Re: ATM "Frozen" liquid mirrors

[Brian Reynolds <reynolds@panix.com>]

Tom WARM wrote:
> 
>    ATM "Frozen" liquid mirrors:
>    
>    I tried making spincast epoxy "mirrors" as described in the SciAm 
>    article.  You may be interested in my experience.  I bought a gallon of 
>    epoxy used to repair car fenders for about $40 at Autozone.  The 
>    hardener comes with it.
>    

After seeing the Opticast homepage I've been thinking of spin casting
a mirror by cutting a 10 inch disk of plywood and using a strip of
wood veneer as a dam to contain the epoxy.  I had figured I would do
multiple pours to build up the surface gradually and avoid excessive
heating problems.  I'll have to go back and re-read the Scientific
American article again.  How would I aluminize (or otherwise coat) an
epoxy mirror?  I'm not sure that the epoxy blank would react well to a
vacuum.

>    I bought a cake cover about 12" in diameter and 6" high for a mold.  It 
>    was made of a flexible plastic, like tupperware.  I was lucky, because 
>    the epoxy didn't stick to it.
>    

You were also lucky the curing epoxy didn't melt the cake cover.  :)
The curing of epoxy (and similar resins) is an exothermic reaction (it
gives off heat).  In the past I have melted (or otherwise distorted)
plastic molds when casting parts with epoxy or resin.

>    It really 
>    stunk up the house with a sickening acrid smell.

WARNING: it is *REALLY* important to use epoxy in a well ventilated
area and to use a proper respirator or mask.  I know of a person who
had to give up his job (he ran a company that made fiberglass parts
for model and high power rocketry) when he became severely allergic to
epoxy.  He attributes this to not using a proper mask and ventilation.

>    The SciAm article had recommended making the epoxy cure as slowly as 
>    possible.  I experimented with the amount of hardener to use, using 
>    various percentages of the amount recommended on the epoxy can - 10%, 
>    25%, and 50%.  I found that the more hardener, the more it shrank.  50% 
>    left a 3/8" gap all the way around.  25% left a 1/8" gap.  The gap for 
>    10% was hardly noticeable, but had to be there, because the mirror 
>    dropped out easily.  I don't know what shape they shrank into, but it 
>    probably wasn't a perfect paraboloid.
>    

Fiddling with the ratio of resin to hardener will adversely effect the
strength of the epoxy.  This might not be an issue with mirrors, but
it may also effect the thermal expansion and stability of the cured
epoxy.

You might want to check out other epoxy brands/formulas in order to
get different cure times.  Good places to look for more information
would be people building models or making fiberglass parts in
rec.models.rockets, rec.models.rc.{air,misc,water}, and
rec.aviation.homebuilt.  Fibre Glast has a good homepage (see
<URL:http://www.fibreglast.com>) with information and a free catalog.

>    I thought maybe I could pour the epoxy into a real telescope mirror, and 
>    get a convex paraboloid, which I could then use as a mold to pour molten 
>    glass onto to make a concave mirror.  But I didn't know if epoxy stuck 
>    to glass.  So on one of my experiments I mixed the epoxy in a large 
>    Pyrex measuring cup.  The epoxy stuck to the Pyrex tightly, ruining the 
>    measuring cup.  I guess I won't making a mirror mold this way!
>    

Using mold release would solve this problem.  The measuring cup should
be coated with mold release before pouring in the epoxy.  I don't
think this would work with molten glass.  Epoxy distorts at relatively
low temperatures, and I'm not sure an epoxy (or fiberglass) mould
would withstand the heat of molten glass.

>    I have not been able to determine the coefficient of expansion of epoxy 
>    due to heat.  Does anyone know?  I also have not found an easy way to 
>    adjust the turntable to other speeds.  Any ideas?
>    

You should be able to get thermal properties from the manufacturers.
If you pick up a used turntable you could gut the electronics and add
a motor speed controller to adjust the turn table speed.

-- 
Brian Reynolds                  | "Humans explore the Universe with five
reynolds@panix.com              |   senses and call the adventure science."
http://www.panix.com/~reynolds/ | - Edwin P. Hubble