To: atm{at}shore.net, ea8yu{at}arrl.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com




At 200mm diameter and 22.2 mm thickness with a plate glass mirror and an
appropriately sized diagonal, Plop says the P-V deformation from a 3-point
cell

will be about 1/42 wave and the RMS 1/190.  These are surface measurements.
The wavefront would be twice these numbers: 1/21 and 1/95.  If you can make
your mirror good to 1/20 wave P-V surface (doable, but not easy) the
corresponding RMS will be roughly 1/100 surface, 1/50 wavefront.  Using the
square root - sum-of-squares calculation predicts that the resulting
wavefront RMS (mirror + cell deformation) will be about 1/44 wave.

This is still a pretty good wavefront.

At 20 mm, the mirror deformation is starting to be more than one would want for

a fine planetary scope.  Time to go to a 6-point cell.

The Plop results tabulated on my web site
http://www.telerama.com/~mdholm/cells/index.html show that the six point
design
is good down to 12.5 mm thickness in 200 mm diameter.

This is of course only considering the back support issue.

Mark Holm
mdholm{at}telerama.com

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