[ATM] Lateral support position experiment

[Dale Eason <atmpob@yahoo.com>]

Before I got David Davis's mirrors I had started
measuring the effect of lateral support position on my
16x1 inch 20 lbs. plate glass mirror.

That is I changed where lateral supports touched the
mirror on its thickness from the front edge to the
back edge.  

I think Nils Olaf Carlin did some experiments with
PLOP along these lines.

My mirror rests on pegs 90 deg apart at the bottom of
the mirror.  I adjusted the supports so that only a
tiny part of the peg touches the mirror.  The support
was like resting it on a knife edge parallel to the
face of the mirror at those two points.

First I tilted the mirror cell back at 45 deg.   By
resting it back at 45 deg (the limit of my test setup)
I'm trying to limit deformation caused by the lateral
supports.  Next I tilted the cell only 20 deg.  I
would like to have used 0 but the mirrror could not be
balanced that way because the supports would not be at
the center of gravity of the thickness.


Here is the resulting contour maps.

http://home.comcast.net/~doeason/lateral_test/contour_lateral_front_to_back.jpg

The images are:

Top left.  Mirror rested back at 45 deg and lateral
support I don't know where exactly but it is using my
normal mirror support system.  This is the control
image.  Strehl .809. 

Top right. Mirror tilted back 20 deg lateral support
in the middle of the mirror.  Not necessarily at the
COG but close.  This is much like the first but a
better strehl of .862.  NOTE TO SELF: find out why
this is better than my normal support.

Bottom left.  Mirror tilted back 20 deg lateral
support at back edge of mirror.  Bottom edge of mirror
is pulled down more than when support is in the
middle.  Strehl is .824 and is closer to my normal
support numbers.  Perhaps my pegs slant downward from
the mirror weight and they only touch at the back of
the mirror.

Bottom right.  Mirror tilted back 20 deg lateral
support at front edge.  Oh, big time difference. 
Front edge of mirror is pushed up a lot. Strehl drops
to .694!  From other graphs of this data I can see
that the edge raised .375 waves.

Bottom line is lateral support matters as Nils Olaf
Carlin has pointed out.  I also saw similar results on
Steve Koehlers 18 x 2 mirror when we were trying to
position the sling last year.  I did not measure it
then.

Dale Eason

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