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."
The perfect shape comes from the spinning liquid: the bowl doesn't have to have any particular shape. You can even use a flat-bottomed bowl, you just need more mercury.
"Flat-bottomed bowls, you make the liquid scope go 'round..." -- Freddie Mercury
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.