High Resolution Radio Source Maps at 73.8 MHz

In 1984 Perley and Erickson proposed a 73.8 MHz [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.