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[ATM] Re: a few spincasting questions
BerichtDear Mr. Treur:
You asked several pertinent questions. I am therefore cc'ing the reply to the atm list.
>1: do epoxies as fluid as water exist?
>2: and with longer potlife (how long??)
>3: do you know any (worldwide) companies where I could obtain them? (I am living in Europe)
In my Stellafane presentation I mentioned the work of Lindblom and Sandberg of Finland who had reported the fabrication of a 1 meter parabolic mirror by spincasting epoxy. They acheived an optical surface of 1/5 wave peak-to-peak at 633 nm. Epoxy used was Ciba-Geigy Harz XB2841, hardener XB 2769. (Lindblom, P. and Sandberg, B. 'A Method for producing concave paraboloidal mirrors', Physica Scripta 21, 828-830, 1980).
>4: concerning possibilities to polish an epoxy-surface: isn't this just a matter of finding an epoxy as hard as possible??
what kind of epoxy should be considered?
The key is to find epoxy systems that do not have fillers. The fillers, while they harden the epoxy, usually also gives the resin undesirable surface textures. Rough surfaces scatter light and are to be avoided in telescope mirrors.
Polishing of the epoxy surface does not have to be mechanical (i.e abrasives on glass). Acrylic plastic, for example, is routinely smoothed and polished by passing a flame over it. I can imagine that epoxy surfaces can be polished by non-traditional, non-mechanical means such as solvents, flames, plasmas, ferro-fluids, magento-rheological fluids, laminar fluid flows, etc. The atm community, with its vast store of knowledge and experience, can surely work up something.
>5: I can imagine that temperature has a degrading influence on an epoxy-layer.Is that the reason you use layers of carbon fibre (To reduce the temperature-effects)???
The layers of carbon fiber are to provide a container (or substrate) that is stiff enough to maintain the shape of the epoxy surface. Epoxy resin by itself is too soft and dense to maintain optical figure (think of tree sap).
>6: In case of irregularities on the spincasted surface (bubbles, micro-scratches), my plan is to fill these with a layer of "filling primer" (used in the carpaint industries. I use a professional filler; Spies&Hecker 2 compound grundfüller. Do you think there is any chance to succeed?? Or do you think this layer will reduce the possibility to polish???
Interesting idea. It's worth a try. Let us know what you find out.
>7: you mentioned a silvering -kit. I have not gained information yet, but is this done by the regular Vacuum-Silver-boiling-process? Or is "your" process easier?
The silvering kit I use is from Peacock Laboratories in Philadelphia, Pennsylvania, USA. It is very easy to use.
a. You put the uncoated mirror in a 'sensitizing bath' for a minute or so
b. rinse it in distilled or deionized water
c. mix in equal parts of the other three solutions and let sit for a couple of minutes
d. rinse in distilled water and gently blow dry.
The process is like developing film, only faster and easier because it is done in light. No heating or vacuum is involved.
> 8: is using cermaic coatings any oiptions (a lot of heat is involved)
To get a good shape and a smooth surface by spincasting, you will need an epoxy system that cures slowly at room temperature. Applying a ceramic coating at high temperature will likely destroy the shape of the resin surface. The carbon fiber laminate substrate, if it is not a high temperature system that has been cured in an autoclave, would likely distort (or be destroyed) as well.
>9: I read an internet-article about using a Ni-coating on shape memory composite mirrors. Does this have advantages?
I am not aware that shape memeory metal (Ni-Ti) alloys can be used to make optical mirrors. I would appreciate a reference to the article.
Hope this helps. Have fun.
Peter C. Chen
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