SIMBAD references

The Ultraviolet Imaging Telescope cameras include two-stage magnetically focussed image intensifiers that introduce small but significant geometrical distortions to the data. These distortions, which create positional offsets as large as 25 arcseconds at the field edges, are corrected by the procedure described here to 2-3 arcseconds, approximately the resolution of the images. The distortion is measured by comparing and correcting UIT images to digitized Guidestar survey plates of the same fields. Two-dimensional third-order polynomials are used to model the distortion. The models assume that the distortion is an instrumental effect, independent of mission elapsed time and target, and that the effect of distortion in the center of each field is minimal. The models are used to improve computed astrometric plate solutions and to remove the geometric distortion while transforming the image to a standard north-up, east-left orientation.