liquid mirror telescopes

[mlcolbert]

Hi everyone,

I am wondering if there are any members using liquid mirror telescopes.

If so, what are the advantages - yes - I know cost is one, but what of any problems etc.

Thanks guys.

michael

[Dooghan]

I would think gravity would be a bigger problem then cost. Trying to move the tube from anything then straight up and the mirror would go out of focus. I remember watch a program on TV and they where saying they would like to use mercury in the in next generation big telescopes. It would make the perfect mirror but they would have to build it on the moon because of gravity.

Dooghan

[programmer]

The moon has gravity too. I admittedly don't know anything about this, but 5 minutes with Google :D and I see they are thinking of spinning the stuff somehow to create a stable parabolic shape. Cutting edge stuff, and I don't think any members would be using it :eyepop:

Not sure how cost could be an advantage as it will probably cost millions to develop. Can't wait for the results in 15-20 years though! I'll have plenty of time to read about it in my retirement.

http://www.abc.net.au/news/stories/2007/06/24/1960080.htm

[bojan]

This was something attempted years ago, back in '70-ies... and before.. and after... it seems every generation tries this idea again and again......
The problem is: how to point such telescope to anything else except the local zenith?

[bojan]

The only way it could be done (on the Moon) to place it on the wheels and move it around to track the object.. Moon rotates slowly enough so this may be possible solution. Or, to build a number of such telescopes there, so the particular object can be tracked by passing it to the next one...
Not a very feasible idea IMHO...

[Glenhuon]

[quote=Dooghan;271524]I would think gravity would be a bigger problem then cost. Trying to move the tube from anything then straight up and the mirror would go out of focus. I remember watch a program on TV and they where saying they would like to use mercury in the in next generation big telescopes. It would make the perfect mirror but they would have to build it on the moon because of gravity.


They have to seal it off from the operators too, the fumes of that stuff are nasty. Used to work on Mercury bath lighthouse lens bearings, dressed up like something out of chemical warfare.

Bill

[JimmyH155]

Cant see how it would be an advantage. You can only ever point it directly upwards to the zenith, and the sky is always moving - so how could they get sufficient time to gather light for photography etc. You couldnt track stuff for even a second, at the accuracies that are worked on these days.
Only thing I can think of is have the mercury spinning pointing up, and then somehow articulate the tube to point or track anywhere by cunning use of strategically placed servo controlled mirrors/reflectors etc. Then you are defeating the object and it gets heavier again.:D
Also spinning mercury, that must involve minute vibrations?? :shrug:There goes your nice smooth mirror.
Naah, put that one in the "failed good ideas" basket.

[Lee]

I thought it was being researched for use in space only..... no gravity issues then..... like a huge aperture Hubble....

[okiscopey]

Liquid mirrors? Read all about them here:

http://www.astro.ubc.ca/LMT/lm/index.html

"The UBC Liquid-Mirror Observatory was established in 1995 to facilitate the development and testing of liquid-mirror telescope technology."

and:

http://vela.astro.ulg.ac.be/themes/telins/lmt/didac_e.html

also:

http://en.wikipedia.org/wiki/Liquid_mirror

Looks like there's a lot of interest but nothing big yet in operation.

[citivolus]

I suspect you could track with a large flat mirror in front of the primary/secondary, but this would almost rule out imaging near the zenith. Also, keeping that flat would be a bit of work.

[Dooghan]

Very true but it's a lot less then Earth. Maybe they think they can tilt it there :shrug: I can't see it being used in 0 gravity as they would have used it in Hubble. I don't know how you would keep it from floating away. If you sit down and think about, they would be using this technology in all big telescopes around the world if it worked. They are not. To me that says the technology is not there yet and may never be there. I still like the idea of it.

[citivolus]

If you could cool a thin mercury mirror below -39C, I wonder if it would remain usable? Granted, this may only be viable in the antarctic.

[Roger Davis]

Think of a Herschellian scope. Only an off-axis portion of the mirror is used. So if you had a 4m mirror the effective diameter would be 2m. By altering the size of the secondary you are not obstructing the incoming light path which means that you could intercept the light cone closer to the mirror and deflect the focusing rays to a convenient position, ie. steerable (a little). The abberations would be calculable and if you were imaging the image could be cleaned up with an algorithm. This could give you a usuable field of maybe around 40° centred on the zenith. Hmmm ....

[GTB_an_Owl]

were'nt they going to spin the mirror (create a vortex) to keep the mirror shape ?

geoff

[g__day]

You can use it to look straight up - shame you can just spin it - freeze it - and use it!

