Slashdot Mirror
Deformable Liquid Mirrors For Adaptive Optics
eldavojohn writes "Want to make a great concave mirror for your telescope? Put a drop of mercury in a bowl and spin the bowl. The mercury will spread out to a concave reflective surface smoother than anything we can make with plain old glass right now. The key problem in this situation is that the bowl will always have to point straight up. MIT's Technology Review is analyzing a team's success in combating problems with bringing liquid mirrors into the practical applications of astronomy. To fight the gravity requirement, the team used a ferromagnetic liquid coated with a metal-like film and very strong magnetic fields to distort the surface of that liquid as they needed. But this introduces new non-linear problems of control when trying to sync up several of these mirrors similar to how traditional glass telescopes use multiple hexagonal mirrors mounted on actuators. The team has fought past so many of these problems plaguing liquid mirrors that they produced a proof of concept liquid mirror just five centimeters across with 91 actuators cycling at one kilohertz and the ability to linearize the response of the liquid. And with that, liquid mirrors take a giant leap closer to practicality."
Why not just spin it, and while it's spinning, lower the ambient temperature so that it freezes? And if you remember your thermodynamics, you'll remember that raising or lowering *pressure* raises or lowers the temperature of a gas -- seal it up, spin it, then freeze it. Easy peasy.
#fuckbeta #iamslashdot #dicemustdie
Sounds like the oil lenses from Dune.
Given a choice between free speech and free beer, most people will take the beer.
Queue the debate:
First!
Okay, I'm in line for a debate. Now what?
Put a drop of mercury in a bowl and spin the bowl. The mercury will spread out to a concave reflective surface smoother than anything we can make with plain old glass right now..
so our bowl making technology exceeds our bowl shaped mirror technology? seems like we could just hire the bowl makers and fire the current crop of mirror makers, problem solved.
Global Vision 2020 is doing something similar to this, creating eyeglasses for people in third-world countries.
They have glasses with special lenses that can be filled with oil. The oil changes the shape of the lens.
The client puts the glasses on and fills the lens with oil until he can see clearly. Then the technician seals the glasses so the amount of oil (and shape of the lenses) won't change any further.
$10 per set of glasses, and no optometrist required to issue them.
If you're looking for a worthwhile charity to donate to, this may be one to consider.
If you're a zombie and you know it, bite your friend!
why can't we just spin it up then freeze it solid?
As long as its kept cold you can use it at any angle, and even make a precision mold from it then make a less temperature sensitive version.
Laslo: Well what would you use that for?
Ick: Making enormous Swiss cheese?
(Chris laughs.)
Mitch: The applications are unlimited.
Laslo: No. With the fuel you’ve come up with the beam would last for what15 seconds. Well what good is that?
Chris: Oh Laslo. That doesn’t matter. I respect you but I graduated.
Mitch: Yeah, let the engineers figure out a use for it. That’s not our concern.
Laslo: Maybe somebody already has a use for it. One for which it is specifically designed.
PS: I'm serious.
Sounds like something perfect for the next generation Hubble (or the next next one - the next one is getting ready for launch). Why fight gravity, when you can just spin it in space?
Course - making it spin for a long time between maintenance visits (on who knows WHAT vehicle) could be tricky.
Okay, I'm in line for a debate. Now what?
Now tell him he is wrong, and why he is wrong.
Don't know something? Look it up. Still don't know? Then ask.
Ignoring the mischaracterization of glass that you're trying to start a debate about, the answer to your question is: No, becuase mirrors are not made of glass.
Bathroom mirrors have a protective layer of glass, but the reflective layer is silver. At best that would be "partially liquid" if we pretend that glass is a liquid. Many mirrors do not have such a protective layer, though; the mirror I use for backpacking is simply a thin metal sheet. Mirrors for lab optics typically don't include a glass layer because it would serve no purpose and would interfere with the mirror's intended use.
The defining element of a mirror is the reflective part, which is made of metal and is usually solid.
You are wrong in 4 ways:
My telescope will have to wait so that I can save up the $$$. This sounds cool in practice, but even at 7 cm the associated control circuitry and actuators would be prohibitive. Cool though!!
I hope this caused some synapses to fire.
