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S T A N / W F P C 2 - Number 44, January 2001


Phase II Deadline is Approaching!:

The Cycle 10 Phase II GO deadline is Feb 16, 2001. The phase II documentation, including policies, proposal instructions, and RPS2 software, can be found at the STScI Website.

In addition, there is a new WFPC2 Dithering Handbook (see next item) for observers considering the use of dithers in their programs and the new Visual Target Tuner (see item below) for overlaying the science instrument apertures on grayscale images.

If you have questions about any Phase II issues or feel you need to have a Contact Scientist assigned, please send email to

New Documentation - The Dither Handbook:

by A. Koekemoer, S. Gonzaga, A. Fruchter, J. Biretta, S. Casertano, J.-C. Hsu, M. Lallo, M. Mutchler, R. Hook (with additional input from other members of the WFPC2, STIS, NICMOS, ACS and OSG groups)

We have made available a new document, the Dither Handbook, which is aimed at consolidating all the information necessary to successfully design and obtain dithered HST observations, and also contains tutorials and examples on how to perform subsequent data analysis using the IRAF/STSDAS "drizzle" software.

Specifically, the Dither Handbook contains information on the following topics:

     - Benefits and disadvantages of using dithering strategies
     - HST/instrument characteristics relevant to dithering
     - Guidelines on designing observational dithering strategies
     - A description of the algorithm behind the "drizzle" software
     - General tutorial on using the "drizzle" software to analyze data
     - Examples of using "drizzle" to analyze WFPC2, STIS and NICMOS data

New CTE Correction Formula Available from Dolphin (2000):

- B. Whitmore

A recent paper has appeared that may be of interest to many WFPC2 users. The paper is written by Andrew Dolphin and is titled "The Charge-Transfer Efficiency and Calibration of WFPC2" (Dolphin, 2000, PASP, 112, 1397). The paper compares WFPC2 observations with ground based observations of Omega Centauri and NGC 2419, using a baseline through March 2000, roughly a year longer than available for a similar study by Whitmore, Heyer, and Casertano (1999, PASP, 111, 1559). In general, Dolphin finds good agreement with the Whitmore, Heyer, and Casertano results, and the longer baseline and more extensive data set used by Dolphin result in less scatter in the residuals. In particular, Dolphin finds similar corrections to within a few hundreths of a magnitude in all cases except for recent (1998 and later) data with low counts. In these case the Dolphin corrections are larger than the Whitmore et al. corrections.

A preliminary comparison of the two formulae using recent August 2000 observations of Omega Cen confirms that the Dolphin formulae results in smaller residuals over most of the range of typical observations. Besides the tendency for the Whitmore et al. formula to underestimate the required correction for faint stars (i.e., in the range 100 - 500 electrons), as reported by Dolphin, we also find that the Whitmore et al. formula overestimates the required correction for very bright stars (i.e., brighter than 15,000 electrons) by a few percent. However, we also found that for extremely faint stars (i.e., 20 - 50 electrons), the Dolphin formulae overestimates the required correction by tens of percent, presumably because he was not able to constrain this part of the parameters space since he used ground-based observations for the comparison, which limits the sample to brighter stars. Hence, at present, the best compromise is probably to use the Dolphin formula for stars brighter than 100 electrons and the Whitmore, Heyer, Casertano formula for fainter stars. An Instrument Science Report (ISR) is currently being written on this comparison and should be out shortly.

Some comments on preflashing: CTE loss can be reduced by increasing the background, hence filling some of the traps before the target reaches them. One can artificially enhance the background by adding a preflash. The problem with this approach is that this also adds noise. Figure 2 [see WWW Advisories site for figure] shows a calculation based on the Whitmore, Heyer, Casertano (1999) correction formula, assuming a very low background for the raw image (i.e., 0.1 electron, appropriate for a very short exposure, a narrow-band exposure, or an exposure in the UV) versus an exposure which has been preflashed with 25 electrons. The ratio of the S/N for the preflashed image versus the raw image is plotted vs the Log of the target brightness. The S/N estimates include the uncertainties in the CTE corrections. The three curves show the effects for a star near the bottom of the chip (i.e., X = 400, Y = 100, where the preflash is never an advantage since CTE loss is low and the preflash adds noise), near the center of the chip, and near the top of the chip (where the preflash is an advantage for the brighter targets). For more typical cases where the background is already sizeable, the gains due to a preflash are even smaller. We also note that preflashing may significantly increase the overhead time for an exposure.

Creating WFPC2 Dark Reference Files:

- J. Mack and M. S. Wiggs

Abstract: In this ISR, we describe the procedures for making WFPC2 reference files for the HST data reduction pipeline. These calculations incorporate several IRAF scripts which are linked by the newly developed task "wkdark". These scripts are currently available for FTP from the WFPC2 website to users who wish to create their own dark reference files.

AAS Posters Available Online:

As presented in the last STAN, two WFPC2 calibration posters were given at the Jan 2001 AAS meeting:

HST Wide Field and Planetary Camera II Status Update and

Charge Transfer Efficiency in the WFPC2 CCD Arrays

The full posters can be viewed online, in postscript or PDF formats, at the WFPC2 Website.

Try the Visual Target Tuner!:

- K. Peterson

The VTT displays apertures superimposed on FITS format sky images (including images from the DSS, NED and your local disk). You can then display primary and parallel apertures, rotate apertures, centroid objects, and display catalog overlays.

For those of you who are working on their Phase 2 proposals for Cycle 10, here are some things the VTT can help with: What are the coordinates for my object using the DSS? How should I place the aperture to best image/mosaic my extended target? Do any of my targets have nearby bright objects that could spoil the observation? What orientation restrictions would avoid the nearby bright object?

We have already had a few Cycle 10 PIs use the tool with very positive results. But we need your feedback (both what you like and don't like) to know how to improve this tool. The VTT is the first of many tools in the coming Astronomer's Proposal Tool (which will eventually replace RPS2). A prototype ACS ETC is also available at this time.


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.

"The Low-Mass IMF -- Deep Star Counts in the dSph
Galaxy Ursa Minor" Modes of Star Formation

KEEL, W.C.; WHITE, R.E. III "Seeing Galaxies
Through Thick & Thin. III. HST Imaging of the Dust
in Backlit Spiral Galaxies" AJ accepted

R.; GREBEL, E.K. "A Critical Examination of
Hypernova Remnant Candidates in M101. I. MF 83" ApJ


Any questions about the scheduling of your observations should be addressed to your Program Coordinator. Post-Observation questions can be addressed to your Contact Scientist. If you do not know who these persons are, you can find the information on the WWW at

Analysis, STSDAS or any other questions can also be addressed to

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