Re: [ATM] Fused Silica 20" blank

[Mark Holm <mdholm@telerama.com>]

William Marriott wrote:
> Hello everyone!
> 
> So, I have comited myself to purchasing the 20" fused
> silica blank, even though it is a bit different than
> what I had thought.
> 
> For some reason it has been ground down .250" to
> remove some surface imperfections that would not of
> bothered me.
> It also has a central hole that tapers from 22mm-44mm.
> 
> It will arrive on Thursday afternoon!
> 
> I have been playing around with PLOP, and have found
> the following: 
> Diameter: 508mm
> thickness: 38mm
> central hole: average at 30mm
> central obstruction: 4.25"

> 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.
Definitely.  I have been using a cut off of 2.5e-06 as the upper limit,
and have been criticized by some for being too conservative.

>  PV seems to be too close to not good
> enough,
7.13e-06 is about 1/70 wave.  You can comfortably go up to 1/40 wave,
12.5e-6 mm.  For the sorts of deformation you typically see in Plop, the
RMS and P-V actually track fairly closely.

You could easily go down to a 12 point design using Tom Krajci's 12h layout.

The Plop file for your mirror is (note: I didn't remember your planned
f-ratio, so I used 5.  You can put in the correct value after you load
this file.)  Just copy and paste the following over whatever is in
Plop's "Edit as Text" tab.

;12 point cell using beams and triangles.
;Inner ring of 3, outer ring of 9...lends itself to 120 degree symmetry
;for final three collimation bolts.
;Similar performance to the 12 point cell using 4 inner and
;8 outer supports.
; http://elib.cs.berkeley.edu/~jonah/krajci12/
var r0  0.337135
var r1  0.756141
var r2  0.729675
var a1  22.1488
var p1  90 + a1
var m1  90 - a1
diameter 508
hole-diameter 30
thickness 38
density 2.22e-06
modulus 7400
poisson 0.17
f-ratio 5
n-mesh-rings 20
rel-support-radii r0 r1 r1 r2
num-support 3 3 3 3
support-angle 90 p1 m1 30
basis-ring-size 3
basis-ring-min 0
obstruction-diam 114.3
optimize r0 0.01
optimize r1 0.01
optimize r2 0.01
optimize a1 0.1
part triangle 3 point 0 0 point 1 0 point 2 0
part bar 3 point 3 0 part 0 0


If you run this 12-point design, you may need some help deciphering
Plop's dimensions.  Here is a web page to help.
http://www.atmsite.org/contrib/Holm/Plop_optimized_cells/12PointCells/examples/12h_drawings.html

-- 
Mark Holm
mdholm@telerama.com

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