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Astron. Astrophys. 318, 997-1002 (1997)

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5. Radio and optical reference systems

The final FK5 optical position of NGC 5929, measured as the point of maximum correlation between the ground -based frame and the "degraded" WF/PC-I continuum image, is [FORMULA] (J2000) = [FORMULA] [FORMULA] [FORMULA].1654 and [FORMULA] (J2000) = [FORMULA] 40' 14".418. This position is consistent with the coordinate system of the contour plots of Figures 2a and 2b, and its estimated total error is [FORMULA] [FORMULA] 62 mas. This error measures the precision of our optical registration. However, a systematic deviation between optical and radio frames is expected. Both, an error in the RA zero-point alignment of the radio frame to the FK5, and local "regional" deviations of the two systems are expected. Assuming that the right ascension zero-point of MERLIN maps has been set via one of the currently adopted procedures for the realization of what is called today the International Celestial Reference Frame (ICRF) [by using the standard source 3C 273 or by an adjustment to known FK5 positions of a set of compact radio sources (see for example Ma et al. 1990)], the local optical-to-radio systematic deviations can be of the order of 20 mas or larger. A recent accurate registration of the radio and optical images of SN 1987A using preliminary Hipparcos data (Reynolds et al. 1995) has shown, thanks to a direct measure of the effect, that the local systematic deviation between the two systems is [FORMULA] 22 mas in declination. We conclude that a systematic shift of about 20 mas should be included in the calculation of the final error of the optical position. This shift adds directly to the error of the optical position and not to its variance.

[FIGURE] Fig. 2a and b. contours from the MERLIN radio image by Su et al. (1996) overlaid onto the HST WF/PC-I contour image in a the continuum and b the line emission. The cross on the lower right corner represents the uncertainty in the registration, 80 mas.

A drastic improvement on these residual systematic deviations is expected following the radio-optical link of the Hipparcos catalog. The final accuracy is anticipated in Lindegren and Kovalevsky (1995) at the 0.5 mas level (for the positional system; 0.5 mas yr-1 for the proper motion system).

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998