What "Ceramic" was used in the telescope mirrors in Chili?

Hi Guest,

Could the "Ceramic" material be made of Al2 O3 fibres, this has a super low coiffeiciency of expansion/contraction, and I have worked with it to produce pistons for my stirling engines!

Spencer.

http://www.mellesgriot.com/pdf/CatalogX/X_04_17.pdf

Produced by Schott, not Corning and it is considered a glass-ceramic composite.

As is so often the case, I found the answer myself, after someone else provided it.

http://etoile.berkeley.edu/~jrg/MATERIALS/node9.html

Both responses were thorough and while they provided the answer I am disappointed.

I would enjoy forming my own ceramic disks using more conventional methods and was hoping that the material used in the ESO would support that. Alas, I can not use Zerodur.

I am puzzled by my mistaking Zerodur as a Corning Glass product. Corning did have a substrate with a very low CTE but I can not remember its name.

Thanks, both of you, for your support

L. J.

I believe your thinking of CerVit which was a product of Owens Corning.

- Dave

I don't recognize Cervit Dave. It appears I have been mistaken about this for a very long time.

Thanks

L. J.

I used Cervit many years ago to construct laser gyros. I was in the Corning plant and watched them make large mirrors out of Cervit. They would core drill large Cervit mirror blanks to lighten them. We purchased the 1.0" diameter cores from them to fabricate small optical components.

Guest wrote: "Corning's product is Macor"

I'll be calling Corning Monday to see how it's sold. If it's as easily machined as their tech sheet suggests, I may have found a way to skin this cat that was not apparent before.

They also provided the chemical composition. That was nice because it suggests I can approximate the recipe from a vendor and have them make a suitable mixture.

Great link!

Thanks!

L. J.

i would go to the website for that info but think it was similar to the university of arizona

site below Tuson az

it is the same material as used at the mt hopkins observitory i saw this on the science

channel

Zerodur is essentially a ceramic since it is a low expansion glass containing random grains formed by sintering. Manufacturer is Schott glas, Durea Pennsylvania.

About twenty-some years ago I was involved with "Star Wars", looking for ways of making large mirrors faster. We looked at lots of different materials, including several ceramics which had inclusions to modify the thermal expansioin. One candidate I recall was beta spodumene, which had been developed for heat exchangers and was thus immune to thermal stress. There was also an Air Force Materials Lab. publication (I gave mine away) which derived various measures of merit for candidate materials. There are, of course, different approaches. The obvious one is materials with a zero coefficient of expansion, but, for example, forged aluminum mirror blanks were a candidate. They would expand when heated, but the high thermal conductivity would make it easier to keep them at a constant temperature.

Hi L.J.,

Macor is not low in expansion but has other very exiting qualities.

The Corning material you are searching for is ULE-quartz-glass, a mixture of silica with 7% titania. Mixture is said to be not too uniform so as overall CTE is near 0.1ppm/K differences up to ? exist. ? depending on specs and price and volume.

Zerodur is made as a molten glass and later ceramised by heat treatment: small crystals growing inside a glass matrix and glass with changed composition remaining.

There is a cheaper substitute made in Russia - often used by amateur telescope makers - seems to be as good but I have no further details.

Another possibility is cordierite - the material used in exhaust catalytic converters to support the platinum.

If you want to make your own material: I had some plans but too many plans had to be postponed, so these too. What size is your intention? Lightweight a necessity? Near net shape or to be ground later? One piece or continuous production, how many per year? Which maximum temperature at production is suitable?

Would be an interesting product for the amateur-telescope community, that I had in my mind.

RHABE

Hi RHABE,

Could you please explain a bit more to me about "Cordierite"?

As a mineralogist I know everything about cordierite exept it's insulation properties;

Chem. Comp; (Mg,Fe)2 Al4 Si5 O18

Crys. Symetry; Orthorombic

Hard; 7-7.5

Colour; Grey, Blue, Blue-Violet

As this matrial is often used as the gemstone "Iolite" because of it's double-refraction properties (colour change depending on view).

So is the man-made equivalent of the same chemical compositian?

And of what form is it produced in?

Spencer.

Hi Scapolie,

the material I have seen (at a Corning factory nearby) is pure white (very fine powder) mixed with a non-disclosed binder (looks like toothpaste and has similar deformation characteristics) and is extruded into very delicate and thin walled objects.

Cross section is: outside circular to oval, inside orthogonal walls at 2 to 3 mm distance, thus formed channels later to be coated with ultrafine platinum black.

Extrusions are cut to length, then dried, then fired thus changing color to slight yellow. (I am using some as support at brazing small objects.)

Resulting material is said to be cordierite with expansion TC below 2ppm/K thus not being cracked by fast temperature variations.

Schott once announced also this material for technical purpose but I don't know if they really produce it.

I can ask a small ceramic company if you need more information.

Back to your original question, I am sure that the material is the same as you state, Mg/Fe not known to me but from color I assume almost no Fe. But very fine crystals and most likely a sintering aid added.

RHABE

Hi RHABE,

Many thanks for the information, I should imagine that you are right about the absence of Fe, or at least very little.

Best regards,

Spencer.