Mirror Cell - # of points

Hi All,
In "The Dobsonian Telescope", the number of support points for a 17.5" x 2" mirror is 9 or 18. Which is the best? I know 18 will support better than 9, but is it overkill?
Thanks,
Tim

Tim, From an engineering point of view: In theory a object will only rest on three points. With that said, the mind-set behind more points is that the glass will deflect under it's own weight to fing more points at which to rest. At every point of contact stress will incure and will influence deflection. with more points added, less stress at each point and thus less deflection at each point. So now you have to play a balancing act. the more points you can rest on the better, however, you don't want your glass deflecting (bending) to acheive rest on these points. So build a mount (cell) that has three solid points of contact. From each of the three branch out with a somewhat flexable moment arm to more points. and so on and so on. This will allow the cell to conform to the glass shape and provide the three point rest. If you do this right, more is always better. Tim

A 17.5" x 2" mirror will sag a maximum of about 1/29th wave with a 9 point support system. (See "Mirror Support: 3 or 9 Points?", Sky & Telescope, Sep 94, pp. 84-87.) There should no need to go to an 18 point system.