From: Mel Bartels 
cc: "'ATM Mailing List'" 
Reply-To: Mel Bartels 


> I don't want to make this a "religious war".  I just want to
point out that
> there are indeed tradeoffs in using refocus on optimization, and I chose not
> to for the stated reasons:  (1a) Refocus-on cells require high construction
> precision to achieve that "additional" RMS rating and (1b)
are seemingly
> more sensitive to construction errors than non-refocus designs, (2) they
> have a different focal length at the zenith and horizion (albeit small), and
> (3) they tend to have less support at the edge of the mirror than
> non-refocus designs.


I've built two PLOP designed cells - for a 6 inch and for a 20 inch.  Both
work quite well and show no mirror figure deviation at high power in the
star test from their former well known patterns.  The 20 inch uses
optimized allowing for re-focus design with rather narrow triangles (it's a
9pt support).  I use simple hand tools and so forth, and regard my
construction as accurate but probably not machine precision (the triangles
and cell are made from wood).  The increased spacing to the edge supports
is a concern, but I've not run into problems so far.

Mel Bartels

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