Quote:
Originally Posted by Satchmo
I'm not sure what element of the Hubble Optics blanks would be patentable...fusing ribs blocks or discs of glass between face sheets for light weighting of glass blanks has been going on in professional circles since the '60's .
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Not sure what you mean by rib blocks. If you mean honeycomb mirrors that is not what he does. What he does is puts very small flat cylinders of glass in the same place as you would put the supports for a mirror cell. The spacers are made of the same type of glass. These spacers are glued or melted to 2 large sheets of glass.
There seems to be less support than a honeycomb but a much greater surface area exposed to the atmosphere, so cooling is apparently never a problem. I understand that they use passive cooling and that the cooling improves with mirror size.
I have downloaded Tong Lieu's (Hubble Optics) provisional patent and another US military patent which is similar to his and which dates back to the 70s.
The US military patent uses hollow cylinders which are much longer than Tong Lieu's. The hollow cylinders have a hole drilled in each one otherwise there would be a cell of trapped air which would expand and contract, creating stress and presumably affecting the figure. The military one was used for aircraft or missiles, I think, so weight and air pressure were more important. Where it also differed from Tong Lieu's is that apart from the primary surfaceof the mirror, all other surfaces were flat.
Tong Lieu, on the other hand used a different strategy. He used much thinner spacers between the plates. His spacers were solid. Tong Lieu's design used multiple sheets (not just 2). Additonally, every surface was curved (including the spacers).
His patent also involved a method of fabrication which involved gluing or otherwise bonding the surfaces which are intially flat. Once they are set, they are placed over a metal template and the whole sandwich is melted and slumped into a curve. The primary surface in then figured. According to Tong Liu, figuring the mirrors is difficult. Not sure if it is difficult to figure because of the heating done to the glass, or if the structure required it to be figured gently.
Another thing in his patent which was interesting was the manufacture of large segmented mirrors. It is hard to describe in words but here goes. Imagine a very thin pizza, cut into four slices only and left on the tray. Then imagine putting some sliced olives in the same place you would put the mirror cell supports. Then put another pizza on top of this, which has been cut into four pieces, but make sure the cut mark don't line up. Keep layering pizza and olives a few more times and baked.
The Hubble Optics/Tong Lieu patent was awarded in China and was lodged in the US in 2007 but has not yet been granted. Not knowing anything about patenting I don't know how to interpret this.
I would like to make a large plate glass blank using this technique, but I have no oven. I wonder if you could get several discs of thickish glass, roughly grind all of them on both sides to the desired shape, get some offcuts of glass from a glass cutter to make the spacers with and glue them using a silicon glue.
I'm not up to that level of skill though. I also have not heard of the opticians who make fast mirrors as to whether or not the claims that are made about these mirrors are true such as-
- the fast cooling (I suspect this is true - surface area to volume ratio indicates they would cool better than a monolith or a honeycomb.
- pyrex being unecessary. This claim hinged on the idea that it is the temperature difference between the air and mirror that was more important than expansion coefficient. He cites a reference in the patent, but apart from this I have seen no confirmation.
To my understanding, very large professional mirrors use honeycomb and not sandwich mirrors. I wonder whether the sandwich mirror is trading structural support for a lightweight quick cooling mirror. I wonder if there is a range of mirror sizes where it is a beneficial design. I've not heard from any ATMs or experience opticians using these techniques.
