Why use interferometry?
Interferometry provides higher test resolution than the zonal Foucault test. The zonal Foucault test assumes that the mirror has perfect rotational symmetry. This is never the case so Foucault results tend to be over-optimistic. Interferometry measures the entire mirror surface. It has limitations because it sees everything, including air currents and test sling errors so it tends to be more pessimistic. There are ways to remove some of the random and systematic errors in the interferometric test to arrive at a very good approximation of the mirrors wavefront. Both interferometry and Foucault result in an approximation of the mirrors wavefront. Both can be used to make a fine mirror but interferometry has better test resolution.
