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Fraser
2007-Jun-22, 12:56 AM
NASA-funded researchers are working on a clever technology that could deploy a gigantic telescope made from rotating liquid... on the Moon! It sounds like science fiction, but they've gotten smaller prototypes to work, and the technology should work even better on the lower lunar gravity. ...

Read the full blog entry (http://www.universetoday.com/2007/06/21/plans-for-a-liquid-lunar-telescope/)

Braungucke
2007-Jun-22, 12:44 PM
Now there's a reason to send humans back to the Moon.

Thats the point! No more setting funny flaggs. Lets do something meaningfull out there.

Ophiucus
2007-Jun-22, 01:01 PM
While I agree on the use of such a large telescope, be it on the moon, in space or on earth (with adaptive optics), there is one big negative point to be told about the liquid mirror:
As of yet, unless Ermano (or Professeur Borra, whatever) is hiding something from his department, his scopes cannot be tilted more than a few arcminutes from the zenith. I.e.: with a liquid mirror, you cannot track, tilt, or point to an object anywhere else than at zenith. This is a major problem, for both astrophotography (you are forced to take a rapid succession of low-time image, which increases reading noise from the CCD) and usefulness (a very limited set of objects can be observed from one telescope).

Being a student in his department, I can tell you that his research is regarded with widespread criticism, at least internally. Oh, and have you thought of the micrometeorites smashing in the scope? I don't think they'd be very good.

Amber Robot
2007-Jun-22, 01:18 PM
As of yet, unless Ermano (or Professeur Borra, whatever) is hiding something from his department, his scopes cannot be tilted more than a few arcminutes from the zenith. I.e.: with a liquid mirror, you cannot track, tilt, or point to an object anywhere else than at zenith.

This need not be a problem for astronomy. There are at least three professional-class telescopes whose primary mirrors do not move during an observation: the Arecibo Radio Telescope, the Hobby-Eberly Telescope, and the Southern African Large Telescope. The tracking is done by an assembly at the focus of the fixed primary. So, in principle this can be achieved. The case of a liquid mirror telescope will be most like the Arecibo rather than the latter two, and although the fixed primary does limit both the length of any individual track and the fraction of the sky observable, the Arecibo has proven to be a highly productive instrument.

Ophiucus
2007-Jun-22, 01:25 PM
This need not be a problem for astronomy. There are at least three professional-class telescopes whose primary mirrors do not move during an observation: the Arecibo Radio Telescope, the Hobby-Eberly Telescope, and the Southern African Large Telescope. The tracking is done by an assembly at the focus of the fixed primary. So, in principle this can be achieved. The case of a liquid mirror telescope will be most like the Arecibo rather than the latter two, and although the fixed primary does limit both the length of any individual track and the fraction of the sky observable, the Arecibo has proven to be a highly productive instrument.

The SALT design calls for a spherical primary, while a liquid mirror produces a parabolic surface. At first glance, it would seem like they are incompatible.

Amber Robot
2007-Jun-22, 07:00 PM
The SALT design calls for a spherical primary, while a liquid mirror produces a parabolic surface. At first glance, it would seem like they are incompatible.

Yes, you're right that those are spherical primaries. :shifty: I would imagine that dealing with a parabola would be more difficult, but may not, in principle at least, be impossible.

dhd40
2007-Jun-22, 08:00 PM
NASA-funded researchers are working on a clever technology that could deploy a gigantic telescope made from rotating liquid... on the Moon! It sounds like science fiction, but they've gotten smaller prototypes to work, and the technology should work even better on the lower lunar gravity. ...

Read the full blog entry (http://www.universetoday.com/2007/06/21/plans-for-a-liquid-lunar-telescope/)

I really donīt understand this.
1. Which liquid could be used without evaporating? Mercury?
2. If functioning on the Moon, why not in space (Lagrangian points). Should be much easier to keep it rotating without permanent power supply

Am I missing something?

cjameshuff
2007-Jun-22, 09:14 PM
1. Which liquid could be used without evaporating? Mercury?

An "ionic liquid". Some sort of highly-polar liquid. Several substances do exist that would remain liquid at the temperatures and pressures required. The big innovation here is vapor-depositing a layer of silver on the surface of the liquid.



