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@Stefan and Rod
Thank you for your interest and I appreciate your advice.
I finished today with 800 grit grinding and await the 1200 grit.
I calibrated the spherometer with its legs at diameter of 70mm on a glass optical flat.
I did measure how flat my 350mm disks are. At 0, 90, 180 and 240deg and at each coordinate at the centre, 70mm, and 140mm and found two disks consistent at each measurement point to be concave by 0.0003”. the third disk is concave by 0.00025 (half a graduation on my spherometer). Which means nothing other than that they are more consistent overall and nowhere near flat-. I am sure the concavity was produced by the way I ground. Tomorrow I will try and increase the stroke and see what happens after one cycle. Maybe next week I will get the 1200 grit and try polishing one disk with cerium oxide. Want to see what it looks like with the interferometer.. I might make a larger spherometer and get it calibrated at the local machine shop. They should have a surface plate.
Thank you Rod for the offer of a blank. At the moment I want to see how far I can go. Like a dog with a bone.
Still need to find someone uphere with a test setup, ronchi and focault. Tried the local Port Macquarie club, but no joy.
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