You really don't need a 9 point cell for the mirror unless the last of the dobs used a much thinner piece of glass for the mirror.
A regular 3 point cell will work just fine and will be distorting the glass a lot less than the probable errors in the mirror surface are by a huge margin.
Unless you have had the mirror tested and found it to be within a `/30th of a wave and want to insure that you are still within a1/37th of a wave when the mirror is in the cell, anything more than 3 points is unneeded.
When you mount the mirror the wisdom today is to put the mounting points at about the 50% zone rather than the old 70% zone as this makes the mirror, when it deforms, a longer radius parabola rather than a different figure. Several guys put together a program called PLOP that does finite element anlysis on mirrors and they found out that the conventional wisdom of where to put the mounting points was wrong.
Sorry to be so late replying but I just got out of the hospital

• Conventional wisdom? - rick_crockett - April 19, 2007 - 08:00 UTC