From: Mark Holm 
To: amul49{at}eth.net, atm{at}shore.net
Reply-To: Mark Holm 


Aditya:

I haven't forgotten my promise to calculate those cells for you.

OD 200mm
Thickness 15mm
Focal Length 1400mm
Diagonal Minor Axis 40mm
Material Plate Glass

A 3-point cell gives 1/80 wave RMS, not good enough.

A 4-point cell gives 1/202 wave RMS with 1/44 wave PV.  This is probably
good enough.  You will have to have a very high quality mirror and diagonal
to justify needing better.  The 4-point cell dimensions are:

r1 = 37.529mm
d = 75.058mm
r2 = 53.074mm

See the 4-point section of my mirror cell web site
http://pong.telerama.com/~mdholm/atm/cells/index.html for the definitions
of r1,
d and r2.  Also, there is a diagram showing the layout of a 4-point cell to help
you understand the geometry.  There will be one bar, with a pivot in the
middle and a mirror support on each end.  There will also be two support
points like the ones in a 3-point cell.

A six point cell gives 1/622 wave RMS.  This is very definitely good enough
for any use.  The 6-point cell dimensions are:

r1 =  51.962
d = 60.000mm
r2 = 60mm

The 6-point cell has 3 bars.

For both the 4 and 6-point designs, the pivot points go exactly in the middle of
the bars.  The physical bars will be longer than the dimension d by roughly
25 mm to allow room for the mirror supports at each end.

Mark Holm
mdholm{at}telerama.com

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