Re: [ATM] 6-points cell voor 20 inch mirror?

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> According to PLOP a 6-points cell for a 20 inch diameter 2 inch thickness
> Pyrex mirror deforms 1/141 lambda RMS. That's a bit less then the 1/128
> lambda limit given by Lewis. I don't know of anyone actually using a
> 6-points cell for a 20 incher, though they are very easy to make. Is PLOP 
> to
> optimistic?

I use a 9pt on my 20 inch of same dimensions.  I also use contact points 
that are 1/4 inch or 5-6mm in size.  That's as low of points as I'd 
recommend anyone going to.  That's because at some orientations and mirror 
placements in the cell, on nights of perfect seeing at > 1000x I think I can 
see slightest deformation due to cell occasionally using out of focus star 
test.

Mel Bartels 

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