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S T A N / W F P C 2 - Number 46, May 2001


OTFR released May 16, 2001 - and a reminder about using uchcoord:

The On-The-Fly reprocessing (OTFR) system for WFPC2 (and STIS) data has been released. As reported in the last STAN, the switch to OTFR should be transparent to users: requests for data are submitted as usual via Starview or WWW and raw plus best-calibrated data are delivered. Observers are reminded, however, not to run the offline STSDAS task UCHCOORD on data processed through the new OTFR system. More details on the OTFR system can be found on the WFPC2 Webpage.

If you any questions or concerns about OTFR data retrievals, please email

WFPC2 Dark Current vs. Time (ISR 01-05):

by J. Mack, J. Biretta, S. Baggett, and C. Proffit


On-going measurements of the dark current in the WFPC2 detectors indicate that the average level of dark current has been slowly increasing over the instrument's lifetime. We have analyzed more recent dark current data, now covering the period from September 1994 to January 2001. This analysis shows that after late-1998, the dark current is increasing more slowly than expected and could possibly be leveling off temporarily. The STIS CCD dark current shows a similar effect. This behavior may be linked to the solar cycle, where the cosmic ray rate is reduced at solar maximum, causing the median dark current to be lower. If this hypothesis is correct, we expect the dark current to eventually continue increasing at its previous rate.

Preliminary Assessment of the FR533N Filter Anomaly (ISR 01-04):

-S. Gonzaga, S. Baggett, and J. Biretta


Analysis of FR533N VISFLAT images has revealed an apparently randomly occuring rotational offset of about 0.5 degrees in some images, a quantity that corresponds to one filter step. The pivot point of the rotation implicates the filter wheel as the source of the inconsistency. We expect no impact on observations as any photometric effect is less than 1%. A cursory check of several other filters (on other filter wheels), shows no similar problem. At this time, the cause of this anomaly, whether it is mechanical or due to a software error, is unknown.

WFPC2 Cycle 10 Calibration Plan (ISR 01-03):

-S. Baggett, S. Gonzaga, J. Biretta, I. Heyer, A. Koekemoer, J. Mack, M. McMaster, and A. Schultz


This report presents the details of the WFPC2 programs planned to maintain and improve the instrument calibration during Cycle 10. The standard suite of calibrations will be continued, including those used to monitor the health of the instrument as well as the programs to collect data for calibration reference files. In addition, several new proposals will be implemented: a measurement of the effect of CTE on astrometry, a characterization of the PSF wings, a calibration check of the clocks ON mode, and a test of the methane quad filter throughput. The total spacecraft time required for the Cycle 10 plan is 61 externals orbits and 2294 occultation periods. This estimate does not include any calibrations associated with the next servicing mission (SM 3b), currently scheduled for Jan 2002; those plans will be presented separately.

Shutter Jitter History Measured from INTFLATs (TIR 01-01):

-A. Riess, S. Casertano, and J. Biretta

Apparent sporadic variations in the closed position of the two WFPC2 shutter blades were detected by examining the light reflected from the shutter in internal flats (INTFLATs). To characterize this phenomenon, we developed a simple algorithm to measure the history of the shutter jitter using INTFLATs. We found the position jitter in the direction of shutter motion to be minimal, indicating stable behavior. In contrast, the positions along the axis perpendicular to the direction of shutter motion exhibit greater jitter, and the amplitude of the jitter has been increasing with time. In addition, much larger sporadic departures from the nominal position are seen beginning in 1999 and continuing to the present time. We hypothesize that these variations in the closed shutter position result from mechanical wear and degradation. However, no direct connection between the jitter history and the shutter anomaly has been determined.

For a paper copy of this report please contact

Preview of June 2001 AAS Poster Abstracts:

Two WFPC2 calibration posters are planned for the AAS meeting in Pasadena; please stop by and see us!

