Astrometry


The astrometric solution has been derived from 12 bright objects randomly distributed over the observed field and for which coordinates are available in the APM and USNO catalogues. A first rough solution ofthe astrometry was computed using a polynomial fit to determine the correct orientation and the relative rotation matrix. Oncecorrectly oriented, the final astrometric parameters for atangent-projection derived from the polynomial fit has been computed.The uncertainties in the solutions thus obtained, which take into account also distortions, are ~ 0.1 arcsec in both RA and Dec. The astrometric solutions are included in the headers of the fits files of the images. SkyCat, Saoimage, STSDAS, can be used to derive the RA and Dec (J2000) directly from the pixel coordinates.SExtractor, Focas and other useful packages are also able to read the astrometric solutions in the header of the images thus providing catalogues including the RA and Dec information.