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WFPC2 STAN

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| STScI Analysis Newsletter (STAN)
| ACS+WFPC2
| 09 September 2009
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CONTENTS:

1.  Repair and Recovery of ACS WFC 

2.  Initial Delivery of WFC Reference Files

3.  WFC Flat Fields

4.  WFC Signal-Dependent Bias Shift

5.  WFC Amplifier Cross-Talk

6.  SBC Operations

7.  WFPC2 Closeout and Data Archive

8. Recent Instrument Science Reports

9. Recent Refereed Publications

10. Recent Press Releases

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1.  Repair and Recovery of the ACS WFC

During the third EVA of SM4 on 16 May 2009, astronauts John Grunsfeld and Drew 
Feustel replaced the ACS CCD Electronics Box and Low Voltage Power Supply that 
incrementally failed in June 2006 and January 2007, causing the loss of the ACS
Wide Field Channel and High Resolution Channel.   The replacement components
(CEB-R and LVPS-R) immediately restored the function of the WFC but not that of
the HRC.  Unfortunately, the damage in 2007 to the circuitry that controlled the
HRC occurred upstream of the location repaired by the CEB-R.  This situation was
not unexpected, as the post-failure analysis of the nature and location of the
short in the HRC circuitry was ambiguous.

The recovery of WFC restores modes of ACS operation used in approximately 80% of
ACS science observations prior to January 2007.   The Solar Blind Channel (SBC)
was not affected by the failure of the CCD circuitry, so its availability and 
usage have been uninterrupted since Janury 2007 (except during SM4).  

Successful aliveness and functionality tests of WFC during EVA-3 were followed 
10 days later by a successful CCD anneal and a campaign to optimize WFC 
performance by exploiting the capability of the CEB-R's Application-Specific 
Integrated Circuit (ASIC) to vary on-board the CCD's bias and clock voltages 
and data-transmission timing.   The optimization campaign showed that the 
performance of WFC CCD under command of the new CEB-R matches or exceeds that of
the old CEB in terms of read noise, linearity, pixel full-well depth, and 
amplifier cross-talk with the same default settings used before January 2007.
The WFC's dark current, hot-pixel fraction, and charge-transfer efficiency (CTE)
have degraded to the levels expected after 28 more months in HST's trapped 
radiation environment since January 2007.  

The CEB-R is equipped with two correlated-double sampling (CDS) modes (clamp and
sample, dual-slope integration) that can be used to measure the charge acquired
by each pixel during read out.   The clamp and sample mode was used successfully
in the old CEB, but the dual-slope integrator (DSI) offers lower read noise at
the expense of more bias structure.  Post-SM4 image analysis confirms that the
DSI yields significantly lower read noise than observed before January 2007 and
that bias frames exhibit a 5-10 DN gradient spanning the rows of each image 
quadrant (see Section 2).  The bias gradient is stable over the time expected 
between the delivery of consecutive calibration reference files, and so will be
precisely removed during normal image reduction and processing.  Consequently,
the DSI has been selected as the default CDS mode for post-SM4 observations.

In summary, the ACS repair during SM4 has fully restored the imaging and grism 
capabilities of the WFC with performance that is equal to or better than its 
pre-January 2007 performance after adjusting for 28 more months of exposure 
to the HST radiation environment.  Table 1 compares the pre-SM4 (January 2007)
detector characteristics with those measured during the recent SMOV period 
(May-July 2009).


Table 1.  WFC characteristics and performance 
============================================================
			Pre-SM4		Post-SM4
Metric			Jan 2007	May-July 2009
------------------------------------------------------------
Read noise 		5.5 e-		4.2 e- (pre-scan)
(e-; gain=2)				4.8 e- (image area)
------------------------------------------------------------
Dark current		10.7		22
(e-/pix/hr)
------------------------------------------------------------
Hot pixels		0.6		1.1
(% of image)
------------------------------------------------------------
Full-well depth		84000		> 80000
(e-; average)
------------------------------------------------------------
Non-linearity		< 0.1		< 0.2
(%)
------------------------------------------------------------
CTE			0.999949	0.99989
(EPER, 1620 e-)
------------------------------------------------------------
Cross-talk		4e-05		(5 +/- 4)e-05
(50000 e- source)
------------------------------------------------------------


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2.  Initial Delivery of WFC Reference Files

The first post-SM4 WFC calibration reference files were delivered to STScI's
Calibration Database System (CDBS) on 9 Sep 2009 in conjunction with the public
announcements of the post-SM4 HST Early Release Observations.  Included in this
delivery are "superbias" and "superdark" reference images and the CCDTAB 
reference file, all of which are used in OPUS processing of WFC images using 
the CALACS calibration software.