2. If functioning on the Moon, why not in space (Lagrangian points). Should be much easier to keep it rotating without permanent power supply

Gravity is needed to pull the mirror into a parabolic shape. It might be possible to attach the whole spinning assembly to a tether and rotate it, though, perhaps using additional optics to compensate for deformation of the mirror due to coriolis forces...or perhaps surface tension alone could pull the liquid close to a substrate, the liquid only providing a smooth surface for the silver deposition rather than producing the required geometry.

As for the zenith-facing problem...a simple flat mirror placed in front of the curved mirror should greatly expand the flexibility, allowing anything near the horizon to be targeted. Situate the telescope near a pole, and observation time for much of the sky would be effectively unlimited. A similar arrangement would be necessary for the rotating tether telescope described above, given the relatively rapid rotation.

edit:
As for micrometeorites...the mirror would be relatively immune to them. Assuming the supporting mesh isn't too badly damaged, fluid could be replaced from a reservoir and the silver layer redeposited.

dhd40
2007-Jun-23, 04:59 PM
Thanks a lot for your response.


An "ionic liquid". Some sort of highly-polar liquid. Several substances do exist that would remain liquid at the temperatures and pressures required.
Interesting, didnīt know this. Could you mention one or two chemicals of that kind (highly-polar liquids)?


The big innovation here is vapor-depositing a layer of silver on the surface of the liquid.
I canīt imagine how this can be done on the Moon, or in space. But obviously it must be possible.


Gravity is needed to pull the mirror into a parabolic shape. (SNIP)
Yes, youīre right. How about micro-gravity doing the job? (slowly, very slowly!)

(SNIP)

Tacitus
2007-Jun-23, 08:02 PM
The biggest attraction of liquid mirror telescopes is the cost. Assuming the technology can be perfected, LMTs would be a tiny fraction of the price of equivalent conventional telescopes, especially if you're talking about building one and transporting it to the Moon.

cjameshuff
2007-Jun-23, 08:10 PM
Interesting, didnīt know this. Could you mention one or two chemicals of that kind (highly-polar liquids)?

Water is the obvious one. Unsuitable for telescopes for numerous reasons...same goes for ammonia.
Acrylonitrile might be one candidate. It melts at -45 C and boils at 82 C.
Dimethylformamide melts at -61 C and boils at 153 C.
Propylene glycol is liquid from -59 C to 188 C.
Butyronitrile is liquid from -112 C to 117.5 C.

I have no idea how volatile these things are at whatever temperatures they'd operate at.

There's a class of salts referred to as "ionic liquids", a term I've seen in a couple other articles on the liquid telescopes. The most notable property of this class of compounds is that their melting points and viscosities are easily tunable by varying their components.
http://www.chemada.com/cat1/default.asp?categ=I



I canīt imagine how this can be done on the Moon, or in space. But obviously it must be possible.

You heat a small blob of silver until it begins to evaporate, and move it across the mirror so the silver vapor condenses on it. A small electrical arc between two silver wires could do the job quite well. Aluminum might also be a good metal, but would require a pretty unreactive liquid...



Yes, youīre right. How about micro-gravity doing the job? (slowly, very slowly!)

You need a gravitational pull in some consistent direction. "Microgravity" typically means freefall, there might be weak tidal forces and the extremely weak gravity of the telescope itself, but nothing predictable and strong enough to use for this purpose.

You could perhaps dangle the telescope from the top of a space elevator (current materials would allow one to be placed on the moon). It seems rather excessive as a way to avoid putting the scope on the lunar surface, though.

dhd40
2007-Jun-24, 08:55 PM
Water is the obvious one. Unsuitable for telescopes for numerous reasons...same goes for ammonia.
Acrylonitrile might be one candidate. It melts at -45 C and boils at 82 C.
Dimethylformamide melts at -61 C and boils at 153 C.
Propylene glycol is liquid from -59 C to 188 C.
Butyronitrile is liquid from -112 C to 117.5 C.

I have no idea how volatile these things are at whatever temperatures they'd operate at.

There's a class of salts referred to as "ionic liquids", a term I've seen in a couple other articles on the liquid telescopes. The most notable property of this class of compounds is that their melting points and viscosities are easily tunable by varying their components.
http://www.chemada.com/cat1/default.asp?categ=I


I think that all the melting/boiling temperatures are valid for "normal" pressures here on Earth. But if there is no athmosphere (pressure) ... ???


