACS-R is designed to restore full, non-redundant operation
to ACS by replacing the original WFC CCD electronic box (CEB)
with the CEB-Replacement (CEB-R) and providing power from a
new Low Voltage Power Supply Replacement (LVPS-R).  The new
LVPS-R will also attempt to restore the HRC function by
providing power to the original HRC CEB through the original
power bus (backpowering). The repair is designed so that
failure of HRC backpowering does not pose risk to the
success of the WFC repair.

As of January 2007, the ACS-R development is proceeding well
and is on-track for SM4.  The team has successfully demonstrated
that the ACS-R design can deliver the expected performance
from the CCDs, and is integrating and testing a fully flight-
representative engineering model.  This testing will be
completed shortly, and fabrication of the flight hardware will begin.

A successful repair will restore the full functionality
of ACS after SM4. All the three channels of ACS are
available for the Cycle 17 Call for Proposals.

More information will be available at the ACS repair web site.


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2.  ACS SBC Bright Object Limit Revisions

As discussed in the ACS Instrument Handbook for Cycle 17,
Sections 4.4.2 and 5.5.2, during 2007 it was found that the
SBC sensitivity to near-UV and optical light (the "red leak")
is significantly greater than previously recognized (or
alternatively, it has increased with time).  Since the
release of the current Handbook, further analysis has shown
that the effect is somewhat less than specified there, but
still much larger than previously recognized.  Further
details will be provided by Boffi et al. in ACS ISR 2008-02.
A revised sensitivity curve has been delivered to the
calibration database, and since November 2007, that has been
reflected in the ETC.  Hence, the V-magnitude screening
limits in Table 7.4, Section 7.2.2 of the Handbook are now
fainter for stars later than type F0 V.  Revised values
are given in the following table.

Revised bright-limit V-band magnitudes for observations
with the SBC filters and prisms (no reddening).
----------------------------------------------------------
Spec F122M F115LP F125LP F140LP F150LP F165LP PR110L PR130L
---- ----- ------ ------ ------ ------ ------ ------ ------
F0 V  4.8   10.1   10.0   10.0   10.0    9.9    8.3    8.3
F2 V  4.0    9.3    9.3    9.2    9.2    9.1    7.6    7.5
F5 V  2.2    7.7    7.6    7.6    7.6    7.5    5.9    5.9
F8 V  0.9    6.8    6.8    6.7    6.7    6.7    4.7    4.7
G2 V  2.2    6.3    6.0    6.0    5.9    5.8    3.2    3.1
G8 V  ...    5.6    5.6    5.4    5.2    4.8    2.2    2.1
K2 V  ...    5.7    5.7    5.2    5.0    4.3    3.2    3.0
KM III ...   4.5    4.5    4.2    4.0    3.6    2.7    2.3
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All values have been calculated with the ETC to satisfy
the local countrate screening limit of 50 counts/second/pixel
(Table 7.3), and all other input sources are also as
specified in the Handbook, except that the KM III values
are now the faintest for each spectral element among IUE
data for 8 objects with spectral types from G8 through M5.

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3.  Calibration Outsourcing

The Call for Proposals for Cycle 17 specifies a category
of "Calibration GO Programs" in Section 3.2.4 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.  By definition WFPC2 calibrations may
only utilize archival data, while ACS may also request
new observations as relevant.  The large number of
WFPC2 'close-out' calibrations obtained in the past year,  
present opportunities for community  involvement. 
If interested in exploring specific calibrations please initiate 
exchanges (e.g. understand our plans) with the instrument team by 
25 February via help@stsci.edu.

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4.  Plans for WFPC2 Data Reprocessing

In preparation for the decommissioning of WFPC2, we have
undertaken a series of calibration and software enhancements
to the CALWP2 calibration pipeline.  These enhancements
include improved photometric calibrations, geometric-distortion
coefficients, and flat fields; an improved UV-throughput
correction; removal of the streaks and correction of the
photometric errors caused by the WF4 anomaly; and the
production of a PyDrizzled image for each WFPC2 data set.
Once these enhancements have been incorporated into CALWP2,
we will reprocess all WFPC2 data in the HST archive.  The
resulting data products, available in both waiver and
multi-extension FITS formats, will provide a uniformly-
calibrated WFPC2 data set, readily accessible to the Virtual
Observatory and related projects.  On-the-fly recalibration
(OTFR) will be discontinued, dramatically improving data
delivery speeds.  We expect reprocessing to begin in
the summer of 2008.

