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Hubble Space Telescope
WFPC2 STAN

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| STScI Analysis Newsletter (STAN)
| WFPC2
| 06 November 2008
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CONTENTS:

1.  PyDrizzled Images from MAST

2.  Final Calibration of WFPC2

3.  WFPC2 Reprocessing and the Static Archive

4.  New WFPC2 Documentation

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1.  PyDrizzled Images from MAST

Users requesting WFPC2 data from the MAST archive now receive an
additional data product, a quick-look mosaic of the images from all
four detectors.  The new files, labeled *drz.fits, are produced by
the program PyDrizzle.  The images are geometricaly corrected and
have a plate scale of 0.1" per pixel.  Because PyDrizzle is run on
each WFPC2 exposure independently, the program cannot perform
sub-pixel sampling or cosmic-ray rejection.  DRZ files are thus
intended as quick-look images, rather than as science data products.
DRZ files utilize a multi-extension FITS format, rather than the
waiver FITS format of traditional WFPC2 data products.

While most WFPC2 image files are in units of counts, the units of
WFPC2 drizzled images retrieved from MAST are counts per second.
Always check the value of the header keyword BUNIT to be sure of
the units.  Note that the keyword PHOTFLAM in the DRZ file is taken
from the first extension (usually the PC chip) of the calibrated
data file.

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2.  Final Calibration of WFPC2

In preparation for the decommissioning of WFPC2, the WFPC2 team has
undertaken a series of reference-file updates and software enhancements
to the WFPC2 calibration pipeline:

WF4 Anomaly:  Since 2002, a temperature-dependent reduction in the
gain has plagued images obtained with the WF4 detector.  Characterized
by low or zero bias levels, faint horizontal streaks, and low
photometry, the WF4 anomaly is thought to be caused by a failing
amplifier in the WF4 signal-processing electronics.  Software to
correct for the anomaly has been added to CALWP2 and the associated
reference files are now in place.  Tests indicate that the corrections
are accurate to 1-2%.  A separate routine to remove the horizontal
streaks that often appear in low-bias frames will be included in
the next STSDAS release.  The program, called "wfpc2destreak," is
a stand-alone routine and not part of the pipeline.  It can also
remove the bias variations that occasionally appear in the other
CCDs.

CTE Keywords:  Three new keywords have been added to the WFPC2 file
headers.  CTE_1E2, CTE_1E3, and CTE_1E4 contain estimates of the
charge-transfer losses (in units of delta mag) for a star at x =
400, y = 400 with intensities of 100, 1000, and 10,000 electrons,
respectively.  The program uses Andrew Dolphin’s (2004) recipe,
which is a function of star brightness, observation date, and
background level.  Note that the pipeline only populates these
header keywords.  No changes are made to the pixel values themselves.

UV contamination & QE changes with time: We have reanalyzed the
calibration observations obtained to track the variation in UV
throughput as contaminants condense on the cold CCD windows — and
are later removed by periodic decontamination procedures.  The
result is an updated SYNPHOT contamination table.  A new header
keyword, ZP_CORR, gives the offset to PHOTFLAM needed to account
for both short-term variations in UV throughput and long-term changes
in the detector quantum efficiency as a function of the observation
epoch and filter.  Users will no longer have to run SYNPHOT to
compute these corrections.

Photometric zeropoints: We are re-evaluating the zeropoints at the
end of WFPC2’s mission using new observations of standard stars and
standard fields. We will update the SYNPHOT filter throughput tables
and detector efficiency tables as needed.

Flat fields for broad-band filters: We evaluated the accuracy of
the flat fields for broadband filters by a new technique: using the
Earth illuminated by the full Moon as a diffuse light source. We
found the existing flats to be quite accurate and plan no further
updates to the flat fields.

Filter red leaks: We measured the leak component directly for the
first time on-orbit by crossing the UV and blue filters with other
filters and observing a bright standard star.  We will use the
results to update the SYNPHOT throughput tables.

Narrow-band filters: We made and analyzed new observations to assess
the wavelength and photometric stability of the narrow-band filters
over the 15-year WFPC2 mission. We will update the SYNPHOT throughput
tables as needed.

Linear ramp filters: We made a new series of photometric calibrations
that will produce substantially more accurate photometry in many
wavelength ranges. We will incorporate the results in new SYNPHOT
tables.

Polarizers. We have made new observations to reevaluate the
calibrations of the polarizers near the mission end.  The analysis
of these data is pending.

Geometric distortion and astrometry. We reevaluated distortion
corrections and generated new, time-dependent corrections for the
offsets between the four CCD chips over the entire WFPC2 mission.
We addressed the 34-row effect, where every 34th pixel row of the
CCD is about 3% too narrow due to a manufacturing error. These have
resulted in new geometry tables — IDCTAB, OFFTAB, and DGEOFILE —
used for "drizzling" data onto a corrected grid. In addition, we
added a new keyword, VAFACTOR, to the header of each image; it
describes the effects of velocity aberration due to Hubble’s orbital
motion.

Bias and dark calibrations: We have generated new super-bias and
super-dark reference files for the last few years of the WFPC2
mission.  We are studying the long-term history of both the bias
and dark calibrations.

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3.  WFPC2 Reprocessing and the Static Archive

At present, WFPC2 images are calibrated, or processed, each time
they are requested from the Hubble archive, thus insuring use of
the most recent calibration files and software. Once final versions
of the calibration files and software are in place, this dynamic
approach will no longer be necessary. The WFPC2 calibration pipeline
will be run one last time for each image, and the results stored
in a static archive.  Subsequent data requests will be fulfilled
from this archive, speeding delivery and reducing costs.  Reprocessing
of the WFPC2 data has already begun and should be completed early
in 2009.

Data files will be archived in both the waiver FITS (WFITS) format
traditionally used by WFPC2 and in the standard multi-extension
FITS (MEF) format used by ACS and other recent HST instruments.
Waiver FITS files will end in "f" as they do now (e.g.,
u9wj600im_c0f.fits), while MEF format files will end in "m" (e.g.,
u9wj600im_c0m.fits).  Both the raw data and calibration reference
files will continue to be available in the archive, should users
wish to perform their own calibrations.

While reprocessing is under way, requests for data sets that have
already been reprocessed will be fulfilled using the new static
archive. For these data sets, files will be returned in WFITS format;
MEF files may be requested by entering the desired extension (c0m)
at the bottom of the MAST Retrieval Options page.  Data sets that
have not yet been reprocessed will be calibrated using the standard
pipeline; for these data sets, only WFITS files are available.  Once
reprocessing is complete, on-the-fly reprocessing will be discontinued,
and calibrated data will be supplied directly from the static
archive.  You can monitor the status of the reprocessing effort
using this web page:

http://archive.stsci.edu/hst/reprocessing.html

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4.  New WFPC2 Documentation

We are working to revise and update the WFPC2 documentation.  A
final version of the WFPC2 Instrument Handbook is now available,
and a final WFPC2 Data Handbook will be released in the spring of
2009.  Additional reports will be produced as calibrations are
updated and finalized.


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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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