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S T A N / W F P C 2 - Number 48, September 2001


WFPC2 Flatfields with Reduced Noise (ISR 01-07):

-E. Karkoschka and J. Biretta

We examine the noise contributed by the pipeline WFPC2 flatfields during normal calibration, and provide new low-noise correction flats for 41 filters. Highly exposed science images (>20,000 electrons per pixel) will show significant noise reduction if these new flats are used; this is especially true for images on the PC1 chip. For some ultraviolet filters, a significant improvement occurs even for much lower exposure levels. Potential photometric issues are also discussed. The new flats are available in the HST data archive as "correction" images which observers can multiply into calibrated science data (i.e. data which have already calibrated with the normal flatfields) to obtain the noise reduction. These corrections may be incorporated in the normal OTFR pipeline flatfields at some future date for selected filters. This work was supported by an HST Calibration Outsource grant from STScI to the University of Arizona.

Creating WFPC2 Dark Reference Files: Addendum (ISR 01-08):

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

In this report, we describe recent updates to the procedures already in place for making WFPC2 dark reference files. These updates include the ability to create dark reference files by using either weekly or daily dark images as input. They also allow the user to specify the year of the WFPC2 superdark image which is compared to the output dark file during processing. The new scripts are currently available for FTP from the WFPC2 website to users who wish to create dark reference files which are tailored to closely match the date of their HST observations. This ISR is written as a supplement to Instrument Science Report 2001-01, and is not intended to serve as a stand alone document.

Beta-test Version 3.0 of New WFPC2 Exposure Time Calculator Available:

-J. Biretta

The WFPC2 Exposure Time Calculator has been upgraded to allow for a more flexible specification of the sky background. There are now three options for determining the background contribution.

The first method uses a rough estimate of "average" or "high" or "low" sky background conditions. The low background (23.3 V magnitudes per square arcsecond) is an estimate of the minimum zodiacal light for sources at high ecliptic latitudes while the high background (21.0 V magnitudes per square arcsecond) is for maximum zodiacal light + reflected earth light. The average (22.5 V magnitudes per square arcsecond) background represents the average zodiacal light only.

The second method requires the user to provide the position of the target and (optionally) an estimate for the heliocentric longitude of the target (sun angle). This method computes the zodiacal light at the time of observation for the specific target. Setting the sun angle to 50 degrees will provide an estimate of the worst-case sky background. The lowest, or best-case, sky backgrounds occur when sources are farthest from the sun, i.e., "sun angle" = 150 to 180 degrees. There is also an option to request "Low Sky", which invokes the formal definition of low sky as used in Phase 1 and Phase 2 proposals. In this case, the zodiacal sky background is estimated at a value 30% greater than the minimum for the target RA and Dec specified. The low sky option overrides the user specified "Sun Angle" and instead selects the appropriate heliocentric longitude for the definition of "Low Sky."

The last option is for the user to explicitly provide a value for the sky background, in magnitudes per square arcsecond.

The new version of the WFPC2 Exposure Time Calculator (3.0) has passed simple tests, but more rigorous testing is still underway. Comments on the new features are welcome (email to

New Archive Search Service for Wide Field Planetary Camera 2 Observations:

Have you ever wondered how many extragalactic observations have been made in both the V and I bands of nearly the same place in the sky? Or how many pointings have at least 3 bandpasses of observations?

Well, now you can find out, by using the Pointings Search for WFPC2 at:

The Pointings Search Interface allows users to explore a new table set created for WFPC2 pointings. The archive group developed this table by defining a "pointing" for the WFPC2 to be a location in the sky circumscribed by a circle with a radius of 40". Then, each WFPC2 observation was assigned to a pointing. A table was compiled which described each pointing: the number of "u", "b", "v", "r", "i", and "narrow-band" observations at each pointing, the number of unique bands, the total number of exposures, and the number of days between the first and last exposure. Each pointing is also identified with a galactic and ecliptic latitude.

Queries with this interface can provide answers to questions such as:

- How many low ecliptic latitude fields have at least 2 distinct
bandpasses of observations and how many unique exposures exist for
those fields? What are those observations?

- How many high galactic latitude observations have at least one
observation in I or R and have at least 2 observations with 10 days
between the first and last observation?

The interface not only allow users to count such observations, but also to inspect previews, observational parameters and to retrieve the data. The next version of this table and interface will also include exposure times as a search parameter and exposure time totals will be reported in the results.

Questions and comments about this new service may be sent to (410-338-4547).


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.

MAIZ-APELLANIZ, J. "Structural Properties of
Massive Young Clusters" ApJ accepted

TOZZI, P. "HST Imaging in the Chandra Deep Field
South.  I. Multiple AGN Populations" ApJ accepted

and Mass of a Young Globular Cluster in NGC 6946"
ApJ 556: 801-812, 2001

Photometry of Globular Clusters in the Sombrero
Galaxy" MNRAS

BARMBY, P.; HUCHRA, J.P. "M31 Globular Clusters in
the HST Archive. I. Cluster Detection and
Completeness" AJ

R.P.; PALTRINIERI, B. "Blue Stragglers, Young White
Dwarfs and UV-excess Stars in the Core of 47 Tuc"
ApJ accepted


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