The primary is made from 25mm Schott Borofloat. It is supposed to be 'flat', so we should not need to grind the back flat. But, we decided to grind the back's flat anyway, just to be safe.

After a few minutes, we could see that the backs of each disc was not flat at all..!! One had the typical hour glass shape that we have sen in float plate glass, the other was not as bad.

It took an hour, using 240 Carbo, to grind out the 'astigmatic' dics so that each was an even smooth surface. Some images below:-