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How would you keep such a thin mirror from warping severely? A 45-point mirror cell?!

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You'd be surprised at how good an 18 point cell is even for relatively thin mirrors. The thing that usually bites you on such mirrors, though, is the edge support - anything that exerts a force perpendicular to the surface (even if it's just through some stiction, or because attachment points for a sling aren't *exactly* in the correct plane) will warp the mirror big time.

http://www.cruxis.com/scope/mirroredgecalculator.htm

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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)

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