Re: [ATM] Fused Silica 20" blank

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

hi Bill,

> If I back up and do 18 pts with variable angles I get:
>
> RMS = 1.12946e-06      P-V = 7.13222e-06
>
> Looking at the RMS value, it still looks like it is
> pretty good.  PV seems to be too close to not good
> enough, but it is noted that RMS should be used, not
> PV.

Rightfully so, I'd say.
>
> Am I seeing  this correctly?

Haven't checked with PLOP. But with my favorite rule-of-thumb, 20 nm RMS
lowers the Strehl ratio by 0.2, the lowering proportional to the RMS
squared, then 1.13 will lower by (1.13/20)^2*0.2=0.00063 = the deformation
will, if Plop and I are right, lower the diffraction peak intensity by about
1/15 of a percent ...give or take some error margin. I'd settle for that<G>
And I can't see that anyone should worry about 1/40 wave P-V <G>

If you want to go even a trifle lower, don't use variable angles in PLOP
(the 18 point cell is so symmetric that it doesn't matter), but instead
variable loads (=1 fixed for the outer ring, 1 but variable for the inner -
the thinner inner part of the mirror needs a little closer support points
carrying a little lower load - I don't expect this to make much difference
unless the mirror is very thin or fast).

But don't forget to be careful with the edge support - it is likely by far
the more critical part....

best of luck,

Nils Olof



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