MIT's Technology Review is 'nalyzin a team's success in combating problems with a-bringin' liquid mirrors into the practical applications of astronomy.
FTFY
.
Prisencolinensinainciusol. Ol Rait!
He has to pay him first.
Okay, liquid mirrors are cool but point straight up. Why not use a pair of flat mirrors to reflect the night sky into the spinning liquid mirror? Yeah, there would be some loss due to the imperfect reflectivity of the flat mirrors, but if liquid mirrors are so awesome, then just make them a little bigger to compensate.
Adding to parent comment..
I am pretty sure that most large "glass" mirrors used in astronomy actually use a very thin top layer of aluminium as the reflective layer, perhaps only 3 atoms thick... And I recall reading somewhere that this layer is cleaned off and applied every couple of years because of corrosion.
No sig. Move along - nothing to see here.
Seems to me that liquid mirrors would be orders of magnitude more sensitive to vibrations than solid ones. (Experiement: fill a glass with water; tap the glass; which has a greater amplitude, the ripples on the surface of the water, or the ripples on the surface of the glass?)
And rotating something large and heavy with a motor, moreso while simultaneously manipulating its surface with several dozen actuators, is a huge source of vibrations.
Step one, manage the forces on a liquid to make a constant bowl -- this allows you to make a telescope and point it anywhere you want.
Step two, manipulate that bowl to alter the parabola. No more adjustable mirrors on solar collection systems, for starters, but this also allows you to direct sunlight on a quickly moving target, like a solar powered space elevator crawler. This gets particularly interesting in space. It allows you to focus sunlight on a satellite, or an object on the ground. A small mirror could charge up the batteries on a satellite, with a wide focal point, and a big one could burn tiny little holes in even the fastest moving target. A big enough lens could do the same gag on people's heads on the ground. Not sure how big it would have to be accomplish that through the atmosphere, though.
Imagine a 200-sq foot mylar fixed mirror doing a crappy job of focusing sunlight on a fixed position high-quality adaptive mirror, which was in turn capable to directing that light down to a few receiver stations on the rotating planet below, constantly compensating for the movement of the receiver.
The key problem in this situation is that the bowl will always have to point straight up.
That, and the fact that mercury is an extremely toxic and hazardous element that has to be carefully handled and disposed of. Accidentally disperse some into the air (splatter) and you could face a very costly cleanup. In short, mercury is a little bit beyond something your average hobbyist should be playing with – not that it would likely deter them, unfortunately.
Alexander Peter Kristopeit bought his basement from his mommy for one dollar.
I thought one of the points was that you don't want to fix the shape permanently. Adaptive optics lets you adjust the mirror to account for atmospheric distortion. Think of it like being able to change the prescription of your glasses. A liquid mirror would allow for near-infinite possibilities to adjust how the light is reflected, with greater precision than current adaptive optics systems.
I'm sure several other people have replied already - and they're not laughing with you.
Confucius say, "Find worm in apple - bad. Find half a worm - worse."
Oh please. You need a good dose, and constant exposure. Otherwise your body will purge it. Don't drink it, but don't drink motor oil either.
Just be careful. You average hobbyist has no problems with it.
The Kruger Dunning explains most post on
Yes, there will be the earths gravity, but the temp of space would freeze it? Then you have an awesome replacement for the Hubble's lens, if it needs to be upgraded (not from what I've seen though).
Or a whole new telescope all together.
Glass isn't a liquid, despite what my (and possibly your) high school chemistry teacher taught.
https://www.eff.org/https-everywhere
Hey, aren't there any optical liquids whose refractive index would depend on electromagnetic field strength?
Imagine a liquid-filled lens for photography, that could be zoomed or focused with no moving parts, perhaps
even reconfigured on the run...
If you think I hadn't seen this debate before and why I said "Queue". I love getting modded down.
Actually, most telescope mirrors are made from glass (some are made of special glasses, that have low thermal expansion and so on, but nevertheless glass), glass being the important "ingredient" of the mirror. The reason is that glass has no crystal structure and can be polished to very high degree of accuracy and achieve the required figure (a paraboloid) with very high precision. Glass is also a very stable medium if prepared (annealed) properly.
