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Technical Instrument Report TIR WFPC2 97-09

The WFPC2 PSF Library
Michael S. Wiggs, Sylvia Baggett, Jean Surdej, and Calvin Tullos -September 23, 1997

ABSTRACT: We describe the development of the WFPC2 PSF Library. The extraction of observed Point Spread Functions from standard calibration observations, as well as the creation of their associated header files are discussed. We then describe the development of the WWWbased PSF Library Search Tool, and use of the Tool.

1. Introduction
The WFPC2 PSF Library is designed to provide observers with easy access to a selection of observed PSFs, taken in most WFPC2 filters, and in various positions across each chip. Many of the PSFs were extracted from images of the clusters Omega Centauri and M67, which were imaged as part of STScI's ongoing WFPC2 Calibration Observations. These observations provided stellar images of various intensities, saturation levels, and positions across the chips. Only isolated, non-overlapping PSFs were selected. In the future, we plan to extract PSFs from the calibration observations of the single white dwarf stars A+81D266, BD+75D325, GRW+70D5824, and Feige110, from selected, public GO data, as well as from the remaining observations of the clusters mentioned previously.

2. Generation of the PSFs
The PSF images were extracted from archival, pipeline processed WFPC2 images, with no further processing, and generally no cosmic-ray rejection. To identify PSF candidates, the observations were examined by eye. The PSF stars had to be well separated from other stars and have a peak intensity of no less than 150 DN; there was no imposed upper limit on any star's intensity. For all stars meeting these simple selection criteria, the IRAF tool imexamine was used to fix the star's position and calculate the peak intensity (a Gaussian fit, in DN). An estimate was made of the background level on the chip, by determining the average within 3 -

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20x20 pixel regions, free from cosmic rays. The resulting estimate is given in the BACKGND keyword in the PSF header. The PSF positions and intensities were used to: 1. Determine the DN value of the peak pixel from image statistics (not the fit from imexamine), and then define the size of the extracted image. For peak intensities less than 3500 DN, the extracted image is 64x64 pixels, centered on the PSF; greater than 3500 DN, the image is 128x128 pixels. 2. Extract the image sections as separate images with individual headers, comprised of the original WFPC2 header, plus new PSF specific keywords, listed in the table below.
ORIGIN XCENTER YCENTER XCORNER YCORNER RA_PSF DEC_PSF PEAKINT PEAKFIT SATURATD BACKGND SPECTRAL REFSPEC T306 T307 T308 T309 V2_RMS V2_P2P V3_RMS V3_P2P GUIDEACT RELFOCUS Data Source (HST, TINYTIM, or COMPOSITE) X coordinate of PSF center on chip Y coordinate of PSF center on chip X pixel of (1,1) corner in PSF image Y pixel of (1,1) corner in PSF image RA of PSF star (deg, J2000) Dec of PSF star (deg, J2000) Peak intensity at xcenter, ycenter Peak of Gaussian fit Is PSF saturated? (Y or N) Background (in DN) Spectral type of source, if known Reference spectrum, if known Secondary mirror temperature (degrees Celsius) Secondary mirror temperature (degrees Celsius) Secondary mirror temperature (degrees Celsius) Secondary mirror temperature (degrees Celsius) V2 Axis RMS (milli-arcsec) V2 Axis peak to peak (milli-arcsec) V3 Axis RMS (milli-arcsec) V3 Axis peak to peak (milli-arcsec) Actual guiding mode at end of GS Acquisition Relative focus (in microns)

Table 1. Keywords added to PSF Image Headers.

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3. Recompute the minimum and maximum data values and the "good" minimum and maximum data values, and update the PSF headers. Any PSF image with saturated pixels (bitflag = 8, in the original calibration image's data quality (.c1h) file), had those pixel values replaced with a value of -100. This will allow users to quickly identify saturated pixels within the PSF.

3. Status of the PSF Library
To date there are 2701 PSF images available in the PSF Library. The table below lists the current distribution of PSFs by filter and chip.
FILTER F300W F336W F380W F410M F439W F450W F467M F547M F555W F569W F606W F675W F702W F785LP F814W PC 8 40 1 1 60 2 1 10 80 12 38 40 12 22 218 W2 9 81 3 1 93 4 3 17 102 18 25 14 27 24 277 W3 14 86 3 1 122 1 3 26 132 20 50 18 18 29 334 W4 9 50 0 0 73 0 1 11 89 11 30 10 17 23 277

Table 2. Status of PSF Library by filter and chip.

4. PSF Relative Focus Information
One of the new PSF header keywords provides a value for the relative focus (RELFOCUS). This was done to provide users with a general idea of the focus of HST at the time of the PSF observation. Thus, users can use PSFs at or near the same focus as their own observations.

