  In a space based gravitational wave antenna like LISA, involving long light
paths linking distant emitter/receiver spacecrafts, signal detection amounts to
measuring the light-distance variationsthrough a phase change at the receiver.
This is why spurious phase fluctuations due to various mechanical/thermal
effects must be carefully studied. We consider here a possible pointing jitter
in the light beam sent from the emitter. We show how the resulting phase noise
depends on the quality of the wavefront due to the incident beam impinging on
the telescope and due to the imperfections of the telescope itself. Namely, we
numerically assess the crossed influence of various defects (aberrations and
astigmatisms), inherent to a real telescope with pointing fluctuations.
