30+inch+F3+Grinding+the+back+flat

Grinding the back flat was done with an 18 inch flat made of plate glass. This had been used to grind a number of mirror backs in the past and was tested to be flat to less the 2 microns PV.

15 micro microgrit was used and the pencil test to determine the surface form of the back of the blank. Many blanks I have worked before (not from Reginato) showed a very astigmatic shape (hour glass) that would take a long time to remove. The Reginato blanks do not show this and tend to be very slightly irregular and do not take much work to grind flat. The surface are diamond generated.

A spherometer was also used to check the surface over a number of areas. No deviation could be detected. Grinding was completed until the pencil marks faded evenly.