7. Summary and conclusions
We present a method to improve the absolute astrometry of HST images. The method has been applied as a test case to the Seyfert 2 galaxies NGC 5929 to register the HST and MERLIN images. The astrometry of the HST image is accurate to 62 mas, while the error in the final registration with the radio image appears 100 mas. Systematic differences between the HST and ground based images are probably the main source of uncertainty in their registration, which can be improved by carefully planning the ground based observations to better reproduce the HST observational parameters. However, the HST absolute astrometry would be only marginally improved given the error of 41 mas in the reference ground based image. Similarly, the registration between HST and radio images is affected by the difference between the radio and optical reference frames and by the accuracy on the positions of the radio phase calibrators. For these reasons, a significant reduction in the error budget of this method is not anticipated until ground based techniques, reference catalogs, and the radio-optical link, are all sensibly improved. The forthcoming Hipparcos catalog will, albeit partially, drastically improve on the present situation.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998