This video documents the design, construction, and testing of a homemade deformable mirror, a key component in adaptive optics systems used to correct atmospheric distortion in ground-based telescopes. The creator explains the principles of adaptive optics, which involve using a laser-generated "artificial star" and a wavefront sensor to dynamically adjust a mirror's shape to counteract turbulence.
The project involves building a mirror with a thin glass face sheet controlled by an array of 25 custom-made electromagnetic actuators. Two different face sheets are tested: an initial 2.3mm quartz plate and a subsequent, thinner 1.3mm Zerodur zero-expansion glass plate. Using a real-time interferometer for measurement, the creator demonstrates that while the thicker quartz plate worked, it was too stiff for fine control. The thinner Zerodur plate proved much more successful, allowing for the creation of complex, higher-order deformations by individually controlling adjacent actuators. However, a significant design flaw was identified: heat generated by the actuator coils caused the silicon support pillars to expand, creating unwanted thermal deformations on the mirror's surface. The video concludes with reflections on this flaw and ideas for future improvements, such as using PCB-integrated coils or piezo actuators.
Building a DIY Deformable Mirror for Adaptive Optics
0:01:46 - The Principle of Adaptive Optics: The video explains how ground-based telescopes use adaptive optics to counteract atmospheric turbulence. A laser creates an artificial star, a wavefront sensor measures the distortion in its light, and a deformable mirror rapidly changes its shape (on a millisecond timescale) to produce a sharp image.
0:07:56 - DIY Mirror Design: The goal is to build a proof-of-concept deformable mirror. The design consists of a thin glass face sheet glued to 25 electromagnetic actuators. Each actuator is a copper coil surrounding a flexible silicon pillar with a permanent magnet on top, allowing the face sheet to be pushed and pulled.
0:09:58 - Choosing the Right Material: The project tests two mirror materials. The first is a 2.3mm thick quartz plate. The second is a much thinner and more flexible 1.3mm sheet of Zerodur, a special glass ceramic with virtually zero thermal expansion.
14:50 - Actuator Physics Explained: A key insight is that for a given power dissipation (heat), the force generated by an electromagnetic coil is independent of the wire thickness or number of turns. The final actuators were designed to have a 1-ohm resistance and produce about 0.1 Newtons of force at 1 watt.
18:53 - Real-Time Measurement Setup: To test the mirror, an interferometer is used with "Wavefront Pro" software, which allows for near-real-time analysis of the mirror's surface shape. This makes it possible to immediately see the effect of activating the coils.
21:56 - Initial Test with Quartz Plate: The first test with the 2.3mm quartz sheet demonstrated basic functionality, such as creating tilt and simple concave shapes. However, the sheet was too stiff to create complex, "higher-order" deformations, as the effects of adjacent actuators blended together.
27:55 - Successful High-Order Correction with Zerodur: After switching to the thinner 1.3mm Zerodur plate, the mirror performed significantly better. It could be deformed much more easily and, crucially, could produce complex shapes (like a "donut") by pushing and pulling adjacent actuators in opposite directions. This proved the concept could work for correcting complex aberrations.
28:59 - A Major Design Flaw Revealed: The primary issue with the design is thermal instability. Heat from running the actuator coils transfers to the silicon support pillars, causing them to expand and push the mirror face sheet upwards, creating an unwanted bump.
30:04 - Future Improvements: The creator concludes that while the proof-of-concept was successful, a future version would need to address the heat issue. Suggestions include integrating the coils into a PCB with a heat-sinking backplate or replacing the electromagnetic actuators with piezo actuators.