Ronchi Pattern
The initial aim is to get a perfectly spherical surface, in which case you’re looking for a diffraction pattern of vertical lines.
Thereafter it’s a small matter of a little more polishing to obtain a parabolic surface, where the diffraction pattern is shown in this picture. (The picture comes from ‘Ronchi for Windows’)
This test allows a parabola to be formed which is accurate to with 1/8th the wavelength of light (approx 70-80nm).
Once it has been completely polished out, tested and found to be a good parabola, the mirror is then ready for silvering which, fortunately, wasn’t a DIY job.