Since the purpose of an astronomical mirror is to collect light in a precise way, the figure of the mirror is of most importance. The role of the metal layer on the surface is only to increase the reflectivity of the glass. There were (and, for some specialized uses probably are) some metal astronomical mirrors (made of speculum metal, mostly before glass got into wide use) but they allow a figure that is no better than the glass ones, and are difficult to polish and maintain.
In fact, metal coating isn't even necessary to use a glass mirror. When you make a telescope mirror, before you send it off for coating you'd perform what is known as "star tests". You'd set up your telescope, put in the uncoated mirror in it, and look at stars to see if the mirror shape is good. I could easily see a lot of planetary detail with my last (40") mirror while I was testing it without coating. Looking at the Moon was blinding.
The key problem in this situation is that the bowl will always have to point straight up.
Not if it's at the bottom of a tube in a centrifuge. Of course, spinning the end and dealing with the gyroscopic forces is a new problem, but you can't have everything. Assuming you can figure that out, you could take snapshots every X microseconds (whenever you're pointing at something you want).
Rather, that should be spelled a-bringin'.
MIT's Technology Review is annal-ma-lyzin' a team's success in combatin' problems with a-bringin' liquid mirrors and such t'the pract'cal applicationin's of the astronomy.
Yikes! That is ONE TOUGH PROJECT. Sure am glad I'm not working on it! *sigh of relief*
Is this not a very expensive way to solve a problem that could be addressed much more simply by just using a large flat mirror to capture the view that we want the telescope to look at?
We do know how to make very reflective large flat surfaces as well as how to point them accurately. The mirror can stay flat on the ground, only the flat reflector moves.
Seen that all the objects are at infinity, any distortions introduced by the reflection should be easy to eliminate.
I know i'm an idiot but why not use this is Zero G? I'm sure with a decent set of thrusters you could make a bowl rotate and point in the right direction plus have enough acceleration to simulate gravity on earth (thus solving the always point up problem). I bet mercury is a lot lighter then the mirrors on hubble...
For a specific example:
http://www.hulu.com/watch/153113/worlds-toughest-fixes-giant-telescope
Most will find it fairly boring, and a little superficial, but it is a specific video example of the process you described.
I won't join Slashcott. OTOH, If Beta goes live, I just won't be back until it's fixed. Sorry Dice.
So why are we still using mirrors? Why not a big CCD?
In a front surface mirror such as in a telescope, the surface of the glass is in fact the mirror because the important part is the shape and smoothness of the surface. You do not need to coat it with a reflective material because the glass itself is somewhat reflective. A large noncoated mirror is good for viewing the moon, which has a lot of detail but is very bright.
After countless hours grinding and polishing or thousands of dollars spent on an optician with a good reputation or even tens of thousands spent on ion milling of the glass, you might want to have your mirror tested with an interferometer. That is done before the mirror is coated. That's because the glass is the mirror. The coating simply makes it more efficient.
It is true that some mirrors are made with non-glass materials such as quartz or zero expansion ceramic. ...or Mercury.
Glass lacks a crystal lattice, thus it is not a solid. When defining phases of matter, we consider in which ways the distribution of matter breaks symmetry. All fluids (gases and liquids) are both isotropic and homogeneous. At equilibrium, every point in the substance has the same conditions, on average, as every other point and everything looks the same when looking in any direction. Gas and liquid have the same symmetry with the only difference being in the incompressible nature of liquids. In fact, you can cause liquids and gases to smoothly transition from one to the other without crossing any phase transition.
Solids have fundamentally different symmetry properties. Starting from a lattice point, one must travel along a special direction by a distance called a lattice constant to reach the next nearest point at which conditions appear the same. Crystals can exist in a number of different symmetry groups.
Condensed matter physicists will tell you that there are actually a huge number of phases of matter including smectics, cholesterics, nematics (e.g. liquid crystals), etc., that all break the isotropy and homogeneity of liquids in different, intermediate ways, compared to the full breaking that occurs in solids.
I am not prepared to accept that glass is a solid without evidence of crystal structure. Crystalline silicon dioxide, however, has another name -- quartz. Quartz is unambiguously solid. It possesses crystal structure as evidenced by the diffraction of probe beams sent through it and has properties quite different from amorphous silicon dioxide.
