Demonstration of how the angular size of source affects the flux difference at the overlap bandwidth for a top-hat (green), a Gaussian (red), and, a Sersic n = 1 (blue) profile. The two vertical dashed lines indicate the largest and the smallest beam FWHM of the FTS at 42′′ and 16.55′′. The horizontal dashed line indicates the percentage difference of the fluxes if a source uniformly fills the beams without taking η_c in account, while the gray band indicates the same value but accounts for the uncertainty introduced by η_c.