Deformable Liquid Mirrors For Adaptive Optics 196
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."
dumb question... (Score:4, Insightful)
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.
Re:dumb question... (Score:4, Insightful)
Some liquids possibly lose the reflectivity as a solid that they had as a liquid.
Re:dumb question... (Score:2, Insightful)
Probably because it would be almost impossible to assure a complete uniform freeze -- you have all kinds of complicated temperature and phase transitions between the underlying bowl material, the ferromagnetic fluid and the reflective film/fluid. The stuff would inevitably crystallize and distort in patches, creating a mess of a surface.
Re:bowl? (Score:3, Insightful)
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.
The liquid takes on a shape that minimizes its surface tension. Small imperfections in the bowl don't affect the surface tension and are smoothed over.
the new mercury astronauts (Score:3, Insightful)
Course - making it spin for a long time between maintenance visits (on who knows WHAT vehicle) could be tricky.
Wobble wobble (Score:4, Insightful)
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.
Re:dumb question... (Score:1, Insightful)
Usually frozen stuff floats, in comparison to its liquid state. (Apples, churches, very small rocks.)
Most things (water, metals) are polarized and will create a crystal lattice when cooled. The slower the cool, the bigger the crystals. This is what makes the "temper" of metals - the size of the crystals. When you crystallize anything, the atoms become locked and pulled into a lattice. This then roughs up the surface and destroys the reflection. By having mobile molecules or atoms, you allow a very fine, uniform surface needed for reflection.
The reason why it floats is because with the lattice mesh, it creates voids, which lowers the density.
Adaptive Optics (Score:5, Insightful)
I thought one of the points was that you don't want to fix the shape permanently. Adaptive optics [wikipedia.org] 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.
Re:dumb question... (Score:2, Insightful)
"Ceptripetal Force" (Score:3, Insightful)
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.
Re:dumb question... (Score:3, Insightful)
And yet, this is how they make glass mirrors used in astronomy -- melting the glass in a large shell inside a furnace, then letting it cool while spinning..
While you can do this to get the approximate shape of the mirror, you still need to do additional grinding and polishing to get a nice reflective surface. I'm not sure if anyone does this, but all of us amateur mirror makers usually grind the desired shape into the glass with abrasives.