[4.01] HST Wide Field and Planetary Camera II Status Update:

-I. Heyer, J. Biretta, S. Baggett, S. Casertano, S. Gonzaga, A. Koekemoer, J. Mack, M. McMaster, A. Riess, and A. Schultz (STScI)

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 recently passed the 100,000 image mark and continues to perform well. Long-term issues with Charge Transfer Efficiency (CTE) in the CCDs (see separate poster) are an on-going concern, but impact only a minority of the current science observations. The photometric, flat field, and PSF stabilities continue to be excellent. The Cycle 9 calibration program is well underway and closely follows calibrations from previous cycles. Three new programs have been implemented as well: a red leak check, a verification of the wavelength stability of the narrowband and linear ramp filters, and an absolute calibration of CTE losses. New plans for Cycle 10 calibration are discussed. The "On the Fly Calibration" system has been highly successful, and continues to supply HST archive users with images with the most up-to-date calibrations; we discuss the recent "On The Fly Reprocessing" upgrade. The WFPC2 Instrument Handbook is being updated for Cycle 11; we expect only minor revisions which are summarized. The recent "Dither Handbook" discusses strategies for pointing dithers during observations, and reduction of this data using the "Drizzle" software. These and other recent works are discussed.

[4.02] Charge Transfer Efficiency in the WFPC2 CCD Arrays:

-J. Biretta, S. Baggett, A. Riess, A. Schultz, S. Casertano, S. Gonzaga, I. Heyer, A. Koekemoer, J. Mack, and M. McMaster (STScI)

We present an overview of Charge Transfer Efficiency (CTE) issues in the WFPC2 CCDs, including results of recent on-orbit tests, and advice on mitigating CTE effects. CTE causes targets far from the CCD readout amplifier to appear fainter than similar targets near the amplifier. For bright targets, the maximum effect is only a few percent, but for faint stellar targets in recent images with very low background, the CTE effects can reach 50% or more. Studies using hotpixels, cosmic rays, and residual images as probes of CTE have revealed at least four distinct components of CTE losses. The largest effect appears related to trapping and release of charge on timescales of hundreds of milliseconds during the readout process. This is manifest as tails on images which extend for dozens of pixels in the Y-direction (parallel register direction) on the CCDs, which have the effect of robbing counts from typical small apertures used for photometry. Extended targets also are subject to CTE effects. Recent work shows that within small apertures, the CTE losses for faint galaxies are roughly similar to those for stellar targets with the same total counts. There are also small effects on the shapes of faint galaxies: the average profiles are asymmetric and consistent with charge being lost primarily from the amplifier side of the galaxy. We present current results of long-term photometric monitoring which show CTE problems steadily increasing with time. There is also some evidence for an acceleration of the effect. Preflashing the CCDs can reduce CTE effects, but the added noise usually makes this unattractive. A noise-less preflash technique has been tested, but only provides modest improvement. We discuss photometric CTE corrections which can be applied during data analysis, including the new Dolphin (2000) CTE corrections and their relation to the Whitmore et al. (1999) corrections.


We draw your attention to these papers, based on WF/PC and WFPC2 data, that will appear in the next few months. This list includes all preprints received by the STScI Library not yet published in the journals. Please remember to include our Library in your preprint distribution list.

CANALIZO, G.; STOCKTON, A. "Quasi-Stellar Objects,
Ultraluminous IR Galaxies, and Mergers" ApJ 7-10-01

Study of the LMC Compact Star Forming Region N83B"
A&A accepted

R.; CONWAY, R. "HST Optical Spectral Index Map of
the Jet of 3C 273" A&A accepted

WALTER, F.M. "The Proper Motion, Parallax, and
Origin of the Isolated Neutron Star RX
J185635-3754" ApJ 549: 433-440, 2001

G.D.; MACRI, L.M.; STETSON, P.B. "Final Results
from the Hubble Space Telescope Key Project to
Measure the Hubble Constant" ApJ 4-10-01

FILIPPENKO, A.V. "An Ultraviolet through Infrared
Look at Star Formation and Super Star Clusters in
Two Circumnuclear Starburst Rings" AJ 6-01

WHITE, R.J.; GHEZ, A.M. "Observational Contraints
on the Formation and Evolution of Binary Stars" ApJ


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