For
Wide Field Planetary Camera II Status Update
M. S. Wiggs, J. Biretta, S. Baggett, S. Casertano, S. Gonzaga, I. Heyer, A. Koekemoer, M. McMaster, C. O’Dea, A. Riess, A. Schultz (ST ScI)
P
Abstract
We review the status of the Wide-Field Planetary Camera II (WFPC2)
onboard the Hubble Space Telescope, as well as summarize the re-commissioning tests performed after the December 1999 Servicing Mission
(SM3a). While WFPC2 itself was not serviced, there is always concern
about contamination or other unexpected effects. Although contamination
rates increased during the mission (as expected), bi-weekly decontaminations immediately after SM3a, and subsequent monthly decons have successfully restored the UV throughput to their pre-SMOV values, and there
is no evidence of any permanent contamination. A photometric sweep covering the entire wavelength range of WFPC2 shows that any changes in the
photometric response are less than ~1% at visible wavelengths, and less
than ~2-3% in the UV. No changes due to SM3a were seen in the flat field
response, read noise, dark current, a-to-d gain ratios, and point spread function tests. In addition to the SM3a results, we also discuss the general status
of WFPC2, including improved documentation for dithering, the now public On-The-Fly-Calibration (OTFC) system, and plans for calibrating
WFPC2 in Cycle 9. CTE results are presented in a separate poster.
SMOV Photometry Check. Observations of standard star
Cycle 9 Calibration Plans. Calibration plans for Cycle 9 are being finalized and, as always,
GRW+70D5824 for the SMOV3a photometry check were made on January 18,
2000. The star was positioned in the center of each camera during four 1-orbit
visits, one visit per camera. Images were obtained in the filters F160BW, F170W,
F185W, F218W, F255W, F300W, F336W, F439W, F555W, F675W, and F814W.
Photometry was performed using the phot task in STSDAS, with the star positions manually identified as input. Apertures of 11 & 5 pixels, and sky subtraction annuli of 11 & 5 pixels were used for the PC & WF cameras, respectively.
The photometric results from the SMOV3a observations were then compared to
standard, pre-SM3a photometric monitoring data, taken before the November 13,
1999 safing. The data indicates that the throughputs for the visible filters, except
for the F814W filter, are consistent with no change exceeding ~1%. The throughput for the UV and F814W filter data is consistent with no change exceeding
~2%. The photometric throughputs show that WFPC2 suffered no ill effects from
Servicing Mission 3a. Additionally, the Lyman-Alpha throughput measurements
for SMOV3a did not show any significant changes compared to pre-SM3a. The
WFPC2 pick-off mirror appears to have been protected from any significant contamination.
users are welcome to suggest new calibrations or comment on existing plans (help@stsci.edu). A table summarizing the programs proposed for calibrating WFPC2 in Cycle 9 as well as detailed descriptions will be
available in the WFPC2 Instrument Handbook:
http://www.stsci.edu/instruments/wfpc2/wfpc2_doc.html#Hand
The proposal suite is aimed at maintaining the calibration accuracy of WFPC2 via monitoring programs,
as well as continuing some previous proposals into Cycle 9, and performing several new tests. The standard
monitoring programs will be continued: decontaminations, photometric monitor, astrometric monitor, darks,
biases, internal flats, and Earthflats. In addition, the CTE monitor, PSF, and photometric checks will be performed.
Three new programs are planned as well: a red leak check (using solar analog standards), a verification of
the wavelength stability of the narrowband and linear ramp filters (using internal VISFLATs, as well as some
external extended line emission sources), and a CTE calibration (using three globular cluster fields). The
CTE calibration, in conjunction with WIYN groundbased observations, will permit a direct verification of the
absolute photometric calibration of WFPC2 in observations that may be affected by CTE, providing a more
robust determination of the zero point for many WFPC2 observations. Table 1, below, outlines the Cycle 9
calibration proposals.
Table 1. WFPC2 Calibration Proposals
Flat Field Stability Check. As part of our post-servicing
check-out of WFPC2 we compared Earthflats taken before and after SM3a.
This was done to test for OTA obscuration and contamination in WFPC2,
changes in the OTA/WFPC2 geometry, and localized CCD QE changes
that may have resulted from the servicing mission.
Earthflats were observed as part of the routine calibration proposals
8053 and 8445, as well as proposal 8495 for SMOV3a. All the flats used in
this test are F502N, gain 15, 1.2 second exposures of the bright Earth.
Eight pre-SM3a images with less than ~1% peak-to-peak amplitude in the
streaks (produced by features on the Earth moving across the FOV), were
selected. Five images from the SMOV3a proposal matching the same criteria were also chosen. These 2 groups of flats were were individually combined using the IRAF task streakflat, to create single pre- and post-SMOV
flats. We then divided the SMOV flat by the pre-SMOV flat, and normalized the resulting image so the central 400x400 pixels of WF3 had a mean
of unity. The resulting SMOV/pre-SMOV image is shown in Figure 1.
Most of the field-of-view shows essentially no change, <0.3%. Changes of
up to ~1.5% are seen at the CCD corners, particularly near the apex, and
are likely due to long-term changes in the camera geometry, rather than
SM3a.
