Borosilicate vs Pyrex Mirror blank?

[long_arms]

Hello, 

Looking for a 12" mirror blank. I originally wanted to splash out on quartz but this is seemingly impossible. 

So looking for other low expansion materials. 

Simply put, why is a Borosilicate blank from here £268. http://www.galvoptics.fsnet.co.uk/telescope3.htm

But a "Pyrex" blank from here only £75? http://www.scientificmirrors.co.uk/Telescope%20Making%20Supplies.html

Is one better than the other? I read somewhere that the "new" pyrex doesnt have the same thermal characteristics it used to. 

Many thanks, 

Dan

[JRM]

Dan,

     Borosilicate is suppose to be the new Pyrex, the thermal qualities from what I have

read are not that different but even without  a price swing like that I would still go with

the Pyrex, so long as you do not mind losing that 1/10th of an inch.

Rick M

[Peter Drew]

There is nothing wrong wity plate glass or pyrex, excellent mirrors have been made from either. Low expansion materials are favoured by manufacturers as the figuring is helped by not having to wait so long for the glass to "settle" after a session of polishing and before testing. The overall benefit to the user is debateable unless the mirror is for a specialist use. 

[Laurie61]

It looks as though the 11.9" Pyrex has a 2" centre hole, would this affect the grinding/ figuring process ?

http://www.oldhamoptical.com/#!glass/cbvr

[JRM]

The 11.9 with the hole would make for a good cass,though I would not attempt one of

those. yet, the other piece is without the hole for a few pounds more.

Rick M

[long_arms]

I've contacted this supplier after finding a recommendation for them on here. 

http://www.stathis-firstlight.de/spiegelschleifen/materialeng.htm

As far as Pyrex goes, this is what I meant. If the supplier uses the name "Pyrex" surely it must be glass produced by World Kitchen LLC? 

"For decades, Pyrex was made of borosilicate glass, a special type of glass in which boron oxide is added to the mix. The added boron allows Pyrex to handle heat much better than typical glass, so Pyrex is commonly found in kitchens, laboratories, and in use with aquarium heaters (as the heaters are, necessarily, submerged in much cooler water). But in 1998, Corning, the company which made Pyrex, sold the brand to World Kitchen LLC. World Kitchen decided to stop the manufacture of borosilicate glass, and since then, Pyrex sold in the United States is made of tempered soda-lime glass, which does not handle heat as well as borosilicate glass does."

Taken from here. http://nowiknow.com/the-war-against-pyrex/

Food for thought if buying a "Pyrex" branded mirror blank surely?

Dan

[long_arms]

Answered my own question. Learnt something new today haha. 

From Wikipedia 
"The European manufacturer of Pyrex, Arc International, uses borosilicate glass in its Pyrex glass kitchen products however, the U.S. manufacturer of Pyrex kitchenware uses tempered soda lime glass."

[Peter Drew]

Not much heat coming from the Moon or stars at this range so the mirror should survive! 

[Astrobits]

If your mirror is at a different temperature from the local environment then the thermals rising from the mirror totally swamp any effects of differential expansion on the figure of the mirror. So the priority during the mirror's use is to get the mirror to the same temperature as the local environment. For that it is best to use the thinnest glass you can comensurate with your support requirements.

The difference in the thermal expansion coeficient between plate/float glass and the original "Pyrex" is only about a factor of 2 so not that much really. One of the borosilicates from Schott has a coeficient of thermal expansion that falls between Plate/float and "Pyrex", the other is the same as "Pyrex". As Peter Drew posts above, low expansion glass helps the mirror maker. At thermal equilibrium there is no difference between the two glasses in actual use.

Nigel

[crashtestdummy]

The German supplier is massively more expensive .I was quaoted over double the price for a 22" blank than I eventually paid from the uk.

Most of the low expansion blanks over here are made from schott borofloat.my supplier imports this as discs from Germany and cuts them to size.if you are doing a 12" grind unless it's quite a short F ratio you may be better off with plate glass if you don't need a low expansion one.