Quote:
Originally Posted by Stefan Buda
Werner,
You don't need an optical flat for setting your spherometer.
Use your spherometer as a comparator: If you get the same reading on all three surfaces, then you are well within what is achievable with your spherometer. In fact you can grind a good flat using only two discs but you need three later on when doing the figuring.
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Think about it until you understand what I said, otherwise you may be wasting your time.
If you grind two discs against each other, you can get the same reading with the spherometer, on both discs, only if both surfaces are flat. We are talking about matched surfaces. If one is slightly convex the other one will be slightly concave and your spherometer will show the difference. That means you can detect an error that is twice smaller than what you could detect by comparing one disc with an optical flat as reference.