Re: [ATM] rubber pad support of ultra thin mirror interferogramanalysis

[hermit <hermit@outofoptions.org>]

This test is only valid for your slumped mirror and the drawer liner.   
Did you measure the drawer liner or are you just guessing?  The stuff I 
have is closer to a 16 th.  Obviously if the back does not fully conform 
to the shape of the mirror, this test was set up to fail before it 
started.  Try this with a flat backed mirror with a flat surface.   Or 
get something that conforms to the shape of your slumped mirror.

Ken Lowther

Dale Eason wrote:

>Ok for all of you who think a 3/4 in piece of plywood
>and a foam pad will support a very thin mirror here is
>what I have found for one example.
>
>I used David Davis's 16 X 3/8 inch slumped mirror. 
>>From previous analysis I know about what the surface
>is at least to 1/4 wave PV surface error.  Thus I can
>place that mirror on other supports, measure the new
>surface deformation and then subtract the old mirror
>surface leaving only the stand induced error.
>
>I used my mirror test stand that is made out of 3/4
>thick plywood.  It has a hole in the center 4.75
>inches in diameter.  I use it to cool my mirror after
>polishing.  The hole probably helps because the mirror
>is slumped so the center is not pushed up too high by
>the stand.  Over that surface I placed a piece of
>non-skid drawer liner.  It is about 1/8 inch rubber.
>It is made up of lots of little rubber beads fused
>together to form a mat.
>
>I tilted the stand back at 45 deg.  I used the tape
>lateral support discussed earlier.
>
>The box in the image is 1 wave high.  This is
>wavefront error.  So the surface error would be 1/2
>this.  Remember we would like the wavefront error to
>be 1/4 wave or better.
>
>http://home.comcast.net/~doeason/DDavis/draw_liner.jpg
>
>The very edge part that turns sharply is an artifact
>caused by diffraction at the edge of the mirror.  It
>is not the real surface.  But about 5 lines in can be
>believed.  The predominate error is astigmatism of
>about .75 wave PV on the wavefront.
>
>The rms wavefront error is 1/5 wave and the strehl of
>the surface is .252.  This is what this support would
>do to a perfect 16 x 3/8 mirror.  Most would consider
>that unacceptable.
>
>Dale Eason
>
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