Re: [ATM] 9pt cell for 16.5 1"-thin quartz mirror?

[Mark Holm <mdholm@telerama.com>]

Hi William,

I think I got pretty close to the 9-point cell you describe with the 
Plop setup below.  I guessed a 2.5 inch diagonal, but that probably 
isn't terribly important because neither the highest nor lowest points 
in the deformation plot occur at the center.

Plop says this nine point cell will result in nearly 10 nm RMS and 42 nm 
P-V deformation.  This is more than one would like for a good mirror, 
but might be OK if the mirror really is only so so.  I would say, give 
the 9-point cell a try.  Do you know how to star test a telescope?  The 
reference on this is "Star Testing Astronomical Telescopes" by Harold 
Suiter.  You can buy it from Willman-Bell  http://www.willbell.com  
Willman-Bell is a reputable company specializing in books, etc. for the 
amateur astronomy market.

If the cell is the limiting factor, you should see evidence of a small 
amount of "trefoil" error, a.k.a. triangular astigmatism, in the star 
test.  If the predominant error is of some other type, such as spherical 
aberration, turned edge or the more ordinary right angle astigmatism, 
then something other than the cell is probably the limiting factor.

Actually, if the cell is the limiting factor, and the mirror and other 
optics are pretty good, you probably won't see this image degradation 
till you push the power up fairly high.  If you are mostly a "faint 
fuzzies" observer, you may do just fine with the 9-point.

If you need a cell and don't have the resources to do metal working 
yourself, I would not throw in the towel.  What you need is pretty 
simple.  If you can make a fairly clean sketch of what you want, with 
dimensions, you may be able to find a small machine shop to do the metal 
work.  Also, California is littered with amateur telescope makers.  We 
ought to be able to scare one up near you that would be willing to help out.

;9 point cell with angles fixed
var r_inner  63.5
var r_outer  177.8
diameter 419.1
thickness 25.4
density 2.22e-06
modulus 7400
poisson 0.17
focal-length 1885.95
n-mesh-rings 12
support-radii r_inner r_outer r_outer
rel-force 1.07 1 1
num-support 3 3 3
support-angle 0 30 90
basis-ring-size 3
basis-ring-min 0
obstruction-diam 63.5
part triangle 3 point 0 0 point 1 0 point 2 2

-- 
Mark Holm
mdholm@telerama.com


_______________________________________________
ATM mailing list http://www.atmlist.net/