Re: [ATM] 8" Diagonal- More info

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: Polaraligned <polaraligned@optonline.net>

>...any opinions out there as to thickness?  or support
>points?  Or should I just "wing" it because it does not
>matter.

Do you have any experience with using PLOP?

My ten inch, 1.3 inch thick paraboloidal mirror rests on only four
supports...no signs of image degradation from the mirror support scheme.

My sixteen inch, 1.5 inch thick paraboloidal mirror rests on only six
supports...no signs of image degradation from the mirror support scheme.

Your 8 x 11 oval...might be 1 inch, or 3/4 inch (have you spoken to any
opticians to see what thinness they are comfortable with?)...and probably
won't have more than about 6 or 9 support points.  But this is just me off
the cuff, not looking at PLOP.
...wait...just did a quick run on PLOP:

8 inch mirror, 18mm thick (about 3/4 inch).  Six points of support.  Pyrex.
Refocus OFF!

RMS error = 1.2e-6 mm.  Not bad!

Then I did an 11 inch mirror, same thickness, six points.

RMS error = 3.5e-6 mm.  Three times worse, but not too shabby.  Yours will
be somewhere in between since it's an ellipse, not a circle.

If you need to be more cautious, use 9 points of support.  For the 11 inch
disk it goes from 3.5 to 2.4e-6mm RMS.

I think a well designed 9 point support will do well for a 3/4 inch thick 8
inch diagonal.

What opticians are willing to step up to the plate to fabricate this thin
flat?

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/



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