Wide Field Planetary Camera II Status Update
I. Heyer, J. Biretta, S. Baggett, S. Casertano, S. Gonzaga, A. Koekemoer, J. Mack, M. McMaster, A. Riess, A. Schultz, M. Wiggs (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 instrument
continues to perform well as it approaches
100,000 on-orbit images. Long-term issues with
Charge Transfer Efficiency in the CCDs (see separate poster), and new anomalies in the shutter
operation, are on-going concerns. But so far, these
issues impact only a small number of science
observations. The photometric, flat field, and PSF
stabilities continue to be excellent. The Cycle 9
calibration program is now well underway. It
closely follows calibrations from previous cycles,
but also contains new calibrations of filter wavelength stability and red-leaks, as well as an absolute calibration of CTE losses. The "On the Fly
Calibration" system has been highly successful,
and continues to supply HST archive users with
images using the most up-to-date calibrations;
soon this system will be enhanced to include "onthe-fly" reprocessing. The WFPC2 Instrument
Handbook has been completely revised for Cycle
10; new material includes updates to many of the
instrument’s basic calibrations, as well as the latest
observation strategies. A new "Dither Handbook"
discusses strategies for pointing dithers during
observations, as well as data reduction using the
"Drizzle" software. These and other recent
enhancements will be discussed.
"A" BLADE
MOTOR
"B" BLADE
POSITION
SENSOR
WFPC2 Shutter Assembly
Encoder Disc and Follower Arm
WFPC2 Shutter Anomaly. In August 2000 WFPC2 began experiencing anomalous exposures in which the shutters failed to open properly, resulting in loss of several science images. In late October, a
related anomaly began to occur where the shutters did not close as commanded; apparently in the process of closing, one shutter would bounce off the other leaving both in the open position. This second type of anomaly
resulted in trailed images since the shutter was open during the CCD clear, or during the CCD readout. There was some small probability that the new anomaly could damage the shutter mechanism, so WFPC2 observations were suspended on Nov 1, 2000, until a software patch could be installed into the WFPC2 computer.
The problems appeared to be caused by the position sensor on the "A" shutter blade (see figures above). The solution was to install a software patch in the WFPC2 microprocessor to modify the way the shutter position
sensor operates. This should eliminate the initial problem and hence also the secondary anomaly. Specifically, the patch turns on the shutter position sensor LED 10 milliseconds earlier than before, providing more time
for the phototransistor in the sensor to respond before the shutter position is read by the WFPC2 microprocessor (the previous software allowed only 30 microseconds). The operational changes should be completely
transparent to observers.
The software patch was successfully installed into the WFPC2 computer Nov 8, 2000, and regression test exposures were run. All test exposures appeared to have produced nominal results; the software patch did not
caused any unexpected effects, nor had any negative impact on normal science observations. Normal science observations were resumed on the evening of Nov 9, 2000. Close monitoring of the >1500 images taken during the following month have shown no recurrence of the anomalies.
There is some small probability that the problem could reappear later (e.g. if on-going mechanical wear was a key factor). An Anomaly Review Board will continue to investigate the causes of the problem, and recommend if further modifications to the shutter management might be needed. Thanks to the efforts of a large number of people at JPL, GSFC, Lockheed, and STScI, the camera was restored to normal operations with minimal science loss. WFPC2 GOs will be kept informed of the shutter status via regular STAN mailings as well as items posted at http://www.stsci.edu/instruments/wfpc2/wfpc2_advisory.html
Please email help@stsci.edu to be added to the STAN newsletter distribution list.
OTFR - On The Fly Reprocessing.
Introduction to OTFC (On the Fly Calibration)
•Until a year ago, 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 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 (Crabtree, et al., 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 the instrument groups.
OTFC will be switched to OTFR
Impact to Users --> NONE
•The switch from OTFC to OTFR is planned for early 2001.
•Changes will be transparent to HST archive users, no need to read further!
•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).
Table 1: Benefits of On-The-Fly Systems
OTFC
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.
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
Dither Handbook, the WFPC2 Handbook, the
CTE paper, and several ISRs.
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.
Drizzling Software and Documentation. Many imaging 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 last release of STSDAS (Summer 2000), the "Dither" package also included 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 have also released "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 cosmic-ray removal for singly-dithered data with the new software. Complete information and software can be found at:
http://www.stsci.edu/instruments/wfpc2/dither.html.
Table 2: Comparison of Dataflow in On-The-Fly Systems
OTFC
OTFR
Request for data is submitted to the archive via Starview or WWW
interface; archive responds with acknowledgement email.
Same as OTFC.
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).
Any problems in the header keywords are fixed.
Same as OTFC, although the OPUS processing steps will have fixed
the majority of keyword problems automatically (i.e., significantly
fewer header fixups required in OTFR).
The best calibration files & switches are determined and header keywords updated accordingly.
Same as OTFC although the best files & switches are already set in
OPUS code (generic conversion).
Images are calibrated by STSDAS calxxx module and sent back to the
archive system.
Same as OTFC.
Archive delivers data and emails completion notification to the
requestor.
Same as OTFC.
References
http://www.stsci.edu
http://archive.stsci.edu
http://www.dpt.stsci.edu/otfc/external/otfc_index.html