[ATM] A interferometric study of plate glass mirror edge heating

[Dale Eason <atmpob@yahoo.com>]

I have been measure the effect of edge heating on thin
mirrors using an interferometer.  It is primarily of
interest to the glass pushers among us.  The mirror is
16 X 1 inch plate glass.  The mirror was supported in
a test stand on two pegs at the bottom of the mirror
and 3 points on the back.  For this series of tests
the pegs only touch the mirror at the front edge. 
That causes the lower surface to be higher than it
should be.  That is the subject of another study that
I’m doing.  So I will not talk about it here.

The ambient temperature was 63 deg F.  I heated the
edge of the mirror by placing the palm of my hand on
the right side edge for 1 minute.  This raised its
temperature to 71 deg.  I took interferograms in the
next 2 minutes.  There are two sets with 5 in each
set.  The images in each set were taken 5 seconds
apart.  The second set was started 1 minute after the
first.  At the end of 2 minutes the edge temperature
of the mirror had dropped to 66 deg.  There was no
forced air cooling.

The first image shows a wire frame of the cool mirror.
Although not very easy to see the light grey circles
define a surface error of 1/8 wave above and below the
zero point.
 
http://home.comcast.net/~doeason/heat/pegs_at_front_cool.gif

 This is an oblique view from the front. You can see
how the lower edge on each side is raised by resting
it on the pegs.

The next image is a wire frame of the heated mirror
taken from the middle of the data sets.

http://home.comcast.net/~doeason/heat/heated_on_right.gif.

The next images shows contour maps of each sample.
Time goes from left to right, top to bottom.  The last
contour at the bottom right is the cool mirror.  The
number in the upper left of each image is the measured
strehl ratio.  That should give you an idea of the
magnitude of the deformation.  The cool strehl is
.555.  

http://home.comcast.net/~doeason/heat/heated_then_cooled_2_min.jpg

The last image is the  surface profiles for each 
sample.  The radius through the largest deformation. 
You can see how each profile changes a little as the
mirror cools.  The thick black line is the cool
profile.  The low zone at 152 mm and the hill at the
edge are something I struggled with throughout the
polishing of this mirror.  I think it might be related
to the heating and cooling of the edge during working.
 Figuring was done using a 8 inch tool all on top. 
Cooling resulted from evaporation of the edge.  I
found the mirror would heat up 2 deg in the center and
only 1 deg at the edge during 5 minute figuring
sessions.  

http://home.comcast.net/~doeason/heat/profile_heated_on_right.gif


Now about that deformation caused by the lateral peg
support.  Oh, that is for another post.  You will just
have to wait.  Coming soon to an email group near you.

Dale Eason



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