It would seem a relatively simpler matter to stretch a thin mylar membrane over a frame, like a drum head, apply a partial vacuum to introduce the basic curvature and then control the surface with a combination of loudspeaker-like actuators, operating at frequencies designed to create standing waves of arbitrary shape according to the needs of an adaptive optics control system. This would have the advantages: lightweight, easy to assemble, use of off-the-shelf speaker technology, very high response time, relatively low power requirements, etc. What's not to like?
Why not just spin it, and while it's spinning, lower the ambient temperature so that it freezes?
I don't know for sure (chemistry isn't my field) but several potential issues occur to me immediately:
Your question isn't a dumb one but it might be very challenging to accomplish if it is even possible.
"Pretend"??? Are you sure that glass is not a liquid?
http://www.phys.ncku.edu.tw/mirrors/physicsfaq/General/Glass/glass.html
Otherwise your body will purge it.
Not entirely true; it takes months or years for your body to eliminate mercury. Otherwise, you are correct, it takes a good dose or constant exposure to cause permanent, serious damage.
http://en.wikipedia.org/wiki/Mercury_poisoning
Even so you are likely breaking all sorts of laws and end up contaminating everything nearby if you are playing with mercury.
It's surely been thought of already, but would it be possible to mount the spinning mercury bowl in a centrifuge?
Set it at the rotational speed where the bowl points to the angle you want, and position the telescope above it. It would line up with the optics once per rotation, use a shutter to control the exposure.
Quite sure. It's an amorphous (non-crystaline) solid. That's the first reference I've ever seen that tried to define all amorphous solids as glasses. Who would consider waxes to be glass?
Socialism: a lie told by totalitarians and believed by fools.
A solid is rigid (a material with a viscosity greater than 10^13.6 Pa s). Some solids are crystaline, some are amorphous. Amorphous solids include glass, wax, some semiconductors, even some food.
Socialism: a lie told by totalitarians and believed by fools.
I can guarantee you can find a 5cm glass mirror superior in cost, quality and usability than this thing. Adaptive optics have come a long way toward eliminating the need for a theoretically perfect parabola. If your only practical outcome is a 100m zenith scope on mars - we already know how to do that cheap enough. So if you find anything in the article(s) that show a real advantage, clue me in. I do see they have come up with new ferromagnetic carriers that are kind of cool. Can anyone explain what percent improvement they get with this design vs. segmented mirrors or even radios and is it enough to offset the response times?
I'm not sure the second point is valid. I took "Queue the debate:" as direction for the debaters to begin lining up after his post.
..every word coming out of the US these days that applies to particular challenges HAVE to be war related? Why are the "combating" this problem? Really guys lighten up?
Now if we can just get these stable when faced horizontally, I see an awesome revisiting of the Archimedes Death Ray.
"Nine times out of ten, starting a fire is not the best way to solve the problem." - my wife
There's got to be an obvious reason that this notion sucks...?
That, and the fact that mercury is an extremely toxic and hazardous element that has to be carefully handled and disposed of.
No more than a hundred other compounds commonly found in a house. Less than many, in fact. There's been a bit of an overreaction against mercury.
Do you use etched glass to protect the aluminum foil hanging in your bathroom ? Or do you just go down to the lake and stare into it till it freezes?
-- I guess he did ask for a debate, I'm just piling on.
How about spinning the bowl of mercury facing up, then swinging the bowl around a horizontal axis? Like a bucket of water swung at arm's length over one's head then back down, then back up, in a cycle, the way kids show each other "centripetal force"? The momentum would keep the concave surface intact as the whole contraption spun and swung around. Then the contraption could be rotated on the other horizontal axis, pointing the concave mirror at whichever direction was desired.
The mirror would point in that direction only intermittently, as the mirror swung past that point in its arc. But the image sensor could be sampled only at that moment, as the position synced with the desired direction.
All of that swinging would have to be calibrated to compensate for the interaction of the various axes of spin. But that all sounds like a set of DSPs could do it, with a laser interferometer keeping the cycles synced and sampling at the right timeslot.
