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Astron. Astrophys. 325, 383-390 (1997)

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2. Observations

We observed these sources simultaneously at [FORMULA] 3.6 cm and [FORMULA] 13 cm for about 10 hours on 18-19 June 1990, using an intercontinental VLBI array of 9 radio telescopes (antenna diameters and locations from East to West in parentheses): Wettzell (20 m, Germany), Medicina (32 m, Italy), Effelsberg (100 m, Germany), DSS63 (70 m, Spain), Haystack (37 m, Massachusetts), Green Bank (43 m, W.Virginia), Pie Town (25 m, New Mexico), Owens Valley (40 m, California) and Fort Davis (26 m, Texas). Right hand circular polarization signals were recorded at each antenna using the MK III VLBI system (Rogers et al. 1983). Recording Mode A (28 MHz bandwidth at each wavelength) was used at all antennas except Pie Town, where the use of a VLBA recording terminal restricted the usable bandwidth to 16 and 12 MHz at [FORMULA] 3.6 cm and [FORMULA] 13 cm, respectively. The observing method in 1990.5 was similar to that used in the previous epochs and is described in detail elsewhere (Marcaide & Shapiro 1983).

Observations of the calibrator source 4C39.25 and the pair 1038+528 A and B were alternated in a repeated cycle throughout the experiment: 2 minutes pointing on the calibrator and 11 minutes on the midpoint between the sources of the pair. This scheme is a very convenient one for calibration of the array.

The data processing was carried out at the VLBI correlator center of the Max Planck Institute für Radioastronomie in Bonn (Germany). A two-pass correlation using different "a priori" geometric parameters was made, to determine separately the interferometric observables for each source. The correlated data were transferred into appropriate formats for further analysis using the Caltech mapping software (Pearson 1991) and the astrometric program VLBI3 (Robertson 1975).

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998