[ATM] 4-point cell for 12.5" x 2.00" Pyrex mirror

[Mark Holm <mdholm@telerama.com>]

Hi,

I am not at the computer that has the original message, so I don't have 
the original subject line, or author to respond to.  The question came 
in on this morning's digest.  I had time then to run a quick check to 
confirm that 4-points is sufficient for this mirror, but not enough to 
copy out the full design.  Now I have it.

Mirror:
12.5 inch x 2.00 inch Pyrex, f/5, with 2.14" diagonal

The 4-point cell, optimized by Plop gives:
1.91 nm RMS residual deformation.  This is plenty low enough.
10.6 nm P-V residual deformation.  Also low.
Focus shift -0.018 mm.  No problem for visual observing,  Imaging 
requires more thought.

The four support points lie on a radius of 0.5081r, or 80.66 mm, 3.176"
The bar length (support center to support center) is 114.08 mm, 4.491"
Your actual physical bar will need to be longer to accommodate the 
support pads.
The bar pivots at its center, 57.04 mm, 2.246" from the support centers.
The bar pivot is located 57.04 mm 2.246" from the mirror center.  The 
bar is perpendicular to the radius from the mirror center to the bar center.

The four mirror support pads are spaced at 90 degree intervals, measured 
from the mirror center.  The bar pivot, naturally, is 45 degrees from 
the two pivots it supports.

Don't be concerned if your diagonal is larger than this.  The design 
won't change enough to matter.  If it is smaller, or if your f-ratio is 
significantly different, let me know.  Might as well run for your exact 
specifications while we are at it.

-- 
Mark Holm
mdholm@telerama.com


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