--
make install -not war
Large Synoptic Survey Telescope, although they had to grind it anyway after it had cooled down (three months later).
http://www.newscientist.com/article/dn13606-giant-telescope-project-begins-with-a-spin.html/
There is already a liquid mercury telescope at UBC in Canada - the Large Zenith Telescope. With a 6 meter diameter, it's one of the largest telescopes in the world. Of course, it's limited to viewing only a narrow range of angles near the zenith, due to gravitational constraints. Even so, it was stunningly cheap compared to other telescopes of its size, and provides decent value for money.
http://www.astro.ubc.ca/lmt/lzt
If freezing the mercury would help, you can be sure they thought of it. It's not just freezing to provide a fixed surface, though. As others noted, the surface reflectivity of a metal changes on freezing, and there are too many geometric distortions associated with the phase change so that polishing would be required after freezing (just like for glass). Furthermore, metals have large thermal expansion coefficients, so a metal-based mirror (whether frozen mercury or frozen silver, etc.) would need extraordinarily good thermal management, which is difficult to provide in a structure which is necessarily open to the atmosphere. Glasses are used for telescope mirrors partly because they have much fewer thermal issues (many other advantages, also).
Those who can make you believe absurdities can make you commit atrocities. - Voltaire
Here's the link to website of the research group at Laval University.
http://wood.phy.ulaval.ca/
They've been publishing on liquid mirrors since the '80...
"Want to make a great concave mirror for your telescope? Put a drop of mercury in a bowl and spin the bowl."
I am not sure that's a good piece of advice for an average consumer who wants to build a telescope, which article indeed suggests.
Mercury is highly toxic, toxic enough in quantities of ca 50 ug/m3 (mercury vapour), so if you are going to be spinning mercury for a mirror, now you have been warned. Exposure through inhalation of said vapour and through the eyes, and exposure through skin absorption are all causes for mercury poisoning.
Drinking it might not be as dangerous as one would think, since you will excrete most of what you have consumed. Mercury vapours *are* more dangerous. Drinking motor oil is also more dangerous. And eating sushi might also be dangerous, because it contains highly toxic mercury organic compounds that your body actually assimilates quite easily (which is why you'd find many advices about how much tuna to eat in a year at maximum).
Mercury is used by dentists, many people (including myself) have tooth fillings with mercury in them. And you know what? I'm not much concerned about the prospect that it breaks and I swallow it, since if I do, it will poison me less than my yearly consumption of sashimi does. Given that it takes years to purge mercury from your body, I guess I have already accumulated more mercury.
Mercury might be extremely toxic, but this is only when it is mishandled. If you know how to handle it, it's not dangerous, and I'd say that shouldn't be a cause for much concern. Also, I hope that these liquid telescopes would be placed in outer space or on the moon, so it is even less a problem. I doubt anyone makes pressurized space telescopes, and mercury separate from your pressurized environment and environmental suit is completely harmless. You can do whatever you want with it with almost zero risk. If it somehow gets inside either, you'd have much bigger problems to worry about.
Keep the mercury spinning straight up, but use high quality mirrors to reflect the wanted image down the tube. There you go.
Glass lacks a crystal lattice, thus it is not a solid.
Glasses have short-range order, not long-range order, and the terms "solid" and "fluid" as used by physicists are based on mechanical properties, not structural properties.
By your proposed definition, which is not used by any physicist anywhere, liquid crystals would be solids, even though they flow easily.
So your proposed definition completely decouples the flow behaviour from the terms "solid" and "liquid". Given the ordinary meanings of these words, it is most epistemologically efficient to adopt new terms, like "amorphous solid" and "liquid crystal" in recognition that ordering and flow properties are only weakly related, rather than to redefine "liquid" and "solid" to be completely unrelated to the flow properties of a substance.
Blasphemy is a human right. Blasphemophobia kills.
Back when I was a kid, they used mercury in fever thermometers. When one would break, we kids would play with the mercury. With all the hazardous crap I've been exposed to (they declared a place I use to go dirt bike riding a superfund site, and as teenagers we'd go swimming in the strip mines) it's a wonder I'm still alive at the ripe old age of 58, let alone more active than some people half my age.
