As frustrating as having an atmosphere can be for physicists, it’s just as bad for astronomers, who have to deal with clouds, atmospheric absorption of certain wavelengths, and other irritations. One of the less obvious effects is the distortion caused by the turbulent mixing of air at different temperatures. To correct this, some larger observatories use a laser to create an artificial star in the upper atmosphere, observe how it appears distorted, and then use shape-shifting mirrors to correct the aberration. The physical heart of such a system is a deformable mirror, a component that (Huygens Optics) created in its latest video.
The deformable mirror consists of a rigid backplate with a set of linear actuators between it and the thin sheet of quartz glass that forms the mirror face. Glass may seem too stiff to flex under the tenth of a Newton that the actuators could apply, but everything is flexible when you can measure accurately enough. Under an interferometer, the glass flexed visibly when pressed by hand, and the actuators created enough deformation for optical purposes. The actuators are made of coils of copper wire placed under magnets glued to the glass face, so that by varying the polarity and intensity of the current passing through the coils, they can push and pull the mirror with an adjustable force. Flexible silicone pillars run through the center of the coils and hold each magnet to the backplate.
A square wave transmitted to one of the actuators caused the mirror to act like a loudspeaker and produce an audible tone, so they were clearly able to distort the mirror, but a Fizeau interferometer gave more quantitative measurements. The first iteration clearly worked and could change the concavity, tilt, and coma of an incoming light wavefront, but adjacent actuators would cancel each other out if they acted in opposite directions. To give him more control, (Huygens Optics) replaced the glass faceplate with a thinner glass-ceramic sheet, like the one he used before, which allows the actuators to oppose their neighbors and shape the mirror in more complex ways. For example, the center of the mirror could have a convex shape, while the rest is concave.
This is not the first time that (Huygens Optics) has built a deformable mirror, but it is a significant step forward in terms of precision. If you don’t need such precision, you can also use controlled thermal expansion to shape a mirror. If, on the other hand, you push the highest performance to the extreme, you can take photos of the sun at very high resolution.