The superbias frame is a median stack of 34 bias frames recorded between the
first two post-SM4 anneal cycles (7 July 2009-3 August 2009).  Inspection of 
the superbias reveals two features not seen in pre-SM4 superbiases: 

(1) The bias level increases by 5-10 DN diagonally across each CCD quadrant from
    the vertical prescan to the amplifier boundary.  The bias gradient is a 
    characteristic of the dual-slope integrator (DSI) used in the CDS stage of 
    the pixel conversion (Section 1).   It is caused by a slow drift of the bias 
    reference voltage during and after the readout of each row of pixels.  The 
    gradients appear stable from frame to frame, and they can be removed via 
    subtraction of the contemporary superbias reference image.

(2) Faint horizontal stripes extend along the rows of the CCD and across the 
    quadrant boundaries.  The stripes are constant along each row of pixels,
    but they are not stable from frame to frame.   They are caused by low-
    frequency (1mHz to 1 Hz) 1/f-noise on the reference voltage generated by the
    ASIC used to offset the pixel signal after CDS is performed.  The stripes 
    are present in all WFC calibration and science images regardless of CDS 
    mode.   The contribution of the stripes to the global read noise statistics
    is small (the peak-to-peak deviation is approximately 2 DN), but the 
    correlated nature of the noise may affect photometric precision for very 
    faint sources.   STScI is developing algorithms for removing the stripes 
    from calibration and general science images, and we intend to release these
    algorithms to the community as standalone software packages that operate on 
    the images independently of CALACS.  As the effect of the stripes on most 
    science programs will be insignificant, STScI does not intend to add 
    stripe-removal to standard CALACS processing in the OPUS pipeline.

The superdark frame is a median, bias-subtracted stack of 24 individual dark 
exposures of 1000 sec.  The superdark pixel values have units of e-/s, and 
the pixels in each image quadrant have been normalized to a common sensitivity
using the relative amplifier gains measured from internal flat field images.
The remaining small discrepancies between the dark signals in contiguous 
quadrants will be resolved soon by refined measurements of the amplifier gains 
from other ACS calibration programs.

The post-SM4 performance characteristics of each amplifier are included in the
newly delivered CCDTAB reference file.   (Note that Table 1 lists the average 
performance characteristics of all four amplifiers.)   The measured values of 
pre-scan bias and read noise for each amplifier were obtained from bias frames
recorded with each of the four gain settings in the CCD functional test during
SMOV (Program 11369) and the SMOV component of the CCD daily monitor (Program
11465).   Precise values of the gain=2.0 setting and average pixel full well
depth were obtained from photon-transfer tests conducted during the SMOV 
Optimization Campaign (Program 11809).  The measured values for the other three
gain settings (0.5, 1.0, and 1.4) were obtained from ratios of flat field images
obtained in the SMOV CCD functional test.  Refinements of these non-default gain
and saturation measurements are currently being determined via measurements of 
astronomical photometric reference stars (Program 11397).   

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3.  WFC Flat Fields

Initial examination of post-SM4 flat field images recorded through the F435W, 
F625W, and F814W broadband filters indicate that the sensitivities of only 
0.5-1.5% of pixels deviate by more than 3 sigma from pre-January 2007 values.
The low number of deviant pixels suggests that the pre-SM4 flat fields are 
presently sufficient for calibrating post-SM4 WFC science images.

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4.  WFC Signal-Dependent Bias Shift

The DSI mode of WFC operation induces a signal-dependent bias shift whose cause
is closely related to that of the bias gradient described in Section 2.   The DC
level of the DSI mode is sensitive to changes in the CCD output voltage in such
a way that the pixel bias level is shifted positively by 0.02-0.30% (depending
on the amplifier) of the signal from the previously integrated pixel.  This
phenomenon is well understood and can be analytically removed using a parametric
algorithm developed by M. Loose of Teledyne Scientific.   This algorithm will be
made available to the community as a standalone package that can be applied to
raw WFC images before CALACS processing.  We anticipate that most users will not
require correction of this small effect, so the correction will not be made part
of the standard OPUS pipeline.

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5.  WFC Amplifier Cross-Talk

Preliminary analysis shows that negative crosstalk exists among the four WFC 
amplifiers at levels similar to those existing before SM4.  At source signals
near the full-well limit, the crosstalk-to-signal ratio is approximately 1e-4, 
or ~10 e-/pix.  Some differences between the manifestations of the pre- and 
post-SM4 crosstalk are currently under study and will be reported at a later 
date. 

