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Lawrence Sayre
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Reged: 10/16/04
Posts: 3706
Loc: N.E. Ohio
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Here is a great source I recently came across which offers some super info on the properties of many of the various types of glass which one might consider for use in the making of a telescope mirror. Everything from plate glass to the exotic zero expansion types is listed here, in detail:
http://www.minerals.sk.ca/atm_design/mirror_substrate.html
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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Lawrence,
Here is an opinion from R.F.Royce inre: the practicality of ULE materials for telescope mirrors:
http://rfroyce.com/pyrex.htm
Reading between the lines, where fused quartz is concerned, the only downside is price.
Here is another perspective on fused quartz:
http://www.fpi-protostar.com/quartz1.htm
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Lawrence Sayre
Post Laureate
Reged: 10/16/04
Posts: 3706
Loc: N.E. Ohio
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I'm fond of fused quartz myself. While I was at Cincinnati Gasket and Packaging (actually the Midwest's largest glass works, despite the very odd company name) getting my 30" Pyrex blank, they gave me a tour of their factory, and I saw a cube of pure fused quartz, about 14" on edge, being cut into nice slabs. Not an imperfection in it, and as perfectly clear as anything I've ever seen. I was drooling just watching the nearly zero expansion stuff being cut, and imagining the possibilities. Their various Blanchard machines, glass cutting machines, and polishing machines had me drooling also, but not like watching that block of fused quartz.....
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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gazerjim
Postmaster
Reged: 02/12/04
Posts: 7728
Loc: About where I thought I was......
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The catch I see in using the low-expansion exotics is that until the mirror reaches near ambient temperature, performance will probably suffer anyway due to the thermal boundary layer. (?)
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Jim Fisher
If you're not part of the solution, you're part of the precipitate.
Henry J. Tillman
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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Jim,
Yeah, but the image degradation from surface issues will be absent. In other words, fans might work faster.
The big advantage to fused quartz is that the surface can be polished much smoother than pyrex. Ergo, it should be easier to make a smooth surface. Add to that the fact its figure won't change as much while figuring due to thermal expansion, and it might be possible to make the mirror faster.
However, though secondary flats can be had in fused quartz, I know of no commonly used mirror maker who uses the material (though some very high end companies surely do).
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
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I thought one of the reasons that fused quartz can be so smooth as a flat is due to the cleavage - it forms square crystals, so a flat surface can be the side of a single crystal and approach atomic flatness (small pieces are used for atmoic force microscopy surfaces due to this). But that might not make them suitable for curved surfaces - the same crystal cleavage might tend to form "steps" along the curve....
Jarad
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 10960
Loc: Los Angeles
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Jarad,
One of the links I posted above shows that fused quartz can be smoother at the molecular level.
It's my guess that the spalling that shapes glass occurs on a smaller level when fused quartz is ground than it does with normal glass.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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gazerjim
Postmaster
Reged: 02/12/04
Posts: 7728
Loc: About where I thought I was......
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Have to admit, a large primary of fused quartz with a strehl of say, 0.99, would be a sweet piece of work!
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Jim Fisher
If you're not part of the solution, you're part of the precipitate.
Henry J. Tillman
Edited by gazerjim (01/06/05 02:40 AM)
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