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Presently, nearly half of the PSFs in the Library have RELFOCUS values determined from an average fit of the focus monitoring data based on the "phase-retrieval" method, with no correction for orbit-to-orbit variations in the focus, (known as "breathing"). No breathing correction could be performed because the observations were made prior to July 1995, when temperature data became readily available. The other half of the PSFs were taken after July 1995, and have the "breathing" correction applied to the averaged phaseretrieval focus values. See Instrument Science Report OTA-18 for details on the focus history of HST and the "breathing" correction (Casertano, 1995). The r.m.s. scatter around the average fit is roughly +/- 2 microns, while the scatter for points with the breathing correction applied is close to +/- 1.3 microns. Within the headers of the PSFs, we have also included the secondary mirror temperatures (T306, T307, T308, and T309 keywords), if known. Thus, if a particular PSF has a value other than 0.00, the breathing correction has been applied. Also available from the PSF Library Help Page is a table containing the Modified Julian Date (MJD), and the averaged phase-retrieval focus values (with no breathing correction applied), to provide users with an estimate for the focus of their science image.

5. PSF Library Search and Retrieval Tool
In order to make the PSF images easily accessible to WFPC2 users, we opted for a Web-based search and retrieval tool, with the PSF images stored and available on-line. The tool was designed to be as flexible as possible, allowing the user to search for desired PSFs using virtually any combination of criteria. The output of this search can also be sorted according to the user's needs . There are four basic steps to using the tool: 1. Define a subset of PSFs using qualifiers on the main page. 2. Select for retrieval a subset (or all) PSFs from the tabular output 3. Request generation of an ftp command file for retrieval of the selected PSFs 4. Save the command file to local disk, and run it at the user's leisure. The Tool is accessible at the following WWW address:

www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_psf/wfpc2-psf-form.html

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The page has an extensive help file as well as a status page, listing the number of PSFs per filter and chip, and also plots showing the X-Y positions of the PSFs on the original images (see Figure 1). The PSF files can be retrieved in either FITS or GEIS formats.

Figure 1. Positions of all PSFs available for F555W on the WF2 chip.

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Figure 2. A portion of the WFPC2 PSF Library Web Page.

Figure 2 displays a portion of the PSF page, with parameters set to search for all PSFs for filter F547M, in the PC, with any gain, and both serials. There are many other keywords that can be used as qualifiers, such as observation date, exposure time, if the PSF is saturated, peak intensity, X,Y position (with search radius), as well as jitter. The user can sort the output according to any criteria, in this example the output will be sorted by dataset name, detector, and gain. The result of this search is in the "default output" format and is illustrated in Figure 3. The output page allows the user to preview the PSF image, by "clicking" on the highlighted dataset name. It also allows for the selection of either all or just a few PSFs to be marked for retrieval. The PSFs can then be downloaded to the user's machine in either GEIS or FITS format.

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Figure 3. Output from a search of the PSF Library.

The PSF Library is being continually updated with new PSFs. However, some users may find no appropriate PSFs in the Library. In this case we recommend using archival data, either calibration or public GO data as a source of observed PSFs. Table 3 lists some of the calibration proposals which could provide useful PSFs. Another alternative is to use TinyTim, a program to generate theoretical PSFs. Tiny Tim can be accessed at:

scivax.stsci.edu/~krist/tinytim.html

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PROPOSAL ID 4616 4749

TARGET Omega Centauri Omega Centauri

NUMBER OF DATASETS 35 14

FILTERS F555W, F814W, F300W,F547M, F569W,F606W, F675W, F702W, F785LP F336W, F439W, F555W, F814W F555W, F814W F555W F336W, F439W, F555W, F675W, F814W ALL (Except UV Filters) F555W F439W, F606W, F675W F439W, F814W F336W, F439W, F555W, F675W F814W ALL BROAD AND MEDIUM BAND FILTERS F555W, F814W F439W, F675W, F814W F336W, F439W, F555W, F606W, F675W, F814W

EXPOSURE TIMES (Sec.) 30 7 - 600

4763 4819 5558 5565

Omega Centauri M67, Omega Centauri Omega Centauri Omega Centauri

35 114 13 70

6 - 400 40 30 3 - 400

5572 5646 5651 5659 5663

Omega Centauri Omega Centauri M67 Omega Centauri Omega Centauri NGC6752 Omega Centauri

18 9 8 18 267

7 - 600 40 2.6 - 230 40 12 - 300

6182

54

10 - 400

6192 6193 6195

Omega Centauri Omega Centauri Omega Centauri

68 80 44

1 - 40 40 100 - 200

Table 3. Calibration Proposals useful for generating PSFs.

6. Acknowledgements
We would like to thank the following people for useful comments and suggestions: Stefano Casertano, Calvin Tullos, Matt Lallo, and Tim Kimball.

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