You heat a small blob of silver until it begins to evaporate, and move it across the mirror so the silver vapor condenses on it. (SNIP)
Agreed


You need a gravitational pull in some consistent direction. "Microgravity" typically means freefall, there might be weak tidal forces and the extremely weak gravity of the telescope itself, but nothing predictable and strong enough to use for this purpose. (SNIP)

Yes, thatīs true. I obviously overlooked the important role of gravity.

kjargirl
2007-Jun-24, 08:57 PM
Please see this earlier original Universe Today article on the same topic
http://www.universetoday.com/am/publish/pristine_view_universe_moon.html
Nancy

ASEI
2007-Jun-24, 09:58 PM
Sounds like an engineering pain to me. Sure, you get a parabolic surface if you rotate an ideal fluid, but what is this rotating drum rotating on? Wouldn't that introduce vibrations? What if your fluid viscocity louses up your parabola? What if you need to look at something off-axis to the gravitational field?

Having something that temporarily rotates as a liquid, then sets and gets polished might be a better idea, IMO.

Tacitus
2007-Jun-25, 02:47 AM
The engineering "pain" is something they've been working on for decades and compared with transporting an equivalently massive conventional scope to the moon, not a big deal at all.

cjameshuff
2007-Jun-25, 12:39 PM
Sounds like an engineering pain to me. Sure, you get a parabolic surface if you rotate an ideal fluid, but what is this rotating drum rotating on? Wouldn't that introduce vibrations? What if your fluid viscocity louses up your parabola? What if you need to look at something off-axis to the gravitational field?

Having something that temporarily rotates as a liquid, then sets and gets polished might be a better idea, IMO.

An engineering pain, yes, but worth it when the alternative is transporting heavy mirrors from Earth's surface. From what I've read, they use a high-viscosity fluid, likely at least in part to damp vibrations. The viscosity wouldn't affect the parabolic shape, only the speed with which it settles into that shape. Magnetic bearings are feasible, especially in the reduced gravity. And off-axis targets are either inaccessible to that particular telescope or require a flat mirror placed in front of the scope.

A material that sets into shape on the moon is a possibility, but you have the issues of thermal and mechanical stability. It likely won't be as rigid as a glass mirror, so it may not work well when tilted from the vertical. There are likely to be problems with dimensional changes as it solidifies, and afterwards as it outgasses or creeps under constant load.

The flat mirror for looking off-axis might be cast from glass on-site. A glass parabolic mirror would have to be ground to correct its shape after the thermal contraction from cooling after being cast, a flat mirror might be able to avoid that. The need for polishing could also be avoided if the surface is smooth enough. Raw glass pellets would be cheaper to ship, fragility not being an issue, or perhaps just soda and lime could be transported, the silica being obtained from the lunar regolith. This mirror isn't necessary for a useful telescope, though, just for aiming it at off-axis objects.

dhd40
2007-Jun-25, 08:48 PM
Please see this earlier original Universe Today article on the same topic
http://www.universetoday.com/am/publish/pristine_view_universe_moon.html
Nancy

Thanks for that link. There are a lot of explanations to my questions.

Nick4
2007-Jul-01, 06:49 AM
I dont understand this

BigDon
2007-Jul-02, 07:38 AM
While I agree on the use of such a large telescope, be it on the moon, in space or on earth (with adaptive optics), there is one big negative point to be told about the liquid mirror:
As of yet, unless Ermano (or Professeur Borra, whatever) is hiding something from his department, his scopes cannot be tilted more than a few arcminutes from the zenith. I.e.: with a liquid mirror, you cannot track, tilt, or point to an object anywhere else than at zenith. This is a major problem, for both astrophotography (you are forced to take a rapid succession of low-time image, which increases reading noise from the CCD) and usefulness (a very limited set of objects can be observed from one telescope).

Being a student in his department, I can tell you that his research is regarded with widespread criticism, at least internally. Oh, and have you thought of the micrometeorites smashing in the scope? I don't think they'd be very good.

Hey Ophi, can't chatting about such things get you in trouble at work? I know of a dozen or so people over a 12 year period who thought that talking about the inner politics of where they work to be fairly safe. They were fired. From 100,000 dollar a year jobs. That's a big ouch just for bragging rights to strangers.

Just a caution from the old guy. I'm not telling you what to do, just what I've seen happen to folks who've done as you've done. Believe me its entirely possible for this to get back to him.

BD