n
Wide Field Planetary Camera II Status Update
I. Heyer, J. Biretta, S. Baggett, S. Gonzaga, A. Koekemoer, L. Lubin, J. Mack, M. McMaster, V. Kozhurina-Platais, A. Schultz (STScI)
Abstract
We review the status of the Wide Field and Planetary
Camera II (WFPC2) onboard the Hubble Space Telescope, as well as recent enhancements to calibration
and user support.
The photometric, flat field, and PSF stabilities continue to be excellent. Charge Transfer Efficiency
(CTE) in the CCDs remains a concern; we discuss the
latest results from on-going monitor programs, as well
as the latest correction procedures. Work is underway
to update the entire set of flat fields; we discuss the
new flats, as well as the low-noise flat fields corrections released last summer.
The "On the Fly Reprocessing" system continues to
perform well, though we mention issues affecting a
few images. The WFPC2 Exposure Time Calculator
has been updated, and is available on our website. A
new WFPC2 Pointings Search Interface tool for the
HST Archive has been released.
The WFPC2 Instrument Handbook has been updated
for Cycle 11, and a new edition of the HST Data
Handbook is currently in progress.
These and other issues will be discussed.
Instrument Handbook Update.
Photometric CTE Monitor. The Charge Transfer Efficiency of the WFPC2 CCD
Here’s what’s new in Version 6.0 for Cycle 11:
arrays has been monitored since the instrument’s deployment aboard HST in 1993. Since then a
significant increase in CTE loss has been observed. We examine photometric data from 28 April
1994 to 11 February 2001. The results from the latest set of observations show that CTE loss has
reached a level of 53% in the worst case scenario (faint target on low background).
The general trends we can see are:
•CTE loss appears to be growing worse in a linear fashion with time.
•CTE loss decreases with increasing counts of the targets. The plots for the higher count bins
show less CTE loss, as do longer exposures.
•CTE loss decreases with increasing background. Filters with higher backgrounds show less CTE,
as do preflashed exposures
•CTE loss is largest for short (F439W) and long (F814W) wavelengths, and less so for intermediate (F555W) wavelengths, which is due to the differences in background level.
The figure illustrates the time dependence of CTE as measured in the F814W filter for targets with
~35 DN (at Y=800). The increase is well represented by the linear fit; any acceleration in CTE
growth is not statistically significant. See WFPC2 ISR 2001-09 for details.
•Anomalies: Added information on the shutter anomaly,
the ORIENT bug, and the FR533N anomaly.
•Dark Current Evolution: New information on the latest
results.
•Dithering: Updated information on dithering from the new
Dithering Handbook.
•CTE: Updated CTE monitor figure to reflect the latest
results. Added section on mitigating CTE effects.
•Photometry: Updated photometry monitor.
•Calibration: Added sections on the "On The Fly Reprocessing System" and the "Cycle 10 Calibration Plan."
•Updated references and index.
This Handbook is on the WFPC2 WWW pages at
http://www.stsci.edu/ftp/instrument_news/WFPC2/
wfpc2_top.html .
HST Data Handbook Update. A revised
version of the HST Data Handbook will be available
around the end of January 2002. Updated sections of particular interest to WFPC2 users include a discussion of:
•New and improved pipeline flatfields, available via OTFR
soon (early 2002).
•Correction flatfields, particularly useful for highlyexposed science images in PC and lower exposure levels in
UV filters on WF chips; these flats are applied after data
are processed through OTFR (On The Fly Reprocessing).
•Advice on generating custom dark reference files, which
could improve hot pixel corrections.
•Summary of the WFPC2 pipeline processing history
(OPUS, OTFC, OTFR)
This document will be available as a link at:
http://www.stsci.edu/instruments/wfpc2/
wfpc2_doc.html#Othr
Dither Handbook Update V2.0.
