This interferometer is designed to measure 75x40 cm laser glass. The entire surface is measured
at a Brewsters angle, 56.57°, with an s-polarization beam. The reflected beam is retro-reflected
by a highly reflective mirror. Thus, a 75x40 cm surface can be tested with a 60-cm aperture.
The most troublesome problem is the ghost reflection from the rear surface of a flat while the
front surface is being measured. After the second surface is polished, both surfaces are reflective
and their beams can interfere. However, the second surface of a flat is to be polished to ensure
the transmitted wavefront quality, not the quality of the surface itself. Therefore, the second
surface does not need to be measured directly. To avoid reflection from both surfaces, the laser
is switched to a p-polarization after the first surface is measured while the flat is still at a
Brewster's angle. Thus, the transmitted wavefront is not affected by the reflection.
We believe that a 60-cm clear aperture, Fizeau phase-shifting interferometer is the most practical
and accurate instrument for testing 75x40 cm optical flats. In this paper, we briefly summarize
the important design factors, and show in theory that the design can meet the required