Better still - from myth busters trying to make Archimedes mirror to burn the Persian fleet. Jamie streched mirro shiny plastic foil over a round 44 gallon drum and vaccuum pumped out alot of air. The resulting shape in the foil is supposed to be perfectly parabolic!

What an easy way of producing a very large, very light, very parabolic mirror - where a change in vaccum achieved would change the focalpoint!

Suprised no one has tried this yet!

[citivolus]

I wonder how much gravity would pull it out of parabolic?

If that were the case, I would think you could do the reverse in orbit using a pressure cylinder under low pressure. Crude drawing attached.

The area behind the primary is vented to vacuum, while low pressure is maintained in the OTA using a non-reactive gas.

[luka]

1. Without gravity as mercury would just float away. No good for space.
2. It can only work pointing straight up as gravity would distort the parabolic shape.
3. You cannot freeze it as mercury will crystallise and hence ruin a perfectly smooth mirror surface.

[Roger Davis]

1. Hg melts at about -38°C
2. Hg boils at aobut 357°C
3. You have to mask the edge of the mirror as it will be turned due to surface tension.
4. You wouldn't want the mirror cell to crack as it would cause an ecological disaster!
5. But then you could always mix it with lead and arsenic (yum yum!) and invent speculum mirrors all over again.

[bojan]

This thing is patented a long time ago :-( I discovered this when I tried it years back, and it sort worked.... My problem then was I could not get hold of aluminized mylar foil).
BTW, this principle (vacuum on one side of the glass plate) is used in a process to grind the Schmidt plate for large appertures (this was published in 80's in Sky&Telescope).

However, the variation of this method may work in space - like inflating a large transparent sphere with low pressure gas.. the sphere must be partly aluminized at one side, of course....

[Dooghan]

One problem I can see with it is the sun is going to heat up the tube. That heats up the gas inside the tube and changes the shape of your mirror. Then when the tube goes into the earth shadow you've got the reverse happening. I also wonder if you air turbulence in the tube would be a problem?

[ving]

liquid mirror! now thats weirding me out!

if they made it out of coffee, you would have to top it up all the time.
how the heck does it work anyhow. zero gravity wouldnt be any good would it? it would just float around?

[bojan]

Oh yes, everything you mentioned would be an issue.. but not necessarily the un-solvable problem... today we have ground-based telescopes with adaptive optics that have better resolution than Hubble... So, I am sure there is a way to compensate for all that :-) .
The beauty of this may be that virtually there is no limit to a size.. Also, if longer wavelengths are used (far IR or even /u-waves) then the distortion of optics is not an issue that much, but the size is, definitely.

[TheCrazedLog]

Bummer if you accidentally turned your scope upside down....

[g__day]

Some interesting links:

http://www.abc.net.au/science/k2/moments/s64751.htm

http://www.math.iupui.edu/m261vis/LMirror/LMirror.html

http://www.astro.ubc.ca/LMT/lm/index.html

What you need is a liquid spinning mirror you can snap freeze without shattering or changing the shape of the material - hard asks. Imagine if you could spin say silver this way at say 400 degrees Kelvin (or whatever) - then span freeze it whilst its still rotating without loosing its perfect surface. If it was bonded to its spinning base - which itself was structurally extremely rigid - you could simply lift this entire structure and mount it like any normal large telescope - hey presto - large perfect mirror very, cheaply formed.

If the mirror device was inside a bessel oven - and continued to rotate at say 3 turns a minute whilst the parabolic arc formed, and could be instantly snap frozen (there is the trick) without rippling the surface - way to go. You might be able to do this if the film of liquid metal was very, very thin - e.g, < 100 microns thick. If you heated and spun say 600 grams of silver in a 3 metre wide circular block that transfered heat very well (e.g. copper) and was already roughly parabolic in shape - you might be able to run collant liquid through the block and instantly snap freeze the silver.

Be really tricky but if you find a way to do it that works - be fantastic!

[citivolus]

I wonder if there are any high viscosity, highly reflectivity fluids that would allow a tilt without shifting.

This brings up a question I have had for a while, and that is are metal mirrors practical at low thicknesses if backed with a supportive substrate? In other words, could you create a metal mirror using this method and then silver it, say using immersion, to overcome the problems associated with crystallisation? Or will using a metal introduce thermal expansion/contraction issues?

[wasyoungonce]

Yeah but spinning a liquid will create an inverted truncus (-1/x^2) shape not a parabola.

I think?