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6.  SBC Operations

The Solar Blind Channel (SBC) was not affected by the ACS repair during SM4 and
is continuing to operate nominally.  During SMOV, the sensitivity, dark current,
encircled energy, geometric distortion, and flat field stability of SBC were 
compared with pre-SM4 values, and no significant differences were found.

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7.  WFPC2 Closeout and Data Archive

WFPC2 was removed from HST in May 2009, bringing to a close its highly 
successful 15+ year mission.  The instrument is currently at Goddard Space 
Flight Center where various post-mission tests are being performed, and 
preparations are being made for future museum displays of the instrument.

Instrument Calibration: 
Efforts continue towards finalizing calibrations for WFPC2.  Since the last STAN 
issue approximately 700 new calibration reference files have been delivered.  
These include new calibrations for the 2008-2009 data, results of special 
closeout observations made over the past year, and various improvements for data
taken throughout the WFPC2 mission.  Among these are: improved bias and dark 
calibration for 2008-2009 data; improved gain corrections for the WF4 CCD 
spanning 2002-2009; improved flats covering the early 1990's; new calibrations 
for AREA mode data; improved contamination and throughput corrections for the 
entire mission; on-orbit photometric calibrations for the Linear Ramp Filters;
on-orbit estimates of UV filter red-leak; and improved astrometric calibrations
for the entire mission.  Details of these improvements will be given in future
STANs, ISRs, and elsewhere.

WFPC2 Archive:
Two important changes were recently made to the WFPC2 data archive.  Previously
WFPC2 data would be calibrated "on-the-fly" when they were requested from the 
archive, thus giving users the best and most recent calibration.  However, as 
most calibrations are finalized, we have transitioned the WFPC2 archive to a 
static set of calibrated images which are used to fulfill new archive requests.
At this time, approximately 99% of the 186481 WFPC2 images have been reprocessed
and placed into this static archive.  The remaining ~1% of WFPC2 images are 
primarily data taken through multiple filters (e.g. polarizers and low-pass 
filters) which are awaiting final calibrations.

We have also changed the default format for the WFPC2 archive output.  Prior to
August 2009, WFPC2 archive images were provided in "wFITS" or "waivered FITS" 
format, which was related to the GEIS format used for many years.  We have now 
changed the default output format to "MEF" or "multi-extension FITS" which is 
more similar to newer HST instruments.  For the moment, the old format remains
available as an option for requests made through the archive web page.  
(Archive requests made through the old StarView interface will continue to be 
delivered in the old wFITS format.)  Information about this change, and tips for
handling the new format, are given on the WFPC2 web pages. 

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8.  Recent Instrument Science Reports 

ACS:

Re-measurement of ACS/SBC dark images (ACS ISR 09-02) 
http://www.stsci.edu/hst/acs/documents/isrs/isr0902.pdf

Updated CTE photometric correction for WFC and HRC (ACS ISR 09-01) 
http://www.stsci.edu/hst/acs/documents/isrs/isr0901.pdf

ACS CCD Image Anomalies in the Hubble Legacy Archive (HLA ISR 08-01) 
http://www.stsci.edu/hst/acs/documents/isrs/hlaisr0801.pdf

HST Focus Variations with Temperature (ACS ISR 08-03) 
http://www.stsci.edu/hst/acs/documents/isrs/isr0803.pdf

A New Geometric Distortion Solution for the ACS/SBC (ACS ISR 08-02) 
http://www.stsci.edu/hst/acs/documents/isrs/isr0802.pdf

WFPC2:

Bandwidth Stability of the WFPC2 Narrow Band and Linear Ramp Filters 
(WFPC2 ISR 09-05)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0905.pdf

Wavelength Stability of the WFPC2 Narrow Band and Linear Ramp Filters 
(WFPC2 ISR 09-04)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0904.pdf

Pipeline Correction of Images Impacted by the WF4 Anomaly (WFPC2 ISR 09-03)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0903.pdf

WFPC2 F343N Filter Throughput Decline (WFPC2 ISR 09-02)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0902.pdf

Rapid Pinhole Growth in the F160BW Filter (WFPC2 ISR 09-01)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0901.pdf

Correcting Background Streaks in WFPC2 Data (WFPC2 ISR 08-03)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0803.pdf

Evolution of WFPC2 Superbiases (WFPC2 ISR 08-02)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0802.pdf

Flat Fields from the Moonlit Earth (WFPC2 ISR 08-01)
http://www.stsci.edu/hst/wfpc2/documents/isr/wfpc2_isr0801.pdf

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9.  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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10.  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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