Instrument Performance

Plate Scale

The plate scale (i.e., the pixel size in arcseconds) had been determined for both of the FOC relays prior to the deployment of COSTAR. This was done by taking a series of images of a pair of astrometric stars, moving the telescope between exposures by a predetermined angular offset. The measured distances (in pixels) between the astrometric stars, combined with the known separation (in arc seconds) then give us the plate scale. The effects of COSTAR on the focal ratio of the image was then applied to these values to obtain preliminary plate scales.

Due to problems observing with the F/48 relay since the deployment of COSTAR, the preliminary plate scale of 0.02825 arcsec pixel-1 remains the only value available, with an error of xb1 0.0002 arcsec pixel-1. Observations of a field of stars already observed prior to COSTAR deployment were used to determine the new plate scale for the F/96 relay. The derived plate scale for the F/96 relay is 0.01435 arcsec pixel-1 (xb1 0.0002). This value has been verified using astrometric observations.

These plate scales will provide separations between close pairs of stars to within 0.4 pixels (or about 0.0057 arcsec for the F/96), while the separations between widely spaced stars are good to within 0.25%. The majority of these errors comes from the stabilization of the FOC image after it is turned on for an observation run. Early calibrations (discussed in Instrument Science Report FOC-045) showed that the plate scale and geometric distortion varies for several hours after initial high-voltage turn on. The plate scale, for example, only changes by about 0.25% in the first few hours then remains relatively stable afterwards. This variation can be important to an observer requiring the same astrometry for all their images, in which case, comments should be made in the Phase II proposal stating that the visit shouldn't start immediately after HV turn-on.

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