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topic 59541

Making a telescope mirror by plating onto carbon fiber backing



February 28, 2014

I recently stumbled across astronomy again and have been drooling over mirrors and larger telescopes. While I understand why metal mirrors are no longer commonly used, I've been wondering if anyone here can think of a reason a high finish metal mirror couldn't be produced by electropolishing.

So that's the general idea, here's a bit more detail.
I'm thinking of a fiberglass or carbon fiber backing for the general form and shape. Most resins I know of will take a reasonably good polish, so it seems that it would be an excellent base to plate on top of. This would provide a reasonable parabolic surface much lighter than glass.

The magic bit would be doing a very high finish nickel plate over a copper strike. The nickel would have to be thick enough to stand up to polishing (to correct any defects in the optics), but that doesn't seem to be that big a difficulty.

So does anyone here think this sounds feasible?

Marc Banks
Blacksmith - Lenoir, North Carolina USA


February 2014

A. Hi Marc. Carbon fiber has a good (low) coefficient of thermal expansion (unlike some plastics), but it's probably directional depending on what the backing looks like. But it's probably not a big issue for an amateur telescope unless it's done really strangely like all the fibers running the same direction.

wikipedia
Visible Spectrum


But nickel will not be a satisfactory top layer, both because of tarnish and poor reflectivity. I think rhodium plating on top of the nickel would work well as it has very good reflectivity across the visible spectrum. There are some good graphs of reflectivity at
http://www.kruschwitz.com/HR's.htm

Luck and Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


February 28, 2014

A. Mirrors have been made from carbon fiber materials. There's a story here worth telling. It was back in the late 70's when a group at a large aerospace company built a very large mirror to reflect high power lasers, with funding from DARPA. Because of the laser power, the mirror had to be water cooled. Mirror came out perfect, so they set up a demonstration for DARPA. With the DARPA people in attendance they turned on the water cooling. As they were setting up for the laser tests water started forming on the surface of the mirror. It was coming right through the carbon fiber material. DARPA was not amused.

jim treglio portrait
Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover - San Diego, California


March 2, 2014

Q. Excellent call on using Rhodium as the final plate Ted. That should provide a much better finish than the nickel in the visible spectrum.
Honestly though, after looking at the graphs and charts there I was half tempted to take a nudge towards silver or aluminum due to their even higher reflectance, even with the oxidation issues that would come with them, it *might* be worth it.

To save on cost I think I'll just prototype the whole thing with nickel.

Jim,
I'm not quite following your point there regarding my fiddling. If there is a cautionary tale in there that I need to be aware of you'll need to beat me over the head with it for me to see it. Are you perhaps referring to water intrusion/permeability?

Oh and credit needs to be made due to the paper that made me wonder if I could do this:

S. Thompson, D. Brooks, and A. Doel, "A nickel-carbon-fibre composite for large adaptive mirrors: fabrication methods and properties," Opt. Express 16, 1321-1330 (2008).

If you do a web search for the paper's title you can find the full text article for free.

Marc Banks [returning]
Blacksmith - Lenoir, North Carolina USA


March 3, 2014

thumbs up signNot a cautionary tale, just an amusing anecdote.

jim treglio portrait
Jim Treglio - scwineryreview.com
PVD Consultant & Wine Lover - San Diego, California

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