This is the least desirable of the three methods because it is slow. However, one can avoid this method if the focal points of the various feeds have been accurately determined because the turret positioning system is extremely accurate. In this case, one determines the pointing with a suitable feed, say one that can see a maser, and then repositions the turret to place the desired feed at the focal point. At low frequencies, the confusing effects of the Galactic emission and background point sources require making pointing measurements on a strong continuum source or a maser. The 1.7-GHz OH masers are particularly good.
Technique is important.
First, we cannot assume that the beam is circular. The beam can be represented as elliptical, with a Gaussian cross section on the minor axis and a skewed Gaussian on the major axis. These parameters are best measured by providing some redundancy in the measurements. A convenient solution is to perform four 'scans' that cut through the astronomical source at relative angles of 45 degrees. If there is a background, as in a continuum measurement, each 'scan' must contain at least five points, and if the background is time-variable then each 'scan' should be a continuous scan instead of discrete points.