Please see picture in attachment.
Given an aperture of diameter D. When a wave originates from a point at distance (A) the assumption is that the wave front is planar and strikes all points in the aperture simultaneously. In reality the wavefront is spherical and strikes the center of the aperture first and then the ends last.
a. Derive the expression for the phase error (the little gap at the end when the wave strikes the middle of the aperture first) as a function of R in units of (2D^2/lamda); i.e just divide the solution by (2D^2/lamda) and note this is for a 2D space case not 3D and lambda = wavelength© BrainMass Inc. brainmass.com November 30, 2021, 4:16 am ad1c9bdddf
The expert examines the wave front planar and strikes for points in the aperture simultaneously.