Re - An atmospheric envelope for ground-based telescopes.

I discussed a method for surrounding large ground based telescopes with a vacuum shroud reaching to the stratosphere in a post to
sci.astro:
Newsgroups: sci.astro, sci.physics, sci.engr.mech, sci.space.policy
Date: 29 Apr 2005 09:55:15 -0700 Local: Fri, Apr 29 2005 11:55 am Subject: An atmospheric envelope for ground-based telescopes. http://groups.google.com/group/sci.astro/browse_thread/thread/d7661ed18e80a693 /
For telescopes of the liquid mirror type arbitrarily large mirrors could then be used without the restrictions on the size of liquid mirrors caused by the winds produced by the rotating surface. Liquid mirrors telescopes have the limitation that they have to be upward pointing. However, they are much cheaper than a comparable sized solid mirror scope, perhaps by a factor of a hundred. There have been proposals for a 100 meter solid mirror telescope, costing in the range of a billion dollars. A liquid mirror telescope of this size might cost $10 million. My suggestion for raising the vacuum chamber walls to high altitude was to have them held in place dynamically with continuously supplied pressurized fluid. However, after a web search I found a report on creating inflatable vacuum chambers, where the walls are filled with pressurized gas for strength. Such chambers could even be buoyant if the walls were filled with a lighter than air gas such as helium:
Stability Analysis of an Inflatable Vacuum Chamber. http://arxiv.org/abs/physics/0610222v4
Bob Clark
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| Date: 29 Apr 2005 09:55:15 -0700 | Local: Fri, Apr 29 2005 11:55 am | Subject: An atmospheric envelope for ground-based telescopes. | http://groups.google.com/group/sci.astro/browse_thread/thread/d7661ed18e80a693 / | | For telescopes of the liquid mirror type arbitrarily large mirrors | could then be used without the restrictions on the size of liquid | mirrors caused by the winds produced by the rotating surface. | Liquid mirrors telescopes have the limitation that they have to be | upward pointing. However, they are much cheaper than a comparable | sized solid mirror scope, perhaps by a factor of a hundred. There have | been proposals for a 100 meter solid mirror telescope, costing in the | range of a billion dollars. A liquid mirror telescope of this size | might cost $10 million. | My suggestion for raising the vacuum chamber walls to high altitude | was to have them held in place dynamically with continuously supplied | pressurized fluid. However, after a web search I found a report on | creating inflatable vacuum chambers, where the walls are filled with | pressurized gas for strength. Such chambers could even be buoyant if | the walls were filled with a lighter than air gas such as helium: | | Stability Analysis of an Inflatable Vacuum Chamber. | http://arxiv.org/abs/physics/0610222v4 | | | Bob Clark`
You haven't taken an engineering degree or done any real engineering, have you? I've heard of castles in the air...
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Rotating pools of liquid will develop spiral standing waves (even in a vacuum) due to imperfections in the bearings and alignment of the rotating plane WRT to the ever-changing local gravity gradient (you forgot about the moon's and sun's gravity, didn't you?).

Air has about 7.25 times the density of helium at the same pressure. The gas laws imply that when helium is pressurized to 7.25 times the pressure of the air it becomes neutrally buoyant - any more and it becomes negatively buoyant (it sinks).
Your vacuum chamber will need an optically flat and transparent lid. This is impossible to achieve in a gravity field. Even a 1 meter span of glass will sag under gravity, resulting in a hopelessly distorted image.
Keck-style mirror arrays is the wave of the future of light-gathering power in earth-bound optical astronomy. The only question will be over how many kilometers will the mirrors be spread out. VLB interferometry is SOO good at improving resolution...
Tom Davidson Richmond, VA

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| > Date: 29 Apr 2005 09:55:15 -0700 | > Local: Fri, Apr 29 2005 11:55 am | > Subject: An atmospheric envelope for ground-based telescopes.http://groups.google.com/group/sci.astro/browse_thread/thread/d7661ed ... | > | > For telescopes of the liquid mirror type arbitrarily large mirrors | > could then be used without the restrictions on the size of liquid | > mirrors caused by the winds produced by the rotating surface. | | Rotating pools of liquid will develop spiral standing waves (even in a | vacuum) due to imperfections in the bearings and alignment of the | rotating plane WRT to the ever-changing local gravity gradient (you | forgot about the moon's and sun's gravity, didn't you?). | | > Liquid mirrors telescopes have the limitation that they have to be | > upward pointing. However, they are much cheaper than a comparable | > sized solid mirror scope, perhaps by a factor of a hundred. There have | > been proposals for a 100 meter solid mirror telescope, costing in the | > range of a billion dollars. A liquid mirror telescope of this size | > might cost $10 million. | > My suggestion for raising the vacuum chamber walls to high altitude | > was to have them held in place dynamically with continuously supplied | > pressurized fluid. However, after a web search I found a report on | > creating inflatable vacuum chambers, where the walls are filled with | > pressurized gas for strength. Such chambers could even be buoyant if | > the walls were filled with a lighter than air gas such as helium: | | Air has about 7.25 times the density of helium at the same pressure. | The gas laws imply that when helium is pressurized to 7.25 times the | pressure of the air it becomes neutrally buoyant - any more and it | becomes negatively buoyant (it sinks). | | Your vacuum chamber will need an optically flat and transparent lid. | This is impossible to achieve in a gravity field. Even a 1 meter span | of glass will sag under gravity, resulting in a hopelessly distorted | image. | | Keck-style mirror arrays is the wave of the future of light-gathering | power in earth-bound optical astronomy. The only question will be | over how many kilometers will the mirrors be spread out. VLB | interferometry is SOO good at improving resolution... |
Not mention his vacuum holding bouncy castle reaches stratosphere. Lordy knows what a hurricane would do to it.
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On Fri, 1 Feb 2008 10:27:21 -0800 (PST), Robert Clark

..
Perhaps you might consider alternate schemes with a lower technical cost. Hoiw about spinning a liquid mirror in a shallow cylinder, which co moves with the fluid, and which could be covered with something not much better than a float glass sheet? This would keep the internal wind to zero, and the external wind would not greatly disturb the glass plate cover.....
Brian Whatcott Altus OK
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