Concrete Mirror?

[jamespels]

A meter scope is not unusable, an American trucker made his own 1.8m f6 recently, works fine, just not very portable. http://m.slashdot.org/story/193923

Just checked the sums on this (should have done this before) and found that even a 1m F4 mirror would only have a meniscus depth of 1.5cm so, agreed, the glass only need be thick enough to maintain its own strength. However, I would expect a thin (say, 40mm) mirror of this size to flex under its own weight so would expect maybe double this. Concrete would allow a mirror this thick near the centre, with ribs for decreased weight and thinning towards the edge. Admittedly, you could achieve the same thing in glass with a suitable furnace but concrete is much easier to mould and will be a lot cheaper - even something like fibrecrete that replaces the stones with organic fibres.

The porosity and anisotropic on hardness will kill you, you would be likely to get scratches all over the place that you'd never shift.

Can you explain what you mean by this please? Concrete CAN be very hard (9 Mohs is achievable, apparently, which is much harder than many glasses - plate glass appears to be between 4 and 5). By anisotropic I think you mean the hardness is directional, or inconsistent. Do you mean the difference between the mortar matrix and the stone? If so, I am curious how the very high shine is achieved on some floors and worksurfaces. Again, needs digging into.

I have heard about the possibility if molded mirrors which might enable low cost large mirrors to be made. Thermal expansion is likely to be an issue, though if it isn't too massive it might thermalise quite quickly.

From my own experience with fibreglass, there is too much flex to be able to finish the mirror sufficiently accurately. However, a few people have mentioned this now and I am no beginning to question my own limited experience and wonder if I dismissed composite structures too quickly.

Not sure where to take this now! Three glasses of wine into the evening so probably little point trying to work anything though now!! Thanks very much for the thoughts on this, though.

J.

[jamespels]

You would have an unwanted random interference pattern (light/dark) if it was slightly out but mostly I guess the image would be simply fuzzy due to the light not being focused properly. Depends how far out it is as to what effect you'd see really.

Makes sense - thanks!

[jamespels]

Starting to think they're might be something in this glass mirror thing - I have been going through quite a few options and not much compares to glass.

I have really struggled to find a decent coating that would work with concrete - potters' glaze is the best I can come up with but concrete won't take the 1000 degrees firing temperature. I either need a lower temp glaze or to make the mirror or if clay (earthenware, not fine bone China!). I now have visions of throwing a mirror on a wheel...!

Not convinced either way about composite mirrors - will investigate further if I get nowhere with concrete and porcelain.

[laser_jock99]

We have used sintered Silicon Carbide for light weight galvo mirrors. Strong, lightweight structures possible but very expensive.