4 Accuracy of galaxy coordinates

According to our previous tests (Paper II) an accuracy of about 5'' to 7'' is obtained for large galaxies (diameter of about 1.7'). This uncertainty comes essentially from the uncertainty in the estimate of the galaxy center. For smaller galaxies we expect better accuracy. In order to test the present measurements with small galaxies we measured 285 faint galaxies measured by Munn et al. (1997). In Figs. 3 and 4 we present the histograms of the differences between our measurements and those given by Munn et al., for Right Ascensions and Declinations, respectively. There is no significant deviation from zero. For the Right ascension we obtained: $\Delta \alpha \cos\delta =0.02~\pm~0.04''$ (n=285). For Declination we obtained: $\Delta \delta =0.08~\pm~0.05''$ (n=285).

The standard deviation is 0.6'' and 0.8'' for Right Ascensions and Declinations, respectively. This confirms that the accuracy of coordinates calculated with the DSS plate solution mostly depends on the accuracy of the definition of the galaxy center. For small galaxies (diameter smaller than 1') the accuracy is better than 1''.

Figure 3: Histogram of the differences between the Right Ascensions given by Munn et al. and this paper. The standard deviation is 0.6''

Figure 4: Histogram of the differences between the Declination given by Munn et al. and this paper. The standard deviation is 0.8''

Acknowledgements

We thank H.G. Corwin for carefully reading and checking the manuscript of this paper. We made use of the ESO facility in Münich for retrieving images from the Digitized Sky Survey produced at the Space Telescope Science Institute under US Government grant NAG W-2166. It is based on photographic data obtained using The UK Schmidt Telescope and the Palomar Sky Survey.

The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright (c) the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permission. We made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech under contract with the National Aeronautics and Space Administration.