Astron. Astrophys. 319, 405-412 (1997)

2. Observations and data reduction

2.1. Global VLBI and MERLIN observations at 5.0 GHz

3C395 was observed simultaneously with global VLBI and MERLIN on September 11th 1991 at a frequency of 5.0 GHz in left circular polarization (LCP). The observing bandwidth was 2 MHz for VLBI (Mark II recording system, Clark et al. 1972 ) and 16 MHz for MERLIN. The strong compact sources OQ208, 1739+522 and BL Lac were observed as fringe finders for the correlation of the VLBI data, and as point source calibrators in the postprocessing.

The global VLBI array consisted of 15 antennas (see Table 1). The VLBI data were correlated in the Block II correlator at the California Institute of Technology (Caltech), and later fringe-fitted using the NRAO Astronomical Image Processing System (AIPS). The calibration of the amplitude visibilities was carried out using the Jodrell Bank OLAF package, from measured system temperatures and gain curves supplied by the individual antennas. For imaging, independent mapping procedures using the Caltech, OLAF (Pearson 1991 ; Muxlow et al. 1988 ) and AIPS packages were used and very similar results were obtained.

Table 1. VLBI arrays

The MERLIN array consisted of six antennas: Jodrell-Mk2, Tabley, Darnhall, Knockin, Defford and Cambridge. Flux density calibration was performed by comparison of OQ208 and 1739+522 with the flux density calibrator 3C286 assuming a value of 7.382 Jy. For the MERLIN data, we followed standard self-calibration and mapping procedures using the OLAF and AIPS packages.

2.2. Global VLBI observations at 8.4 GHz

We observed the quasar 3C395 at 8.4 GHz in right circular polarization (RCP) on September 12th 1992 using the Mark II recording system. The 12 antennas participating in the observations are listed in Table 1. The data were correlated in the Block II correlator at Caltech. The main delay and fringe rate residuals were removed using the AIPS package and then the data were exported to the Caltech package for calibration and mapping. The data were initially calibrated using the measured system temperatures and antenna gains. The compact source 1739+522, also observed during the experiment, was then used to refine the amplitude calibration. The final image was obtained after several cycles of phase and gain self-calibration.

2.3. VLBA observations at 5.0 and 8.4 GHz

We observed 3C395 for 12 hours with six antennas of the VLBA on August 12th 1991, cycling between 5.0 and 8.4 GHz every 15 minutes (see Table 1). The data were recorded in LCP, using the Mark II recording system with a synthesized bandwidth of 2 MHz. The correlation of the data was carried out in the Block II correlator at Caltech. The data calibration was performed with the AIPS package, following a similar scheme to that described in sections 2.1 and 2.2. The sources 1739+522 and 0016+731 were used as calibrators.

2.4. VLA observations at 5.0 GHz

Observations of 3C395 were made with the VLA on August 18th 1991 over a 3 hour period. Two standard 50 MHz IFs were used with frequencies of 4835 and 4885 MHz. 3C286 was used as a flux density calibrator with assumed fluxes of 7.426 and 7.379 Jy, respectively. The phases were calibrated with the nearby source 1751+288. The calibration and mapping of the data was carried out with the AIPS package.

2.5. VLBI test observations at 22.2 GHz

Test VLBI observations of the quasar 3C395 were made on May 1st 1993 at 22.2 GHz in LCP with a three baseline array (see Table 1), using the Mark III recording system (Rogers et al. 1983 ) with a synthesized bandwidth of 112 MHz. The correlation and fringe fitting of the data were carried out at the Mark III correlator in Haystack (MA, USA), where the data were exported to the Caltech package format for calibration and mapping purposes. The strong point source 0954+658 was used as a flux density calibrator with an assumed flux density of 1.3 Jy. The final image was produced after several cycles of phase self-calibration. Some scaling errors may be present since 0954+658 is known to be variable at this radio frequency.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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