WFPC2 Bright Star Avoidance Regions

Last Update: May 1998, (addition of STFOV information, see below).

Bright stars near the WFPC2 field of view can cause various artifacts to appear in science images. To completely avoid artifacts in broad band filter images, stars brighter than ~15th magnitude should be moved at least 10 arcseconds outside a 150 x 150 arcsec.² enclosing the nominal field of view.

When writing Phase II proposals, observers will sometimes attempt to avoid bright stars by placing them in the un-imaged "L" shaped region adjacent to CCD PC1, or in the region just outside the nominal field of view. This is ill-advised. In both cases the optics can image this light back onto the CCD's. Bright stars must be placed at least 10 arcseconds outside the field to avoid artifacts.

After Phase II proposal submission, STScI routinely provides Palomar Sky Survey charts (so called GASP charts) with the target positions marked. The accompanying template (PostScript) can be used with these GASP charts to check for stars in the problematic regions. This template can be printed or copied onto a clear vu-graph sheets for use on the GASP charts. It should print close to the correct scale on most postscript printers. The 120 arcsecond scale bar can be used to check the linear scale. Stellar magnitudes can be estimated using the star images printed on the chart. Since different Sky Survey plates have different exposure times, there are three series of star images for Sky Survey exposure times of approx. 5, 20, and 70 minutes; the correct one should be chosen based on the "Exposure (min.)" value printed on the GASP chart.

To properly locate the template on the GASP charts, place the desired aperture (e.g. dot marked WF4) on the target. Then rotate the template CCW by the "ORIENT" angle requested in the Phase II proposal. If the "ORIENT" angle in not known, then bright stars should be avoided for all rotations of the template.

There are three different avoidance regions near the WFPC2 field of view. Stars in the "PC direct ghost region" produce an arc of light on the PC CCD affecting the region nearest the pyramid appex (CCD row+column number < 500). This is caused by light passing directly from the WFPC2 articulating fold mirror to the PC CCD, without reimaging.

The second avoidance region, the "PC diffraction ghost region" encompasses the entire "L" shaped un-imaged region surrounding the PC CCD. Stars in this region cause broad arcs of light which can appear anywhere on the PC CCD. This effect probably results from light missing the perforation in the PC relay camera primary; this light reflects off the relay primary, back to the relay secondary, and onto the CCD in a severely defocused Airy pattern.

The third avoidance region is the "Dragon's Breath" region, which is a 6 arcsecond wide border surrounding the WFC CCD's. Stars in this region produce spikes of light extending into the adjacent CCD. The mechanism is poorly understood, but is likely to involve reflections off aperture stops early in the WFPC2 optical train.

More details, and examples of image artifacts, can be found in the Field Guide to WFPC2 Image Anomalies by Biretta, Ritchie, and Rudloff.

New HST field-of-view plotting task available in STSDAS:

by J.-C. Hsu and S. Baggett

STFOV, a new STSDAS task to overplot the science instrument apertures of the Hubble Space Telescope (HST) on a gray scale image, is now available. STFOV is basically a "wrapper" script based upon the existing tasks disconlab and siaper, but with a more user-friendly interface. The input image can be any image with WCS (= World Coordinate System) parameters in the header or a Digital Sky Survey (DSS) image; the input aperture can be any one of the HST apertures (i.e., not limited to WFPC2 apertures). The orient parameter (spacecraft roll measured in degrees east from north) is the same as that specified in the Phase II proposal template.

Other features include:

(1) Appending a new FOV, with user-specified color, to an existing chart, thus allowing convenient multiple overlays on the same gray scale image.

(2) Providing rudimentary estimates for magnitudes of stars in the field, which is useful when planning exposures, e.g., to avoid bright stars near WFPC2 which may result in scattered light.

STFOV is now available in the newest STSDAS release (2.0.1), under the graphics.stplot package; please refer to the STSDAS help for more details and examples.