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5.  Update on WFPC2 - WF4 Anomaly

Since early 2002, the WF4 detector has exhibited an anomaly
characterized by low or zero bias levels, faint horizontal
streaks, and low count levels. The problem is thought to be
caused by a failing amplifier in the WF4 signal-processing
electronics. The anomaly is temperature dependent and since
2006 January has been mitigated by reducing the temperature
of the instrument at regular intervals.  The next temperature
adjustment, scheduled for 2008 February 26, should ensure
normal bias levels through SM4.  To correct archival data
for the effects of the anomaly, two new calibration steps
have been added to CALWP2. The first rescales the counts in
each image pixel, correcting the photometry and restoring
the bias to its normal level.  The second removes the
streaks seen in low-bias WF4 images.  Testing of both
programs is ongoing, and we expect them to be completed in
time for the WFPC2 data reprocessing this summer.

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6.  Recent Instrument Science Reports

ACS:

Updated Flux Calibration and Fringe Modelling for
the ACS/WFC G800L Grism (ACS ISR 08-01)
http://www.stsci.edu/hst/acs/documents/isrs/isr0801.pdf

ACS PSF Variations with Temperatures (ACS ISR 07-12)
http://www.stsci.edu/hst/acs/documents/isrs/isr0712.pdf

Calibration of Ramp Filters Using the ACS Grism (ACS ISR 07-11)
http://www.stsci.edu/hst/acs/documents/isrs/isr0711.pdf

ACS Polarization Calibration - Data, Throughput,
and Multidrizzle Weighting Schemes (ACS ISR 07-10)
http://www.stsci.edu/hst/acs/documents/isrs/isr0710.pdf

Two Astrometric Fields for UV Observations (ACS ISR 07-09)
http://www.stsci.edu/hst/acs/documents/isrs/isr0709r.pdf

Variation of the Distortion Solution of the WFC (ACS ISR 07-08)
http://www.stsci.edu/hst/acs/documents/isrs/isr0708.pdf

Calibration of ACS/WFC Absolute Scale and Rotation for Use
in Creation of a JWST Astrometric Reference (ACS ISR 07-07)
http://www.stsci.edu/hst/acs/documents/isrs/isr0707.pdf

Photometric Calibration of the ACS CCD Cameras (ACS ISR 07-06)
http://www.stsci.edu/hst/acs/documents/isrs/isr0706.pdf

Detection of Optical Ghost in the HST ACS Solar
Blind Channel Filter 122M (ACS ISR 07-05)
http://www.stsci.edu/hst/acs/documents/isrs/isr0705.pdf

ACS/WFC: Differential CTE Corrections for Photometry
and Astrometry from non-drizzled Images (ACS ISR 07-04)
http://www.stsci.edu/hst/acs/documents/isrs/isr0704.pdf

ACS CCDs UV and Narrow-band Filters Red Leak Check (ACS ISR 07-03)
http://www.stsci.edu/hst/acs/documents/isrs/isr0703.pdf

WFC Zeropoints at -80C (ACS ISR 07-02)
http://www.stsci.edu/hst/acs/documents/isrs/isr0702.pdf

Pixel-to-pixel Flat Field Changes on the WFC (ACS ISR 07-01)
http://www.stsci.edu/hst/acs/documents/isrs/isr0701.pdf

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

WFPC2:

Temperature Reductions to Mitigate the WF4 Anomaly (WFPC2 ISR 07-01)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0701.html

For a list of all WFPC2 ISRs please see:
http://www.stsci.edu/hst/wfpc2/documents/isrs

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7.  Recent Refereed Publications

A list of refereed publications for ACS or WFPC2 may
be obtained using the STScI Library associated 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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8.  Recent Press Releases

Press releases, many of which are based on data from
WFPC2 and/or ACS may be identified 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:
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| WFPC2: http://www.stsci.edu/hst/wfpc2
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| or http://www.stsci.edu/hst/wfpc2/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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