Wide Field Planetary Camera II Status Update
J. Biretta, S. Baggett, S. Casertano, S. Gonzaga, I. Heyer, M. Wiggs, M. McMaster (Space Telescope Science Institute)
HST SAA Models - SEU’s - S/C Trajectories
Abstract
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We review the status of the Wide-Field Planetary Camera II (WFPC2)
onboard the Hubble Space Telescope as well as recent enhancements to
calibration and analysis methods. The instrument continues to perform
extremely well. Low level effects such as dark current and CTE continue to
increase with long-term radiation exposure (see accompanying poster), but
have minimal effect on most observations. The long-term photometric stability appears to be excellent, with most filters showing changes of a few
percent or less. Recent work shows that aperture photometry for small
apertures (1-2 pixel radius) will be affected at the 5% to 10% level by target position in the field-of-view due to small focus variations. New flat
fields are available which correct long-term changes in the illumination
pattern; corrections are typically 0.5%, though small dust spots have corrections up to 12%. The Dither Package software has been upgraded to
include mosaicing of all 4 CCDs, as well as cosmic ray removal from single image pointings. These and other topics are reviewed.
median for central 400x400, darks, gain 7
DN/sec
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2.50E-4
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Drizzling Software and Documentation. Most imaging observations made with WFPC2
Long-range HST Schedule. Through most of last year, approximately 20% of HST time was devoted to WFPC2 observations. With the suspension of NICMOS operations, this rate has increased to 40% - 50%. About 160
Cycle 6 and 7 proposals are in the scheduling queue, as well as 111 new cycle 8
proposals. A recent study of the HST long-range plan shows that opportunities
for long, contiguous observations (> 5 orbits) are over-subscribed. This is due to
several factors, including a trend towards longer observations of fewer targets
and the recent suspension of NICMOS observing. A number of steps are being
taken to alleviate this situation for WFPC2 observers. Some WFPC2 PIs have
been asked to split their long visits into pairs of shorter ones wherever possible.
This will avoid scheduling delays and help observers obtain the same data sooner
than otherwise. We have also made the Southern Atlantic Anomaly avoidance
region smaller for WFPC2 (M26 contour in figure). As a result, about 0.1% more
time becomes available for scheduling long visits. Finally, for programs with
short exposures, more effort will be made to utilize short visiblity periods which
are often available in the schedule, rather than requiring the usual ~56 minutes of
visibility per orbit. We believe these changes will be transparent to most observers and will mitigate the over-subscription. Distribution of cosmic ray rate
around the SAA with the revised WFPC2 SAA avoidance contour indicated (thin
solid line). Curved lines indicate the HST orbit at points where it runs tangent to
the old WFPC2 contour (M25, dotted line) and the new contour.
(and other HST imagers) now use position dithers to aid in the removal of detector artifacts, as well as for
enhancement of spatial resolution. The observed images are aligned and combined using the "Drizzle" technique with software written by Andrew Fruchter (STScI) and Richard Hook (ST-ECF). The basic Drizzling routines are now incorporated in STSDAS. A recent upgrade (the "ditherII" package) is available from the WFPC2
website; this suite of tasks allows removal of cosmic rays when there are only single images at each pointing.
We have recently completed a detailed, hands-on guide to the Drizzling software package, called "The Drizzling
Cookbook." The Cookbook gives detailed examples of the drizzling procedures for WFPC2, STIS, and NICMOS data for a wide range of targets. Input images, command scripts, and final images are also available on our
WFPC2 site, so that users can practice with our examples before drizzling their own data. Work is underway to
add additional capabilities to the drizzling software, including the ability to mosaic the four WFPC2 CCDs onto
a single output drizzled image. An update to both STSDAS and the Cookbook for these added capabilities is
planned for next Summer. The figures are "before" and "after" images of the Hawaii Deep Survey Field SSA22
(HST program 5399, PI: Lennox Cowie). The left image is one of 12 single image dithers taken in the WF2. The
right image is the result of shifting and drizzling the 12 input images.
References to published results of the data discussed in this
poster are available in a handout. If there are no handouts to
be found beneath this poster, please ask at the STScI booth. At
the booth we also have limited copies of the Drizzling Cookbook, the Flat Field ISR, and the Cyle 8 Calibration Plan.
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Dark Current and Hot Pixels.The number of permanent
hot pixels at all intensity levels has increased by about 2.5 times since
1995. These still represent only a very small fraction of the total number of
pixels (0.2%). Low level dark current has doubled since 1995; it now
ranges from approximately 6 to 10 x 104 DN/s on the different CCDs (see
plot). This has little impact on most observations, except for some noise
increase for long exposures in narrow-band and UV filters. Both these
effects are caused by on-going radiation damage to the CCDs. New superdarks will calibrate-out these long-term changes.
Flat Field Evolution. Most changes in the flat fields are due to
small shifts (~1 pixel) in the camera’s optical alignment, and have the
effect of moving dust spots and camera obscurations relative to the pixel
grid. The strongest changes are pairs of bright/dark spots a few pixels in
size with +/- 12% amplitude on WF4; these are coincident with previouslyknown strong dust spots. Each CCD has about two dozen such features
where milder errors (> 4%) are seen. All cameras also show a pattern of
broad (40 pixel wide) diagonal bars crossing the CCD with 0.5% amplitude, which are caused by changes in the alignment of the OTA spider and
WFPC2 repeater spiders. The only other notable change is the appearance
of three new dust spots on the WF2 field flattener lens. Work is underway
to update the flat field calibration files to account for these changes. The
figure is a ratio image of new vs. old flats for F502N in WF4.
Geometric Distortion
and Inter-chip Position Errors. The geometric
Photometric Monitor. The standard
star GRW+70D5824 is regularly observed to monitor photometric throughput. These observations,
conducted since May 1994, indicate that the
throughput for most filters has been stable to ~2%.
However, the far-UV throughput in the PC appears
to be increasing with time. This effect, seen in
F160BW and F170W filter plots, is probably due
to the slow outgassing of some contaminants.
Note: the bi-modal appearance of the UV filters is
due to the decontamination procedure, where the
WFPC2 is warmed to approximately 20 degrees
Celsius for 6-12 hours to remove the build-up of
contaminants. High points indicate high-throughput measurements taken after decontamination.
distortions for each chip, caused by
the camera field flattener, remains
stable. An improved plate solution
with new geometric distortion correction coefficients will be available in the near future. The figure
illustrates the distortions over the
WF3 field-of-view for three different solutions: the Trauger solution
for F555W (top image), the Holtzman solution (middle image), and
the new solution derived by Stefano
Casertano (STScI WFPC2 group).
The positions of the WFPC2 chips
with respect to each other appear to
shift slowly by on the order of 100
mas (~1 pixel). The WFPC2 group
is monitoring this effect and will
post updates as needed to the
WFPC2 Advisories webpage. The
inter-chip position errors affect the
STSDAS WFPC2 tasks metric and
wmosaic; these tasks will be
updated in a future version of
STSDAS.