To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


> From: Ralph Seguin 

>...but what about using flex tubing with some sort
> of fluid (water?? :-) coolant?   Could be woven into
> the carpet, or just used in place of the carpet.
> I'm sure that tubing probably has more than its fair
> share of problems that I am not seeing.

If the mirror were metal, then there would be fewer problems, but glass is
a lousy conductor of heat.  Water pipes cool the area right around
them...but unless you move air around the distrubute the cooling
effect...you'll now have local cold spots that will further distort the
mirror figure due to temp. expansion/contraction effects.  So now you need
water cooling...*and* moving air.  I'll stick with just moving air for now.
 Simple, and no risk of leaks.

>...I don't see the need for
> ultra flat surfaces when you are using very flexible
> material like carpet....

There are flexible materials, and then there are flexible materials.

As I said in my last post...all those rubbing carpet bristles mean there is
friction present.  Friction implies stiction, which implies the carpet,
while it is flexible...will NOT return to the true equilibrium point
because of this friction.  There is hysterisis in this moving system.
Depending on what direction you apply forces, and then release them...the
carpet support will come to a different 'equilibrium' point every time.
That's not good.

Try this experiment.  Put a mirror (even your shaving mirror will
work...don't need an astronomical mirror for this test) on some carpet.
Shine a laser point onto it and project the dot a long way onto a wall or
screen.  Note the initial position of the laser spot.  Now, disturb the
mirror by pushing a bit on one side, and then release.  Did the laser spot
return to the same location on the wall?  Push on the mirror again, but
from a different location, and again release.  Where is the laser spot this
time?  I bet you'll see hysteresis in action.

This is why I am becoming a fan of compliant hinges.  See:
http://overton.tamu.edu/aset/krajci/cell-bearing.htm

Tom Krajci
Tashkent, Uzbekistan

Hysteresis \Hys`te*re"sis\, n. [NL., fr. Gr. ? to be behind, to
   lag.] (Physics)
   A lagging or retardation of the effect, when the forces
   acting upon a body are changed, as if from velocity or
   internal friction; a temporary resistance to change from a
   condition previously induced, observed in magnetism,
   thermoelectricity, etc., on reversal of polarity.

--- BBBS/NT v4.00 MP