liquid telescope mirrors

[Daniel-K]

http://www.forbes.com/sites/brucedorminey/2012/12/30/liquid-mirror-telescope-technology-finally-going-mainstream/

[JamesF]

So I could then see the undersides of clouds in exquisite detail?

James

[madatter]

great if your looking stright up but have they tried tipping it yet?

[JamesF]

great if your looking stright up but have they tried tipping it yet?

Towards the end of the piece they do talk about being able to point them at places other than the zenith.

James

[freeflyjay]

this was done as far back as the 19th century i think, i remember reading about some chap who filled part of a well with mercury then had a motor to keep it spinning

[Capricorn]

Isn't glass defined as a super cooled liquid?

If so then all scopes are liquid.

There is no crystaline structure and no defined melting point, the visocsity just alters with the temperature, just never actually sets.

In old churches the glass has had sufficent time to flow down and the top is thinner then the base now. Only takes a thousand years or so. :grin:

[Naemeth]

I should think the only way they can be used for a non-zenith position would be if the liquid was kept in some sort of box that didn't allow the liquid to move around, but the whole box would rotate in order for the liquid to be a parabola.

Surely they would lose a lot of light using mercury though, we manage around 90% with glass don't we? That's 15% light loss.

[swamp thing]

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In old churches the glass has had sufficent time to flow down and the top is thinner then the base now. Only takes a thousand years or so. :grin:

Afraid this is a myth. The glass has not flowed at all. The glazier would fit the glass round that way if he noticed a variation in thickness to make a better weather seal.

There has been numerous studies done of old buildings and it has been found that whilst half of the glass is thicker at the bottom, the other half is not, with some having the thicker edge at the top.

The variation in thickness was down to the manufacturing processess available at the time.

One last thought, if glass was truly running then according to this myth, if it takes just a few hundred years for it to move in a window then, the glass discovered in the Egyptian tombs should have been no more than puddles. They weren't. They still held their form after thousands of years.

[JamesF]

Afraid this is a myth.

Quite so.

James

[Daniel-K]

pretty cool though that its can reflect 75% of light as a liquid,

[foundaplanet]

Afraid this is a myth. The glass has not flowed at all. The glazier would fit the glass round that way if he noticed a variation in thickness to make a better weather seal.

There has been numerous studies done of old buildings and it has been found that whilst half of the glass is thicker at the bottom, the other half is not, with some having the thicker edge at the top.

The variation in thickness was down to the manufacturing processess available at the time.

One last thought, if glass was truly running then according to this myth, if it takes just a few hundred years for it to move in a window then, the glass discovered in the Egyptian tombs should have been no more than puddles. They weren't. They still held their form after thousands of years.

The times I have tried to explain this to folk. They look at me like I'm stupid so firmly is the myth implanted.. Still, I suppose that's how we end up believing in all sorts of nonsense.

[DaveS]

I've set up demos of this for school physics classes using a polyprop food bowl 175mm dia containing 2Kg very clean mercury (Enough to give a reasonable depth) sitting on a turntable spun by a variable speed motor. Trouble is, any vibration at all sets up zonal ripples. It's still a pretty demo though, to see a flat surface start to focus light!

[cantab]

I believe the actual liquid mirror telescopes use a parabolic tray, so that only a small amount of mercury is needed, enough to make a thin layer when it's spinning at the right speed. You might want to try that, just so you don't need so much mercury for your demo.

[andyboy1970]

I remember reading about this back in the 80's when the US Navy were experimenting with the idea.

[Peter Drew]

I think you would soon have the Health & Safety "police" swooping in if you were to use mercury for this purpose these days . As already mentioned, it is difficult to get a smooth enough drive to the mercury container to avoid zonal ripples. I'm sure I've read somewhere about molten glass being spun to at least get close to a suitable parabola to speed up production time. On the subject of glass "flowing", there was an article in Sky & Telescope some years ago concerning the permanent sagging of glass coffee table tops and the possible long term affects on large telescope mirrors. Had a few prople worried for a bit!

[cantab]

Yes, they do spin cast the telescope mirrors. I came across a photo where they started from basically a load of broken glass pieces. Because the glass contracts as it cools, and the desired mirror shape isn't a parabola on modern large telescopes, there's still a lot of figuring work to be done afterwords, but it saves grinding out the gross curve.