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Bent glass
Checking out the slumped blanks, there appears to be about 1mm of astigmatism across the disk with the edges less (?!) slumped and a deep centre. The depth was even greater than I thought, and they are about f1.8.
With the uneven edge difficult to get them to seal properly for vacuum trials, but I'm sure they "could" work.
The trick seems to lie in the successful design of the steel slump mould, Adrian is only using a thin spun steel shell which I think is not sitting perfectly flat nor is 100% symetrical. Will continue......
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