[ATM] Re: RTV, You Will Comply

[sho at surfnetUSA.com (Steve Houlihan)]

Hi Jeff,

Sorry I called you John in an earlier post.

Your last paragraph has me confused.

"Thus, a 1/4 inch steel backplate should be stiff enough to support a 16
inch mirror on 9 RTV blobs, with no wiffle tree. Then again, a 9 point
cell is easy enough to make with just RTV, so why bother?"

I'm beginning to believe that RTV and a flat plate can make a good cell, but
what do you mean by "a 9 point cell is easy enough to make with just RTV".
How is this different from the 1/4 inch steel plate with 9 RTV blobs?

Then the next obvious question is......"How do you support the 1/4 inch
steel plate?    It never ends!!!

Steve Houlihan
Santa Cruz, California

sho@SurfnetUSA.com
http://www.surfnetusa.com/sho


----- Original Message -----
From: "Jeff Anderson-Lee" <jonah@eecs.berkeley.edu>
To: "ATM" <atm@atmlist.net>
Sent: Sunday, February 15, 2004 12:41 AM
Subject: Re: [ATM] Re: RTV, You Will Comply


> I wrote:
>
> > F = EA(dh)/H  or E = FH/A(dh)
> >
> > (7.2 lb) *  (0.125 in)  / (0.82^2/4/pi in^2) / 0.004 in  = 427 lb/in^2
> > 27131.14
> >
> > That makes a *big* difference on the viability of RTV on a stiff plate
> > backing.  I'm a little tired tonight, so perhaps another time.
>
> Can't get to sleep so...
>
> Back of the (Excel) envelope calculations for a 0.25 in steel backplate
> and a 16 x 1+5/8 inch f/4 pyrex mirror:
>
> Assuming 9 RTV pads 3/16 inch thick by 3/4in diameter each place along
> the flex contour lines of the backplate when supporting the backplate by
> three collimation points.  The P-V for the backplate is 6.35e-4 mm, so
> assuming that we can reduce that difference by a factor of five by
> following the contour lines, we have a displacement threshold of 1.27e-4
> mm.  Using an elasticity of 450 psi, a 1.27e-4 mm displacement of the
> RTV means a relative force difference of  0.135 lbf.  Since the average
> force on each mirror support point is 0.929 lbf, that works out to less
> than a 15% variance in forces.  However, the 9 point support pattern
> should tolerate up to a 25% force difference, so we are within spec.
>
> Thus, a 1/4 inch steel backplate should be stiff enough to support a 16
> inch mirror on 9 RTV blobs, with no wiffle tree. Then again, a 9 point
> cell is easy enough to make with just RTV, so why bother?
>
> Jeff Anderson-Lee
>
> --
> ;Plop file for 9 point mirror support:
> ;9 point support for use with steel backplate
> var r_inner  0.351279
> var r_outer  0.727273
> diameter 406.4
> thickness 41.275
> density 2.23e-06
> modulus 6400
> poisson 0.2
> f-ratio 4
> n-mesh-rings 12
> rel-support-radii r_inner r_outer r_outer
> num-support 3 3 3
> support-angle 0 30 90
> basis-ring-size 3
> basis-ring-min 0
> obstruction-diam 0
> optimize r_inner 0.01
> optimize r_outer 0.01
>
>
> ;Plop file for steel backplate
> ;9 point mirror support on 0.25in steel backplate w/ 3 collimation points
> var r_inner  71.3344
> var r_outer  147.782
> var r_supp  122
> var r_supp0  109.1
> diameter 321
> hole-diameter 50.8
> thickness 6.35
> density 8.03e-06
> modulus 193100
> poisson 0.29
> sagitta 0
> n-mesh-rings 20
> support-radii r_inner r_outer r_outer r_supp
> rel-force -0.929 -0.929 -0.929 5.41115
> num-support 3 3 3 3
> support-angle 0 30 90 0
> basis-ring-size 3
> basis-ring-min 0
> obstruction-diam 0
>
>
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