Multi-Frequency Synthesis - a New Technique in Radio Interferometric Imaging

Details Multi-Frequency Synthesis - a New Technique in Radio Interferometric Imaging Multi-Frequency Synthesis - a New Technique in Radio Interferometric Imaging

Oct 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.246..490c&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Vol. 246, NO. 3/OCT1, P. 490, 1990

Astronomy and Astrophysics

Astrophysics

41

Scientific paper

We describe an extension to aperture synthesis, multi-frequency synthesis or MFS, whose principal goal is to allow high-quality radio images to be obtained from spars interferometric arrays. The idea is to use data from a relatively narrow range of frequencies (typically +/- 10 per cent) to produce a major improvement in the aperture-plane coverage of an array. From these data one can reconstruct an image at a single reference frequency in which the 'reconstruction errors', which normally occur as a result of large 'holes' in the aperture plane, are much reduced. The obvious problem to -be overcome is that the source brightness distribution is slightly different at each frequency. We analyse the 'spectral errors' introduced into the reconstructed image by this frequency-dependent structure. For a +/- 1 2.5 per cent range of frequencies we show that typically, spectral errors will be below the thermal noise level provided that the ratio of the peak brightness (excluding any compact component such as a radio core) to rms noise is less than a thousand-to-one, Unresolved features can be identified easily and effectively removed from the data a each frequency and hence they need not be a source of spectral errors. When this spectral errors are above the noise they can be recognized and removed by an extension to the well-established CLEAN deconvolution methodi We call the new method 'double deconvolution', or DD, and demonstrate its effectiveness 0 simulated data. The primary motivation beyond narrow-band MFS observations is to produce improved intensity images, rather than to determine spectral index distributions. Although the DD algorithm effectively estimates the spectral index distribution in the course of removing spectral effects, these estimates will be crude and not generally of astrophysical use. We briefly consider the practical problems involved in making MFS observations and conclude that the major limiting factor is likely to be radio-frequency interference. However, MFS is currently feasible between 1.4 an 1.7 GHz and at higher frequencies in the UK.

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