A vertically-mounted black disk with a concentric pattern of reflective disks is illuminated under a red light. A large number of copper wires run away from the the disk to a breadboard.

Deforming A Mirror For Adaptive Optics

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 air at different temperatures turbulently mixing. To correct for this, some larger observatories use a laser to create an artificial star in the upper atmosphere, observe how this appears distorted, then use shape-changing mirrors to correct the aberration. The physical heart of such a system is a deformable mirror, the component which [Huygens Optics] made in his latest video.

The deformable mirror is made out of a rigid backplate with an array of linear actuators between it and the thin sheet of quartz glass, which forms the mirror’s face. Glass might seem too rigid to flex under the tenth of a Newton that the actuators could apply, but everything is flexible when you can measure precisely enough. Under an interferometer, the glass visibly flexed when squeezed by hand, and the actuators created enough deformation for optical purposes. The actuators are made out of copper wire coils beneath magnets glued to the glass face, so that by varying the polarity and strength of current through the coils, they can push and pull the mirror with adjustable force. Flexible silicone pillars run through the centers of the coils and hold each magnet to the backplate.

A square wave driven across one of the actuators made the mirror act like a speaker and produce an audible tone, so they were clearly capable of deforming the mirror, but a Fizeau interferometer gave more quantitative measurements. The first iteration clearly worked, and could alter 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 frontplate with a thinner sheet of glass-ceramic, such as he’s used before, which let actuators 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 was concave.

This isn’t [Huygens Optics]’s first time building a deformable mirror, but this is a significant step forward in precision. If you don’t need such high precision, you can also use controlled thermal expansion to shape a mirror. If, on the other hand, you take it to the higher-performance extreme, you can take very high-resolution pictures of the sun.

Tiny Tesla Valves Etched In Glass

While it’s in vogue right now to name fancy new technology after Tesla, the actual inventor had plenty of his own creations that would come to bear his namesake, including Tesla coils, Tesla oscillators, Tesla turbines and even the infamous Tesla tower. One of the lesser known inventions of his is the Tesla valve, a check valve that allows flow in one direction without any moving parts, and [Huygens Optics] shows us a method of etching tiny versions of these valves into glass.

The build starts out with a fairly lengthy warning, which is standard practice when working with hydroflouric acid. The acid is needed to actually perform the etching, but it’s much more complicated than a typical etch due to the small size of the Tesla valves. He starts by mixing a buffered oxide etch, a mix of the hydroflouric acid, ammonia, and hydrochloric acid, which gives a much more even etching than any single acid alone. Similar to etching PCBs, a protective mask is needed to ensure that the etch only occurs where it’s needed. For that there are several options, each with their own benefits and downsides, but in the end [Huygens Optics] ends up with one of the smallest Tesla valves ever produced.

In fact, the valves are so small that they can only be seen with the aid of a microscope. While viewing them under the microscope he was able to test with a small drop of water to confirm that they do work as intended. And, while the valves that he is creating in this build are designed to work on liquids, [Huygens Optics] notes that the reason for making them this small was to make tiny optical components which they are known for.

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