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There is also a new technique for making mirrors (at least I thought it was a new technique) where round sheets of plate glass are joined together by small spacers. Tong Liu of HubbleOptics has an outstanding patent which I managed to find which shows the basic idea but he is not the first person to come up with the idea, the US military had been using it for airborne reflectors since the 1970s. There are actually a number of designs.
While I have never done it myself, I intend to look into it when I start to fabricate bigger mirrors (I am currently finishing off my 1st 6 inch reflector). There are two advantages to this approach. One is a reduction in weight, the other is an increase in surface area. Tong Liu, in his patent cites a study (can't remember where), where it said that there were no intrinsic benefits to using pyrex and that mirror seeing (inability to cool the mirror) was the limiting factor.
Tong Liu claims that performance improves with size rather than degrades for larger mirrors (probably because the way that he scales it, larger mirrors have more glass elements sandwiched together and therefore for spaces between them, which means more surface area to lose heat. It's all fairly new but if you want me to dig up some of the patents, let me know. Unfortunately, I can't give you the benefit of any experience.
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