  An in-depth analysis of GWs, LISA and its telescopes has been carried out in
this paper, especially for the case in which all aberrations, except
astigmatism and tilt are optimised. Though it is popular to neglect these two
aberration types for their minute effect on the wavefront, a test of this
negligence has been carried out in an extreme scenario of high phase noise. The
relationship between phase noise and fringe visibility has been analysed,
resulting in the discovery of an upper bound for allowed phase noise in LISA.
