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Up: A photometric catalogue

5. Astrometry

  We performed a standard transformation on our CCD coordinates to the reference system defined by Godwin et al. (1983, GMP). In this system each object has (tex2html_wrap_inline1054) coordinates in arcsec, given relatively to a center defined at tex2html_wrap_inline1056 - located between both largest/brightest central galaxies, NGC 4874 and NGC 4889 (see Fig. 1 (click here)). For spectroscopic purposes, the frames were taken with a large superposition in Y (of the order of 40%, while almost negligible in X), which caused double observation of many objects. We carefully eliminated these double entries, both for stars and galaxies. In order to estimate the final precision of our positions, we compared our star catalogue with the Guide Star Catalogue (GSC) of the Hubble Space Telescope limited to magnitude m<15.5, which has a 0.3 arcsec accuracy. The positions of the same objects in both catalogues coincided, after final tuning, within less than 3 arcsec (median result for 17 stars identified in the field of our observations).

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