h on each of 12, 13, 16 & 17 February 1997 and 36 PNe were observed during 10 h on each of 24, 25, and 26 May 1999. In 1997 each subsample contained 10 PNe and each PN was observed for 5 min every hour. In 1999 each subsample contained 6 PNe and each PN was observed for 8 min every hour. The total quality integration time per PN was at least 50 min in 1997 and 80 min in 1999. To avoid artifacts at the center of the field we offsetted the positions 30 in declination. At the beginning and the end of each 12 h shift the primary flux density calibrator 1934-638 was observed for 5 to 10 min. The phase calibrator VLA 1730-130 and VLA 1748-253 were observed about every 20 min for 2 min.

Data Reduction {#reduction}
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The data were reduced using the package [Miriad]{} following standard reduction steps as described in the reference guide by Bob Sault and Neil Killeen (Sault et al. 1995) . The data were loaded, bad points flagged, and then split into single source files. The bandpass function and the antenna gains were determined as a function of time to 