Figure 2. Photometric monitoring of standard star GRW+70D5824. The observations span the interval from
May 01, 1994 to January 31, 2000. The F170W data points are marked with a cross-hair. (The bimodal distrubution of the data points results from pre- and post-decon measurements.) The UV sensitivity is
unchanged.
.
OTFC - On The Fly Calibration.
Introduction:
• OTFC calibrates data as it is requested by users from the HST archive, using latest calibration software, files,
and recommended parameter values.
• CADC/ST-ECF developed an OTFC system circa 1996. OTFC at STScI requires high throughput capability
and integration with existing data processing systems. In addition, it provides improved values to parameters
(keywords) in archived datasets as they are determined by each instrument group.
Motivations - Data Volume & Improved Calibration:
Drizzling Software and Documentation. Many imaging
Figure 1. Ratio of SMOV/pre-SMOV flats taken in F502N. The greyscale display rasnges from
0.985 (black) to 1.015 (white).
New WFPC2 Instrument Handbook! Version 5.0 of
the WFPC2 Instrument Handbook will be released June 15, 2000. Major
revisions since Version 4.0 are summarized as follows:
• Comparisons to ACS, STIS, and NICMOS.
• CCD Performance: Material on long-term dark current increase and CTE
(Charge Transfer Efficiency) has been updated, including results presented at
the January 2000 CTE workshop, and latest CTE time-dependence and corrections.
• PSF: new material on PSF subtraction and the F1042M filter PSF anomaly.
Also aperture corrections as function of field position.
• Updated focus history.
• Astrometry - new geometric corrections.
• Throughput tables: Values have been fully revised using current SYNPHOT
results. Sample SNR calculations fully updated. New material on long-term
photometric stability.
• Calibration: Material on UV throughput variations, dark current calibration,
polarization calibration, flat fielding, and impact of focus variations on photometry has been updated. Cycle 4-9 calibration proposals are described.
• Changes in HST scheduling system.
• Advice on current dither techniques and strategies.
• WFPC2 Clearinghouse.
• Updated references, index, and acronym list.
programs carried out with WFPC2 now use position offset dithers to aid in the
removal of detector artifacts, as well as enhancing spatial resolution. These
images can be combined using the "Drizzle" software written by Andrew
Fruchter (STScI) and Richard Hook (ST-ECF). The basic Drizzling routines are
incorporated in the STSDAS "Dither" package. In the next release of STSDAS
(Summer 2000), the "Dither" package will also include tasks that allow the
removal of cosmic rays from singly-dithered datasets that have only one image at
each dither pointing (previously these tasks had to be downloaded separately
from STScI as the ditherII package).
We are also releasing "The Dither Handbook" (Koekemoer, et al., 2000), a
complete guide to planning, obtaining and analyzing dithered HST data. This
document provides general discussions on dithering strategies with HST instruments (currently including WFPC2, STIS, and NICMOS, and in the future
ACS), together with a "Cookbook" section that gives detailed examples of drizzling procedures. All the input images and scripts for these examples, as well as
the final output images, are available from our WFPC2 website, to enable
observers to gain experience in the use of the drizzling software through practicing with these examples. As an example, the two figures shown above are
"before" and "after" WFPC2 images of the Hawaii Deep Survey Field SSA22.
The left image is one of 12 single-image dithers, while the image on the right is
the result of shifting and drizzling all 12 images, showing the success of cosmicray removal for singly-dithered data with the new software. Complete information and software can be found at:
http://www.stsci.edu/instruments/wfpc2/drizzle.html.
• HST optical disk jukeboxes are near maximum capacity. Some data can be stored off-line and a transition to
magneto-optic technology upgrade is underway, but these are limited solutions.
• ACS will significantly increase the storage requirement.
• Data volume in the archive can be substantially decreased by storing raw data in compressed form and not
storing calibrated data.
• Nearly all datasets can benefit from recalibration: improvements come from better calibration files, calibration
parameters, and calxxx software.
Components and Dataflow:
• User interface (Starview or WWW) accepts and authenticates user requests.
• HST archive (DADS) provides raw data to OTFC and returns raw + calibrated data to users.
• Data cache holds all calibration files and some calibrated datasets.
• Exceptions system updates/adds to dataset keywords in DADS catalog.
• Bestref/bestswitch determines best calibration files & switches for each dataset.
• Getref/upref updates headers with proper calibration keywords and values.
• OPUS blackboards coordinate OTFC pipeline and calxxx.
Current Status: OTFC is now available for WFPC2 and STIS!!!
• For more details, see the top-level STScI page: http://www.stsci.edu/, or the WFPC2 instrument page: http://
www.stsci.edu/instruments/wfpc2/wfpc2_top.html.
• Support for NICMOS and ACS is planned for the future.
WFPC2 WWW Resources:
http://www.stsci.edu/instruments/wfpc2/wfpc2_top.html
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 PASP CTE paper, and other WFPC2 documentation.
http://www.stsci.edu/instruments/wfpc2/wfpc2_doc.html
http://archive.stsci.edu/hst/otfc/