WFPC2 Miscellaneous FAQs

Q: Is there an astrometric equivalent of shutter shading?

A: No, there does not appear to be any such effect, down to 0.01". WFPC2 image pairs, taken back to back at the same pointing, 0.11 sec for one exposure and 1.0-1.2 sec exposure for the other, were examined for possible effects. On 10 image pairs (4 WF3, 6 PC), there was a good point source with adequate signal in the 0.11s case and no saturation in the longer image. The results show an average offset of only 0.003" (no difference WF vs. PC) and a range of 0.001"-0.005". Since this is rather precisely what would be predicted from nominal, well-behaved HST jitter, the conclusion is that there is no astrometric equivalent of shutter shading (to well under 0.01" level).

Q: What are the "dead" areas seen at the edges of the chips?

A: The "dead" areas are due to the pyramid being in the aberrated part of the beam. The aberrated image of the pyramid edge is 40 ± 20 pixels from the edge of the chip along the two edges of each CCD which butt against the fields of view of neighboring CCD's (e.g. WF2 is butted against PC1 and WF3). Also, because the beam is aberrated at the pyramid, there is a vignetted area at the inner edge of each field with a width of 4 arcseconds. The vignetted light ends up focused in the corresponding position in the adjacent channel (with complementary vignetting).

Q: I can't find the information I need such as HST's period, radius, and orbital elements.

A: You can find the information you need in a file that has the extension .shh. Within this .shh file are the following keywords:

      HSTORB  = 1/2 the orbital period of HST in seconds.
      SINEINCL = The sine of the inclination of the angle
         of HSTs orbit.
      POSTNSTX = Position of HST from the center of the
         Earth in kilometers, (in x).
      POSTNSTY   =  (same but in y)
      POSTNSTZ   =  (same but in z)
      VELOCSTX   = Velocity of HST around the Earth in
           km/sec.
      VELOCSTY   = (same but in y)
      VELOCSTZ   = (same but in z)

Note that the values for the keywords are those of HST at the beginning of an observation.