An update of "The Dither Handbook" (V2.0;
Koekemoer, et al., 2001) is now available. It includes
clarified and updated descriptions of how to design
dithered observations for Cycle 11, including information on ACS and NICMOS, as well as WFPC2 and
STIS. It also includes updated examples of how to
reduce and analyze dithered HST observations,
including a section on how to combine dithered ACS
images using the new PYRAF-based version of drizzle, known as "pydrizzle". The updated Dither Handbook V2.0 is available at the following website:
http://www.stsci.edu/instruments/wfpc2/dither.html.
& model
May 2001: OTFR replaces OTFC Impact to Users --> NONE
Wavelength (angstrom)
PC1
8000
Low Noise Flat Field Corrections. We have also
.8
•Changes are transparent to HST archive users.
•Requests for data are submitted as usual (Starview or WWW); calibrated data delivered.
•All requests for WFPC2 data will be handled by the OTFR system, no exceptions.
•OTFC processing started with raw files and performed only calibration.
•OTFR processing will start with the original telemetry files (“POD” files) received at STScI from Goddard and run the entire pipeline
(OPUS & calibration). See Table 1 below.
•OPUS processing steps will have fixed the majority of keyword problems automatically (i.e., significantly fewer header fixups
required in OTFR)
•OTFR has trouble with about a dozen WFPC2 images (out of ~115,000 total in the archive). It has recently been setting the
FGSLOCK keyword incorrectly to "unknown" in a majority of images. Fixes for these problems should in in place soon
NOTE: Do not run UCHCOORD (version prior to May 2001) on OTFR data.
1
.9
6000
provides observers access to a database table summarizing key characteristics of all WFPC2 pointings. It can be used to search for
images and fields with many user specified properties. For example,
one could construct a list of low ecliptic latitude fields with at least 2
distinct bandpasses of WFPC2 observations. The WFPC2 Pointings
page is at http://archive.stsci.edu/hst/pointings.html
updated the WFPC2 exposure time calculator WWW tool. Changes
include more flexible specification of the sky background light -observers may now specify "low-sky" conditions or explicitly give the
sky magnitude. We have also added signal-to-noise estimates for simple aperture photometry (where the user can input aperture radius in
pixels), in addition to the previous optimal PSF weighting method.
1.1
Ratio (clocks-on/clocks-off)
Ratio (clocks-on/clocks-off)
1.1
New Archive Search Service for WFPC2
Observations. The Archive’s new Pointings Search Interface
WFPC2 Exposure Time Calculator. We have
1.2
4000
OTFR - On The Fly Reprocessing - Released May 16, 2001.
•Until April 2000, data were calibrated as they were received at STScI. Results were stored in the HST archive (DADS), and could be
retrieved via Starview from DADS.
•OTFC calibrates data when it is requested by users from the HST archive, using latest calibration software, files, and recommended
parameter values.
1.2
2000
•2001-06 “WFPC2 Cycle 8 Closure Report“, Baggett et al.
•2001-07 “WFPC2 Flatfields with Reduced Noise“, Karkoshka and Biretta.
•2001-08 “Creating WFPC2 Dark Reference Files: Addendum“, Mack, Baggett, and Biretta.
•2001-09 “The WPC2 Photometric CTE Monitor“, Heyer.
•2001-10 “An Improved Geometric Solution for WFPC2“, Casertano and Wiggs.
•2001-11 “Summary of WFPC2 SM3B Plans”, Koekemoer, Gonzaga, Heyer, Lubin, and Kozhurina-Platais.
•2001-12 “Updated WFPC2 Flat Fields for 1995 - 2001”, Koekemoer, Biretta, and Mack.
Introduction to OTFC (On the Fly Calibration)
WFPC2 is monitored by observing the white dwarf standard star GRW+70D5824 (HIC 66578;
DA; V=12.77; B-V=-0.09). Interspersed among the normal observations with the serial clocksoff (CLOCKS=NO,ATD-GAIN=15) are observations with the serial clocks-on
(CLOCKS=YES,ATD-GAIN=15). The figures show the ratio of the clocks-on to clocks-off
photometry (ON/OFF) for PC1 and WF3; there is no significant difference in the photometry
for CLOCKS ON vs. OFF. Other tests show CLOCKS ON causes a modest increase (5 - 10%)
in the dark current. CLOCKS ON is of potential interest to observers since it reduces by 1 min.
the overhead time on exposures longer than 180s.
