This paper proposes the optimum combination of a compact interferomete
r with a reverse circular path of the working and reference beams and
a two-frequency phase modulator. The interferometer is a modulated Aug
ustyn interferometer in which the usual beam-splitting cube is replace
d by a polarization beamsplitter. Tests of a prototype of the proposed
system showed that it is promising as a wavefront sensor of an adapti
ve telescope. The experimental resolution of the system was lambda/100
.