The low frequency waves observed on optical inserts which cause color separation on image had been eliminated by ruling, a post diamond turning process, which Delta Optics had developed! The charts below illustrate profile found on diamond turned hardened steel inserts before ruling (~30 nm) amplitude and after ruling.
A ruled insert is produced by diamond turning a pair of inserts, one with concave and the other convex profile (of mirror image) and subsequently matched together on these surfaces and honed with each other by rotating them at a differential speed. Such action with appropriate pressure and lubricating setting, enable the reduction of the waviness on these surfaces to a minimum on the diamond cut surfaces.
The picture illustrates a couple of spherical (R 70mm) ruled-inserts with radius deviation of 0.015% and surface finish < 5nm. The size of the square insert is 10×10 mm whilst the round ones is 19 mm diameter.