In this paper a simulation for a mathematical two dimensional model was conducted
to assess the presence of defocusing due to telescope aperture quality. The point
spread function (psf), Modulation transfer function (MTF), and Fourier frequencies
of binary star are computed in the absence and presence of defocusing. The results
show that the total frequency components of MTF decreases very sharply as
defocusing values goes to 6 and after this value the total frequency changes very
slightly afterward. The relative heights of the second to third peaks decrease rapidly
as defocusing values goes to 3 and slightly changes after that.