STScI Logo

Hubble Space Telescope

S T A N / W F P C 2 - Number 4, February 1995



by Brad Whitmore

As part of the recent STScI reorganization the WFPC2 Instrument Team and the WFPC2 Analysis Team have been merged. This change should be largely invisible to observers. The WFPC2 Team will now handle both the user support and the technical support for the instrument. The Group Lead is Brad Whitmore while the Technical Lead is Chris Burrows. Several people previously involved with the WFPC2 effort have moved on to new challenges as part of the reorganization, including John MacKenty (NICMOS), Bill Sparks (Data Quality Project), and Rick White (STSDAS).

Some of the items on our agenda for the coming months are: update the Instrument Handbook and Data Handbook, assign a "single point of contact" to each cycle 5 program, begin implementing the programs that use the polarization and ramp filters, develop a data quality plan, update synphot using the latest photometric calibrations, develop new software for cosmic ray removal, hot pixel repair, and CTE corrections, simplify the WWW homepage and develop an extensive set of Frequently Asked Questions.

Let me take this opportunity to solicit YOUR INPUT concerning these plans. Are these the items you think we should be spending our efforts on? What could we be doing that would be most beneficial for you? Any comments would be greatly appreciated.


by Karla Peterson (PRESTO)

A new version of RPS2 was released on February 21. It corrects most of the posted advisories and has enhanced schedulability tables (targets will appear to be easier to schedule, in particular those with low declination).

We strongly recommend that you upgrade to this new release unless you have already finished and submitted your proposal. There is _no_ need to reprocess the submission in this case.

You can find more information about the specific advisories which are corrected in this release in the document. General information about RPS2 (and the links to the software) are in:

Two configurations of this release of RPS2 are available:


The first one (which is the one we recommend to install) is a stand-alone version and will run all the tasks locally in your computer. As this version requires 75MB to install (38MB installed and ready to run), and is computationally intensive, you can choose the second one that only runs certain tasks locally. The rest of the processes are run remotely on STScI computers. The results of both configurations are the same, you should not notice any differences in the output.


Dithering Strategies

By Brad Whitmore

In the January issue of this newsletter we outlined some of the strategies people are using in an attempt to increase spatial resolution of their WFPC2 observations. We would like to draw your attention to a set of articles in the February issue of the ST-ECF Newsletter on this subject. Several images are included in the articles that demonstrate to what level improvements can be made.

While it is clear that for programs where spatial resolution is the limiting factor, and high signal-to-noise is expected, such techniques may have much to offer. However, it is important that observers realize some of the potential difficulties and limitations. Here are some of the pros and cons to consider.

  • Increased spatial resolution
  • Simultaneous removal of bad pixels
  • Reduction in the RMS of the background
  • Increase in complexity of analysis
  • Increase in computational requirements
  • More difficult to remove cosmic rays (generally should take at least two exposures at each position)
  • Overhead of more readouts and potential reduction in detection threshold due to being limited by read noise, due to shorter exposures, instead of being background limited.
  • There is limited experience using these techniques

Hence, it is necessary to consider the specific scientific goals of the program, the amount of observing time allocated, and the resources available for the analysis.

One of the dithering strategies discussed in the January issue of STAN was an offset of 0.50", which corresponds to nearly integer pixel shifts for both the PC and WF (i.e., 10.98 PC pixels and 5.02 WF pixels), providing an easy way to treat both cosmic rays and hot pixels while reducing the RMS of the background. A similar combination for those who are considering fractional pixel offsets is 1/2 this values (i.e., 0.25") which translates to 5.49 PC pixels and 2.51 PC pixels. Most of the gain is accomplished by using two different positions, or three if enough observing time is available. According to experiments by Hans-Martin Adorf and Richard Hook, little is to be gained beyond three separate positions.

An important point to keep in mind in all this is that the accuracy of the actual telescope offset is about 3 mas for short offsets during the same orbit and about 10 mas after a reacquisition in the next orbit, as long as offsets on the order of 0.50" or less are used. Accuracies during separate visits using the same guide stars are about 20 mas, while differences on the order of an arcsec can be expected when different guide stars are used. Hence it is important to combine the exposures into a single visit when using these techniques. In general, the POS TARG special requirement should be used rather than modifying the RA and DEC.

New software releases

By J C Hsu

A new release of STSDAS (1.3.3) will be available early next month. The changes affecting the WFPC package are:

(1) The task UCHSCALE to update the plate scales of WFPC2 (or WF/PC) has been added.

The plate scale values used by the pipeline to populate the keywords have undergone changes since the instruments were first commissioned. This task can be used to update the CD matrices of earlier observations to the more accurate plate scales.

(2) The task CALWP2 has been modified. It now propagates the information in the reference file keywords PEDIGREE and DESCRIP to the standard output device and the header history. The new version of this task is

(3) The tasks WMOSAIC and METRIC were modified. They now run on WFPC2 images as well as WF/PC images. The WFPC2 coefficients for these two tasks are still preliminary, but should give satisfactory results (better than 0.5 WF2 pixel).

(4) The task ENGEXTR (used to extract data from the engineering files) has also been updated and now accepts WFPC2 or WF/PC images.

New and Improved WFPC2 Frequently Asked Questions file

By Krista Rudloff

A new version of the WFPC2 Frequently Asked Questions file has been posted on STEIS. Its new format includes separate postings for Proposal Preparation Questions and Post-Observation Analysis Questions.

Proposers who are completing their Phase II proposals may be particularly interested in browsing the Proposal Preparation part which covers topics such as Scheduling and Overheads, Photometry, and Special Observing Modes and Strategies.

The Data Analysis portion of the WFPC2 FAQ covers Calibration, STSDAS Analysis Tools, the WFPC2 PSF, Photometry, and other miscellaneous items.


by Richard Griffiths, Kavan Ratnatunga, Stefano Casertano and the MDS Team

The Medium Deep Survey (MDS) catalog of 11,500 objects from Cycles 1-3 has been completed and can be downloaded from:

The README file in the same directory describes the contents of the catalog in detail.

Briefly, the Medium Deep Survey Catalog file ( contains classified objects from over 100 WFPC1 parallel fields obtained from Cycles 1--3. It lists the field name, position, object classification (in the I an V bands), parameters of the fitting models, sky levels, several I and V-band properties of the objects and some comments on the fits.


Any questions about the scheduling of your observations should be addressed to your PRESTO contact. If you do not know who this person is, PRESTO's Mosaic page ( contains that information.

Post-Observation questions should be addressed to

Comments, questions, requests for issues, additions or deletions to the mailing list, etc. can be e-mailed to

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.