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S T A N / W F P C 2 - Number 26, June 1997


A New Version of Drizzle:

by Andy Fruchter

Richard Hook and I have completed work on a new version of the code, Drizzle, which we developed to combine dithered WFPC2 data. The new version, Version 1.1, is more flexible and user-friendly than the earlier public release (version 1.02) and can be easily installed for use with IRAF on any UNIX machine. Version 1.1 also includes the first public release of the program BLOT which is the primary component in the suite of scripts we are developing for the removal of cosmic rays from dithered data. However, no significant bugs have been found in Version 1.02, so users can safely continue to use the earlier version, should they wish to do so.

Individuals wishing to retrieve the new code should set their web browsers to and follow the instructions from there. However, patient sorts may wish to wait a month or so for the next release of STSDAS. A new STSDAS package, called "dither", will contain a number of helpful tasks for processing dithered data, and among these will be Drizzle.

For more information on the subject of dithering WFPC2 images, users can either go to the above web page, or look for articles on the subject by Hook and Fruchter in the most recent issues of the STScI and ST-ECF newsletters.

Happy Drizzling!

Progress Toward Understanding the WFPC2 Charge Transfer Efficiency (CTE) Problem:

-Brad Whitmore, Inge Heyer, John Biretta, Max Mutchler

During the past year a fair amount of progress has been made toward understanding the Charge Transfer Efficiency (CTE) problem (e.g., see the CTE/Long vs. Short page on the WFPC2 homepage). In addition, two new Instrument Science Reports (ISRs) are nearing completion and will be available in late June or early July. These can be obtained via the WFPC2 homepage or by contacting the help desk at STScI.

The first new ISR, by Biretta and Mutchler, shows that residual images apparently left by poor CTE have a half life of about 11 minutes. The dataset consists of 30 minute dark observations following observations of planets. The typical brightness for the planet is a few thousand DN while the residuals are a few tenths of a DN. These results suggest that a "noiseless" pre-flash (i.e., a strong pre-flash which is read out just before an exposure) may be useful for reducing CTE for short exposures. Tests of this concept are planned for later this year.

In the second ISR, by Whitmore and Heyer, an empirical approach designed to correct for poor CTE has been taken. They find that CTE loss can range from about 2% to 15% for a star at the top of the chip that must traverse all 800 pixels during the readout. The amount depends on the how bright the star is and whether the background is faint or bright. There also appears to be a weak CTE problem along the X axis (1 - 5 %), with stars on the right side of the chip being fainter than stars on the left side. This is probably due to CTE loss in the shift register. A set of formulae has been developed to correct for these effects to a level of about 2%, for cases where an aperture with a 2 or 5 pixel radius has been used to perform aperture photometry.

The WFPC2 Clearinghouse:

-Brad Whitmore, Michael S. Wiggs, Inge Heyer

The WFPC2 has now been operating for over three years, and in this time a great deal has been learned about the characteristics and calibration of the instrument. The WFPC2 CLEARINGHOUSE has been developed in an effort to make this material more readily available to WFPC2 users.

The Clearinghouse contains references to:

     1) STScI documentation,
     2) publications from the astronomical literature,
     3) user submitted documentation.

The user specifies one of a few dozen keywords (e.g., aperture photometry, dithering, cosmic rays, etc) and the tool returns references to articles on these subjects, arranged into three categories from:

     3 = fundamental references,
     2 = contains some new material on this topic,
     1 = does not contain new information on this topic.

The tool also provides a handy set of search engines to access the various articles and software tools once they have been identified. At present (end of June) the WFPC2 Clearinghouse is only about 50% populated, but will eventually contain any articles using WFPC2 data that we are aware of. One important point to note, however, is that inclusion in the WFPC2 Clearinghouse does not necessarily mean that the WFPC2 group at STScI endorses the results of a given reference, since there are simply too many items for detailed review.


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.

CAPETTI, A.; AXON, D.J.; MACCHETTO, F.D. "The ionization of the NLR
of NGC 1068 and its relationship to the extended radio emission"
ApJ 11-97

RIDGWAY, S.E.; STOCKTON, A.  "Deep WFPC2 and ground-based imaging
of a complete sample of 3C quasars and galaxies" AJ 8-97

YANNY, B.; JANNUZI, B.T.; IMPEY, C.  "HST imaging of the BL Lacertae
object OJ 287"  ApJ accepted

B.; ROOD, R.T.  "HST UV observations of the cores of M3 and M13"  
ApJ accepted


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