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R. A. Perley (National Radio Astronomy Observatory)
Although the foundations of interferometric imaging were developed with instruments operating at meter wavelengths, most of the development of the techniques, and the subsequent science, have been done at increasingly shorter wavelengths. Two of the key reasons for the lack of a high-resolution imaging capability at meter wavelengths are the extreme difficulties in correcting for ionospheric phase corruption, and the complication and expense of image formation from the non-coplanar data taken by the Very Large Array.
However, the techniques of self-calibration and image formation which have been developed for interferometry at shorter wavelengths can be modified and extended to handle the more difficult problems posed by long-wavelength, long-baseline interferometry. The differences between long- and short-wavelength imaging are only a matter of degree, not of principle.
To test these techniques, and to demonstrate the potential of arcsecond resolution imaging at meter wavelengths, the Naval Research Laboratory and the National Radio Astronomy Observatory have outfitted the Very Large Array with receivers operating at a wavelength of 4 meters. Extensive tests with this new capability have shown that even simple extensions of the self-calibration methods developed for higher frequency data are sufficient to permit good imaging at this very long wavelength. More sophisticated variants of these techniques are now under development, and will soon allow full phase correction over the entire eleven-degree primary beam at the VLA's full resolution of 25 arcseconds.
In addition, modern computers have provided the necessary capability to permit development of sophisticated, multi-faceted imaging techniques, allowing full primary beam imaging while avoiding the aberrations which accompany simple two-dimensional inversion of non-coplanar visibility data. Thus, both historical limitations have been bypassed, and true diffraction-limited, wide-field imaging at meter wavelengths is now possible.
In this talk, I will review these historical limitations to imaging interferometry at meter wavelegths, and show the modern solutions. Examples of recent images made with the VLA's 74 MHz system will be shown to demonstrate the excellent results from this system, and to illustrate the scientific potential of long-wavelength radio astronomy. I will conclude with a look to the future with the proposed LOFAR facility.
The author(s) of this abstract have provided an email address for comments about the abstract: rperley@nrao.edu