RE: ATM Epoxy spin-cast mirrors: comments for experimenters

["Andersen, Clark R." <clanders@utmb.edu>]

>IMHO spin casting to make an *accurate* paraboloid
>by spin cating would be anything but *simple*. Refer
>to the literature on how spinning mercury mirrors
>are made - it works but the anti vibration and 
>stability problems are formidable, not to mention
>viscosity effects. The 3m NASA Hg mirror (currently
>located here near Cloudcroft NM) had a spin cast
>epoxy surface, but this was made to minimize the
>quantity of mercury required and to reduce large
>amplitude 'slosh' modes. The spin bearings are typically 
>air bearings with servo controlled motors.

I would speculate that, as the epoxy progresses through a range of
viscosities from a liquid to a solid, any non-resonant vibration effects (of
sufficiently low amplitude) will damp-out and not affect the figure.  Also,
I'm really envisioning mirrors of under 0.5 meter diameter (amateur scale),
which simplifies elimination of vibrational problems.  Nevertheless, I still
anticipate the need of some sort of damping mechanism between motor and the
epoxy container.

>UV cured epoxy mught be a way to quickly solidify
>a mirror if the spin problem can be solved, perhaps
>with real-time figure testing to know when to 'freeze'
>the epoxy.
 
That's a really good idea, assuming that a UV cured epoxy is available with
reasonable properties.  I'm not that familiar with them.

Clark Andersen


>Andy Saulietis