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Well, I have to say I have a 27-point cell in the 22" (that's a big hunk of BVC) and am currently running up a 27-point cell for a 20" for a friend. If any of those 27 points varies, then each millimetre is worth 1/100 wave on the mirror. You can see it would be easy to ruin a good figure rather quickly here - but as you say, smaller mirrors = less required accuracy.
Arthur
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