A method of testing figure quality of a convex mirror in which an
optical quality substrate material is used. The substrate has a front convex surface that has to be polished and figured to high accuracy, typically λ / 10 (HeNe), and a rear
concave surface which only needs to be polished and figured to moderate surface figure accuracy, typically about 1-λ (HeNe). Three separate interferometric null tests are then carried out to produce three independent sets of
optical path difference (OPD) data. Null lenses, or nulling computer generated holograms (CGHs), are designed and used as needed in each
test setup so that
spherical aberration is corrected. All three separate interferometric tests are either ideal null tests, or near null tests. From the three sets of OPD data, surface figure errors on the rear and front surfaces of the test optic, h1(xi,yj) and h2(xi,yj), are calculated as well as the OPD error OPDI(xi,yj) introduced by
refractive index inhomogeneity in the substrate material. The rear surface is then corrected, generally using a computer-controlled
polishing machine, to reduce rear surface errors hi(xi,yj) to a manageably small level. The front convex surface of the test optic is then corrected to reduce surface figure error h2(xi,yj) to within some specified amount. The front mirror surface can be figured to any required accuracy in
spite of residual surface figure error in the rear surface, h1(xi,yj), and the effects of
refractive index inhomogeneity in the substrate material OPDI(xi,yj). The convex front surface is then coated with a reflective
coating to produce the finished convex mirror.