My dad had a bit of wisdom he imparted to me: Don't believe anything you hear (or read) and only half of what you see. Speaking of him, he was an electrical lineman for 40 years, exposed to intense electrical fields up on those towers, PCBs they used for transformer oil, and all sorts of other things people are scared shitless of these days. He's 79 and still goes square dancing every Saturday night.
I guess it runs in the family.
Free Martian Whores!
Quite sure. It's an amorphous (non-crystaline) solid. That's the first reference I've ever seen that tried to define all amorphous solids as glasses. Who would consider waxes to be glass?
Well, since waxes are liquid, they must be glass by definition, right? ;-)
No he doesn't.
*boggle*
Socialism: a lie told by totalitarians and believed by fools.
Because you want a lot of smaller mirrors as they can adapt to a lot more situations. Only one of these mirrors can be parallel (well, tangential, really) to the ground.
The fact that you are modded informative is just more proof of this weird trend to value armchair common sense more than hard science.
Let's look at what some obscure website named Wikipedia has to say:
Case control studies have shown effects such as tremors, impaired cognitive skills, and sleep disturbance in workers with chronic exposure to mercury vapor even at low concentrations in the range 0.742 g/m3. A study has shown that acute exposure (4 8 hours) to calculated elemental mercury levels of 1.1 to 44 mg/m3 resulted in chest pain, dyspnea, cough, hemoptysis, impairment of pulmonary function, and evidence of interstitial pneumonitis. Acute exposure to mercury vapor has been shown to result in profound central nervous system effects, including psychotic reactions characterized by delirium, hallucinations, and suicidal tendency. Occupational exposure has resulted in broad-ranging functional disturbance, including erethism, irritability, excitability, excessive shyness, and insomnia. With continuing exposure, a fine tremor develops and may escalate to violent muscular spasms. Tremor initially involves the hands and later spreads to the eyelids, lips, and tongue. Long-term, low-level exposure has been associated with more subtle symptoms of erethism, including fatigue, irritability, loss of memory, vivid dreams, and depression.
And now the kicker: Mercury constantly vaporizes at room temp. Just keep an open jar around and be eligble for all of the above. Alternatively, just spill a bit of this stuff to get the same long-term benefits. And not for a few weeks, only. We are talking about years and decades. And _that_ is why something as simple as a cleaning lady wiping up a broken thermometer with her rag and continuing to clean the rest of the building can result in 9 months of complete renovation.
As for your statement that the body somehow purges mercury? Not happening. Heavy metals are keepers. Which is why human bodies are considered toxic waste, at least in Germany.
I am the first to agree that this one doctor calling frankfurters a high-risk food because you can choke on them is ridiculous. But that does not mean you get to disregard every word of caution.
Seems the wiki paste ate some chars. The most notable being that the constant exposure of mercury with which said workers experienced symptoms is
0.7-42 micrograms per m^3
More reading:
http://en.wikipedia.org/wiki/Mercury_(element)#Safety
http://en.wikipedia.org/wiki/Mercury_poisoning
What short range order do they have? They are isotropic and homogeneous. They have no broken symmetry.
Actually, my definition is common in the field of condensed matter physics -- the branch of physics that concerns itself with phase transitions, symmetry and order parameters.
Here is one page that delves into some of these ideas. I can dig up some peer-reviewed articles if you still don't believe me.
My proposal is that liquid and solid both refer to particular phases of matter with well-defined symmetry properties. That is, if I conduct a diffraction experiment I can predict the properties of the diffraction pattern that emerges. Liquid crystals are an intermediary phase of matter, as I believe I pointed out in my prior post. They possess an intermediate degree of symmetry breaking and will only exhibit broken symmetries in special directions.
"Amorphous solid" simply refers to a fluid whose flow rate is insignificant to us. Given full control over temperature and pressure I could cause it to enter a gas phase without crossing any phase transition. In what way is such a substance solid?
Liquid crystal is an informal term for material existing in an intermediary phase between solid and liquid so that it possesses both crystalline properties and fluid properties. Any serious scientist will specifically refer to the phase of matter that they are talking about (e.g. Smectic A, twisted nematic, etc.).
If you read the "liquid crystal" article you linked, you will find many references to other phases of matter. This could also have suggested to you that defining phases of matter in terms of broken symmetries is not a foreign or unique idea in condensed matter physics.