2. Observations and processing
Observations of NGC 4261 were made at 21 cm with the European VLBI Network (EVN) on February 22 1999 and lasted 10 hours. Participating antennas were Effelsberg, Jodrell Bank (Lovell telescope), Medicina, Noto, Onsala, Torun, and Westerbork (phased array). Unfortunately one of the three large collecting areas, the 100m at Effelsberg, was unable to observe due to heavy snowfall. The observing mode consisted of 4 frequency bands, all observed with dual circular polarisation, 4 MHz wide and 2-bit sampled. The second frequency band was centred on the H I line of NGC 4261 at a velocity of 2237 km/s (heliocentric, optical definition) corresponding to a frequency of 1410 MHz.
The data were processed on the EVN MkIV data processor at the Joint Institute for VLBI in Europe (JIVE) and constitute the first scientific experiment to be carried out with this new facility. The data were correlated between July 26 and August 6 1999 in two passes; the first resulting in 128 spectral channels on both polarisations of the line data, the second pass yielding sensitive continuum data, by processing all basebands with modest spectral resolution. For the spectral line dataset the resulting spectral resolution from uniform weighting was km/s. Data quality was good, except for some bad tape passes from Torun and a large fraction of the data for Medicina which was corrupted by interference. Amplitude calibration was carried out in the standard way using the values and gain curves from the stations. The continuum was imaged using standard self-calibration and CLEAN deconvolution methods. From variations in amplitude gain factor in the final step of amplitude self-calibration we estimate the uncertainties in the overall flux density scales of our images to be of order 15%.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000