Ronchi Tester
During the latter grinding and whole polishing process, frequent use is made of the Ronchi (pronounced “Wron-key”) tester.
The tester shines light through a slit. The mirror, which is placed at a distance of its ‘radius of curvature’ from the light source, reflects the light back through a diffraction grating.
The pattern in the diffraction grating shows how spherical the polishing process has made the mirror surface. It also shows any defects clearly.