.8
WFPC2 Instrument Science Reports. New ISRs since the last AAS Meeting:
The ISRs can be accessed on the web at http://www.stsci.edu/instruments/wfpc2/wfpc2_bib.html
Comparison Clocks-on vs. Clocks-off. The photometric performance of
.9
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 monthly decontamination procedure,
where the WFPC2 is warmed to approximately 20oC
for 6-12 hours to remove the build-up of contaminants.
High points indicate high-throughput measurements
taken after decontamination.
ter understanding of its effects and better photometric corrections. Hot pixels provide an interesting probe of the CTE
effects at the single pixel level. The figure illustrates "tails" observed on hot pixels as a result of CTE, and shows these
tails increasing in strength from 1994 to the present. These tails rob counts from the target image, and displace counts
to distances which are large compared to aperture radii typically used for stellar photometry.
major update of the WFPC2 flat field reference
files for all the standard filters redward of
300nm, based on Earth flat data obtained during the time period September 1995 - May
2001. We divide the flat fields into epochs
depending on the appearance of new dust
spots, as well as long-term changes in existing
features. The changes also result in an
improvement of the r.m.s. noise of many of the
flats by about a factor of two, and may benefit
high signal-to-noise (SNR >100:1) images.
Starting in early 2002 the new flat fields will
be automatically applied to any WFPC2 data
in the relevant filters when the data are
retrieved from the archive.
Details are given in WFPC2 ISR 2001-12.
1
Photometric Monitor. The standard star
Physical Effects of CTE. We continue efforts to study the physical effects of CTE so as to derive a bet-
WFPC2 Flat Field Updates:
1995 - 2001. We have completed a
References to published
results of the data discussed in this poster are
available in a handout. We
have limited copies available of the Dither Handbook, the WFPC2
Handbook, and several of
the latest ISRs underneath
this poster. If you don’t find
what you need please either
check our website, or leave
your name, address, and
list the document(s) you’d
like mailed.
2000
4000
6000
Wavelength (angstrom)
WF3
8000
derived a series of correction images which reduce the pixel-to-pixel
noise contributed by the flats. They give the largest benefit to UV
observations on PC1 (5% - 16% noise), but may give some marginal
improvement (<1%) for other filters. While reducing noise, they also
introduce some low-level photometric uncertainty. Hence these corrections are not included in the flat field reference files, but exist as a
series of correction images observers may manually apply to their
data. Further information is contained in WFPC2 ISR 2001-07.
Table 1: Benefits of On-The-Fly Systems
OTFC
OTFR
Improved calibration files are used by calwp2.
Same as OTFC.
Most recent calibration software is used, allowing for improved algorithms, new capabilities, and software fixes.
Same as OTFC. In addition, the most recent OPUS pipeline software is
used (pre-calwp2 processing steps of data partitioning, data editing, and
generic conversion).
Header keywords can be corrected if needed.
Same as OTFC. Extra benefit: significantly fewer fixups will be
required in OTFR than in OTFC.
Only uncalibrated data need be stored in the archive.
No calibrated or uncalibrated data need be stored in the archive. Only
the “POD” file (original telemetry data received at STScI from Goddard) is required for OTFR.
Raw files are retrieved from the HST archive and passed to the OTFC
system. For WFPC2, the raw files include d0, q0, q1, x0, and trl files.
POD file (original telemetry file) is retrieved from HST archive and
passed to OTFR system. For WFPC2, there is typically 1 POD file for
each image. OPUS processing is performed: data partitioning, data
editing, and generic conversion (generation of raw files).
References
http://www.stsci.edu
http://archive.stsci.edu
http://www.dpt.stsci.edu/otfc/external/otfc_index.html