Specifications for Selection of WFPC2 for Scrapbook

MISSION:   WFPC2 (HST)  

General goal: show WFPC2 images in five bands: U, B, V, R, I.

Include everything with a preview which is non-proprietary. When more than one image with the same filter is available, pick the one with the longest exposure time. If that includes more than one image, pick the most recent one.

Definition of individual pointing

All observations within 40" from a given observing position. This roughly corresponds to: 1. a circle centered on the "V1" position (the middle of the WFPC2 field of the view) and extending to the top-left corner of the Planetary Camera (PC) detector (38.5"); 2. a circle centered on the "V1" position with radius corresponding to 1/4 the size of the WFPC2 detector (37.5").

Observations of the same area will be more or less at the same observing position for dithering/cosmic-ray removal purposes. Pointings [for a given wavelength] shifted by more than 40" but less than 75" (half the size of the WFPC2) are rare, given also the STScI policy against repeat observations. (An obvious exception are programs which use mosaics of WFPC2 fields.) An examination of the choice of WFPC2 apertures, in fact, shows that in ~ 85% of the WFPC2 images the target is centered either on the V1 position or on the center of the PC.

Note that the RA and DEC in the FITS headers for WFPC2 coincide with the V1 position. If the target was positioned in the field of view at a distance > 40", then the target will not be included in the unique pointing (i.e., a search for that target with r<=40" won't find any images). This will happen, in particular, if the WF2, WF3, or WF4 aperture positions are selected (only <~ 15% of the time; see above). However, if a search radius equal to at least 1/2 the size of the WFPC2 detector (75") is used this problem will be avoided.