Search

In 1984 Perley and Erickson proposed a 73.8 MHz 1_4 meter wavelength] observing system at the VLA site (NRAO Scientific Memo #146). They proposed a stand-alone antenna system that would feed its signals into an existing spare channel of the VLA waveguide and utilize a separate correlator. Over the 35 km VLA baselines this system would produce images with 20 arcsec resolution, unprecedented at this frequency. The major technical problems are ionospheric refraction and interference. Some doubt existed as to whether or not it would be possible to cope with the large, rapidly-changing ionospheric phase fluctuations to be expected over 35 km baselines. Thus it was proposed, as a first step in the development, that 73.8 MHz feeds be installed in the present VLA dishes and that trial observations be made to prove that techniques such as self-calibration can be successful. Eight dishes now have 73.8 MHz instrumentation and a number of radio source images have been made with this initial system.

There are now approximately two dozen operating interferometers in astronomy, with more under construction or in planning. The wide distribution of interferometer arrays, and their acceptance as essential tools in astronomy are a result of the remarkable flexibility and imaging capabilities of modern arrays. Much of the success of these arrays’ is due to advances in data processing and imaging brought about by innovative and powerful algorithms.

The jet in the radio galaxy NGC6251 (Waggett et al. 1977) has been observed with the VLA using resolutions from 1.15″ to 15″ at 1370, 1480, 1662 and 4885 MHz, and a resolution of 50″ at 1446 and 1662 MHz. The resulting maps (e.g. Figure 1) give exceptionally detailed information on the distributions of the total and polarized intensities over the jet.

NGC 6251, a 14th mag elliptical galaxy, was shown by Waggett et al. (1977) to have large-scale radio emission features with a total angular extent of ~ 1.1°, which corresponds to a projected linear size of about 1.7 Mpc (HO = 75 km s-1 Mpc-1). A bright radio jet links a central core source embedded in NGC 6251 to the extended emission on the northwest side of the galaxy.

As part of the VLA calibration program, 404 small angular size sources have been observed in the “A” array at both 6 and 20 cm with resulting resolutions of 0.4″ and 1.2″ respectively. Use of self-calibration techniques has allowed a search for associated extended structure to a level of ~0.3% of the peak. Here we report preliminary analysis of the results.