Telescope surface measurement with two feeds

A method is suggested where a reference feed is fixed at the telescope focus, and a second feed has a lateral offset and scans the focal plane. The telescope tracks a celestial radio source, and the receiver measures the phase difference at the two feeds and their (voltage) amplitude ratio. A set of coordinate transformations is applied, after which the surface deviations \sigma from a paraboloid can be obtained by a Fourier transform. The measuring error is discussed, and the best wavelength is found to be between 14 and 21 rms \sigma .