Re: ATM "Frozen" liquid mirrors

["Ken Carter" <kencar@worldnet.att.net>]

Peter,
  You and I have been talking about this and I think I may have part of a
solution for the vibration issue, when spin casting.
  What I had in mind was...
        - Solid platform
        - Oversized container with water in it.
        - Attached to the bottom of the container would be a floating bearing
positioned in the center.
        - Riding in the bearing would be the center shaft of the mold barge for
lack of a better name. The barge would be round. This barge would be able
to float on the water surface and be kept stable in position by the
bearing.
        - On the outside edges of the barge would be fins and LED's evenly spaced.
 The LEDs are for accuratly determining the rotational speed of the barge
and the fins are to drive the barge.
        - Positioned in the center of the barge is the mold for the epoxy
composite.
  The spinning of the barge would be by light air jet. Little air pressure
would be required to maintain a given speed.  The air presure would be
regulated to maintain the proper rotation of the barge. A light sensor
would pickup the pulses of light as they passed and determine the
rotational speed of the barge. A circut would have to be designed to
process the pulses and adjust the air pressure to maintain the barge
rotation at the exact speed you need to result in the focal lenght desired.
  Another way of removing the bubbles from the epoxy is to apply heat to
the surface of the epoxy.  I've done very well with a blast from a blow
torch. I have yet to confirm that is would also be true of a heat lamp. 
You don't have to keep it on the surface and each application will result
in the removal of bubbles for about five minutes.  It doesn't take long to
remove all of them.
Ken...

I don't mind being wrong, so long as I can learn.
Visit my home Page: http://kencar.simplenet.com



----------
> From: CHEN@uit.gsfc.nasa.gov
> To: atm@shore.net
> Subject: ATM "Frozen" liquid mirrors
> Date: Thursday, February 20, 1997 8:28 PM
> 
> Hi Folks:
>       There seems to be some interest in this subject, so I'll
> get on the soapbox and give a quick and brief summary on
> spincasting epoxy mirrors.  My purpose is to provide a little
> history and background, plus what I think are the potentials and
> challenges of this technique.  Then I'll sit back and hopefully
> we can toss some ideas around.
>     Spincasting is based on the idea that the surface of a
> liquid, in uniform  rotation about a vertical axis and under
> gravity, assumes a parabolic shape.  The shape of the parabola,
> and hence the focal length, is a function of the rotational
> speed.  The size, of course, is that of the container.  Therefore
> if the liquid can be gradually and uniformly turned into a solid
> while rotating, then one can generate parabolas of arbitrary
> sizes and focal lengths.  As someone has pointed out (sorry, I
> lost you post), this is a very nifty way to generate off-axis
> parabolas.  
>        The power of spincasting is tremendous. Most of us know of
> and are awed by the very large mirrors in use or currently being
> produced by Dr. Roger Angel and colleagues at the U. of Arizona's
> Mirror Lab.      
>     Spincasting epoxy mirrors uses a chemical reaction to
> solidify the liquid.  Epoxy is usually chosen because it is easy
> to handle, is quite viscous, gives a very smooth surface, and
> cures evenly.  (Freezing water won't do.  The ice forms from the
> air/water interface and expands downward, causing a bad figure
> and very rough texture).  The material is cheap.  A quart of
> Shell EPON 828, the preferred medium, costs somewhere between
> $15-$20 from an industrial plastics supply shop.    
>      The basic process goes as follows: 
> 1. mix the epoxy with hardening agent according to manufacturer's
> recommendation.  
> 2.  Place mix in an airtight container.  Use vacuum pump to get
> out gas bubbles.
> 3.  Pour mix into rotating container.  Spin overnight until epoxy
> hardens.
> 4.  Take off the hardened epoxy.  Put it in an oven and cure at
> low heat.
> 5.  Vacuum coat with aluminum, or silver, or whatever.
>      The process is so simple.  So what are the problems?
>     To begin with, the rotational axis must be vertical (to
> within a few arcminutes, if I recall correctly).  This is not a
> serious problem since a nonvertical axis will cause a small
> vortex to form at the center.  On can adjust the tilt of the
> apparatus until the vortex disappears.  Of course the chemical
> mix must be uniform and be free of bubbles and dust.  Takes work,
> but not impossible.     
>     The killer problem, which have thwarted previous efforts to
> make diffraction limited (oops, I mean Rayleigh criterion)
> astronomical mirrors, is uniform rotation.  If the speed is not
> held constant, or if the apparatus vibrates, the resulting
> surface is covered by ripples.  For those with access to a good
> reference library, there is a picture of a 36" f1 mirror made by
> this process in Modern Plastics, August 1957, page 116. The
> author stated that, despite all efforts, 'the mirrors are not
> perfect...ripples 40 millionths of an inch in height are visible
> to the naked eye".
>      Well, it has been forty years.  Many new technolgies are now
> available to solve the problem.  Lasers, quartz oscillator
> controlled motor drives, etc.  And we know the problem is
> solvable.  After all, Dr. Borra and colleagues at the U. of Laval
> (& UBC, UWO, and others) have successfully built and observed
> with large diffraction limited rotating mercury mirrors.  And
> bear in mind that mercury is very much more sensitive to
> vibration (it's not called quicksilver for nothing!).
>     So here's the challenge.  Does anyone know how to make a
> turntable that is vibration free, cheaply?  The liquid mercury
> mirror telescopes use air bearings which are too expensive ($8-
> 15K), at least for me.  Since epoxy is lighter than mercury and
> more viscous (damps out vibrations better), it should be possible
> to use a less than perfectly quiet apparatus.  A high grade ball
> bearing motor, perhaps?  Come to think of it, something off of an
> old jet engine that is not smashed up might be a good candidate.
> Hmmmm.  Any suggestions?
>     For those who are going to try spincasting, please be sure to
> have good ventilation, stay away from food areas, use disposable
> gloves and containers, and read the MSDS (material safety data
> sheets) for the chemicals.  And have fun.         
>     
> 
> Peter C. Chen
> Astronomer, optics, detectors, and dreamer
> chen@uit.gsfc.nasa.gov
> homepage http://snoopy.gsfc.nasa.gov/~lunartel/lunar1.html