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8.2 Astrometry

The astrometric accuracy of FOC data depends on two factors. The first is the pointing accuracy of the FOC. The second is the internal geometric accuracy of an FOC image itself, including the correctness of the distortion model, the plate scale, and the image rotation.

Pointing Accuracy: Positional errors in the HST Guide Star Catalog contribute most of the error in the RA and Dec assigned to the center of an FOC image. Typical 1 errors in guide-star positions are +/- 0.33 arcsec in the northern hemisphere and +/- 0.5 arcsec in the south. One might expect that the (unknown) proper motions of guide stars in this catalog gradually add to these errors. The accuracy with which HST places a target in the FOC field of view depends in a complex way on the target coordinate uncertainty, the positions of the guide stars in the FGS fields of view, and the alignment of the FOC imaging aperture with respect to the FGS reference frame. This FOC-to-FGS alignment is maintained to better than 0.2'', and experience with the overall pointing accuracy of the FOC when GASP coordinates are used has shown that 1 sigma error in the absolute pointing is approximately 0.5 arcsecond. On top of these errors, different filters induce different target shifts within FOC images (Table 8.1 lists known filter shifts). In most cases, the translation of the image due to the filter is small (1-3 pixels, or 0.015-0.05 arcsec), but some filters do introduce a large shift. Particularly notable are the F320W (shift=88 pixels) and F486N (shift=20 pixels) filters.
Relative Positions: The best estimate of the accuracy of the relative positions within an FOC image comes from the rms residuals of star positions in the crowded field used for calibrating the geometric distortion. Typical values are 0.3 pixels (0.005 arcsec) for the 512 (zoomed) x 1024 format and 0.2 pixels (0.003 arcsec) for the 512 x 512 format. These uncertainties are compounded by the uncertainty in the plate scale, which is subject to time variation. The absolute calibration of the plate scale and rotation has been accomplished in two ways; firstly, by observations of an astrometric star field using astrometric guide stars, and secondly by using the programmed offsets between observations in the crowded-field geometric distortion analysis. Typically, these different measurement methods give consistent results in cases where the pointing system operates without anomalies. However, the FOC plate scale can vary from switch-on to switch-on. Comparisons of images of the same field taken several months apart have shown plate-scale variations as large as 0.7%. These time-dependent drifts of the FOC plate scale have never been studied in any systematic way.
Based on all the above, the best estimate for the f/96 plate scale is:
f/96 plate scale = 0.01435 +/- 0.00007 arcseconds/pixel
Recent measurements of the f/48 plate scale which compare images of the same crowded field from both the f/48 and f/96 cameras show that the plate scale of the f/48 is:
f/48 plate scale = 0.02870 +/- 0.00029 arcseconds/pixel.
The best estimates for the pre-COSTAR f/96 and f/48 plate scales are:
f/96 plate scale = 0.02217 +/- 0.00010 arcseconds/pixel
f/48 plate scale = 0.04514 +/- 0.00012 arcseconds/pixel.

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