RE: [ATM] Testing the limits of ATM technology

["lou krajci" <loukrajci@comcast.net>]

>From: "Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>

>> See: http://www.aura-nio.noao.edu/book/ch4/4_5_C.pdf
>>
>> Especially look at fig. 5 (three bipods for lateral support) and fig. 10
>> (view of bipods and axial support system)...and note that they are
>> supporting a 1 meter piece of glass that's about 50mm thick on only 18
axial
>> supports. Looks promising.

>From what I see they still have to figure out how to connect these bipods
to
>the support frame. This seems a challenge, since you don't want to excert
>any normal forces to the mirror, which should be taken by the 18 points
>cell.

In the case of the professional scopes...actuators and computer control
(with load cells to provide feedback on the amount of force applied by the
actuators) provide just the right amount of normal (axial) force to the
mirror so that the bipods provide zero normal (axial) force.  This way the
bipods only provide lateral forces.  I'm not aware that there are
requirements for special/critical connection schemes for the bipods.  Just
make sure they are mounted in position, and bonded to the mirror, so that
they don't put any radial stresses on the mirror.

In our case I propose using levers, in a manner similar to astatic mirror
supports.  See:
http://astrosurf.com/altaz/astatic.htm and note the section of the web page
near the very end...the section titled "Adjustments".  It describes a 12
point support.  Three points are fixed/hard points that define the mirror's
plane/collimation.  The other 9 points provide the axial supporting force
(due to levers, in the astatic design scheme) so that the three fixed/hard
points only provide enough force to keep the mirror constrained in one
plane.  In my proposal bipods are used instead of the post-shaped fixed/hard
points.  Again, I'm not aware that there are requirements for
special/critical connection schemes for the bipods...but if this is a
critical issue...please tell me what makes it critical.

One thing I think you should do is choose a cross section of the bipod
members that makes them stiff in one direction (which is tangential to the
mirror's circumference), but not stiff in the perpendicular direction (which
is parallel to the mirror's radius that runs through the bonding point of
the bipod to the mirror's rear surface).  Figure 5 in
http://www.aura-nio.noao.edu/book/ch4/4_5_C.pdf shows this well...the bipods
are stiff in some directions, but not in others.  That is by design, and it
allows the bipods to apply only the desired forces to the mirror, as shown
in fig. 6.  The bipods are not stiff in other directions, so really can't
apply forces to the mirror in other directions.  Fig. 10 shows the bipods
connected to the rear of the mirror, and the back plate/frame of the mirror
cell.  It does not look like they are using any special/critical method of
attachment.

I hope this helps.

Tom Krajci
Albuquerque, New Mexico

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