The location of the center of the beam pattern for each antenna in the array must be determined to an accuracy of less than 0.1 FWHP of the beam pattern. Larger errors can reduce the sensitivity of the observations and introduce distortions in extended objects. This tolerance must be even smaller if a large part of the primary beam contains emission.
References
Napier, P. J. "The Primary Antenna Elements." Ch. 3 in Synthesis Imaging in Radio Astronomy: Third NRAO Summer School, 1988 (Ed. Perley, R.; Schwab, F.; and Bridle, A.). San Francisco, CA: Astronomical Society of the Pacific, 1988.
