4 Astrometry

The center of the frame is taken from the header of the FITS file. From the Guide Star Catalog (GSC) and from the LEDA database we extract all objects (stars or galaxies) known in the corresponding square. A cross-identification between matrixes and stars is made exactly as described in Paturel et al. (1996, Sects. 3.3 and 3.4). Galaxies are also used but only when they have accurate coordinates (i.e. typically better than 10 arcsec). A 6th order polynomial fit converts (x, y) positions on the frames to Right Ascension and Declination. The number of GSC stars varies from one frame to another. A histogram of number of GSC stars per frame is given in Fig. 3. If this number is smaller than 7 or if the standard deviation of the polynomial fit is greater than 4 arcsec, the solution is rejected and we adopt the "header" solution calculated from the coordinates of the center and the pixel size as given in the header. If the GSC-solution seems acceptable but differs from the header-solution by more than 30 arcsec, the header solution is preferred and coordinates are flagged to recall that they may be inaccurate.

  

Figure 3: Histogram of the number of GSC stars by frame

In Fig. 4, we show the differences between the GSC-solution and the FITS header-solution. Most of them are in good agreement within 15 arcsec. Note that more recent measurements have been astrometrically calibrated to better than 1 arcsec by cross-identifying with the PMM database.

  

Figure 4: Histogram of the differences between the two astrometric solutions (GSC- and direct-solution)

Galaxy coordinates will be compared directly with coordinates of LEDA galaxies.