[ATM] 4 point mirror cell

["Rod Brackenridge" <rjbr@lycos.com>]

Hi,

I am in the process of converting my 12 inch F5 Dob to Mel's scope drive system.  I want to create space inside the rocker box for the azimuth drive (similar to James Lerch's design for his 8 inch binoculars). 

To help create this space I would like to re-build the mirror cell.  It currently has a 9 point flotation cell which works OK as is but the cell sits rather high. I thought if I switched  to a 3 or 4 point cell sitting on metal crossbars (Krieg and Berry style) I could reduce its height and save some weight (by shortening the mirror box).  My mirror is 12 inches in diameter and 2 inches thick (pyrex).  The diagonal is 3 inches in diameter (too big really, one day I will buy a smaller one)

I have some questions:

Am I right in thinking a 4 point cell is adequate?

Mark Holm's site describes a 4 point cell for a plate glass mirror 2.125 inch thick.  I am wondering whether the cell specifications given for it would be OK for mine: the radius upon which the pivot bar sits is 0.357 and the radius on which the support points lie is 0.505.  Would that also be OK for a 12 inch pyrex mirror?

I would like to RTV the mirror to the cell if possible.  Would four blobs of silicone support the weight of this mirror?

Thanks,

Rod Brackenridge
Mount Martha
AUSTRALIA
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