2 Observations

We observed the complete S5 polar cap sample at 8.4 GHz on 6 December 1997 (epoch 1997.93) and on 28 May 1999 (epoch 1999.41) with the VLBA, each time for 24 hours. We used each case the recording mode 128-8-1 (1 bit sampling, 16 channels per each of the 8 IFs, with a total of 128 channels), yielding a total bandwidth of 64 MHz, in right circular polarization. Data were correlated at the VLBA Array Operations Center of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, using a basic integration time of 4 s. All radio sources were detected and provided fringes for all baselines. We observed in groups of 3 or 4 radio sources in a cyclic way with duty cycles of about 6 min. Every scan was 78 s long, and the small (a few seconds) time gap between different scans was used for slewing the VLBA antennas. We replaced one or two members of the group of observed sources by new members about every 2 hours, until all 13 members of the sample were observed. This observing scheme resulted in a time coverage for each single radio source of about 4 hours. We tracked the clock behavior - of relevance for the ulterior astrometric analysis - by including some scans of BL0454+844 in all groups observed during the first 12 hours, and of BL2007+777 in all groups observed during the second 12 hours. After having fringe-fitted the correlator output, we obtained correlation amplitudes for each source. We then constructed the visibility amplitudes in a standard manner, using the antenna gain and system temperature information provided by each station. We exported the data for use with the Caltech VLBI Package (Pearson 1991) and the difference mapping software DIFMAP (Shepherd et al. 1994).