The delay pattern is the fringe visibility envelope (also known as chromatic aberration) which is radial and increases as the distance from the phase tracking center is increased. It occurs because a real receiver accepts frequencies of a finite bandwidth instead of a single frequency. This introduces an instrumental envelope function. The delay can be though of as occurring because radio telescopes form an image by adjusting the phase of the correlator signal from each point in the image plane instead of the arrival time at the wavefront.
Field of View, Phase Reference Center
References
van Gorkom, J. H. and Ekers, R. D. "Spectral Line Imaging II: Calibration and Analysis." Ch. 18 in Synthesis Imaging in Radio Astronomy: Third NRAO Summer School, 1988 (Ed. R. A. Perley, F. R. Schwab, and A. Bridle). San Francisco, CA: Astronomical Society of the Pacific, 1989.
