Compact, reflective, easy to manufacture mirrors are a critical component for advancing astronomical technology in space. Mirrors are a key component in most telescopes, though they are notoriously hard to manufacture with the necessary precision, especially at large scales. A new paper from researchers in the UK uses additive manufacturing to make a thin, flexible, and lightweight mirror out of aluminum and analyzes its properties to see if it will be useful in applications such as CubeSats.
Before the researchers could make the actual mirror, they had to make several decisions about its design. In choosing a lattice, they decided to use a “split-p internal lattice”, which basically looks like a honeycomb. In finite element analysis (FEA) modeling, it was shown to be the most robust. The team also designed a mounting structure that utilized four printed circuit boards (PCBs) as mounting rods, which was part of the overall CubeSat bus design the system was going for.
In running further FEA models, the researchers found they could reduce the weight of the mirror by about 56%, close to the 60% it was originally designed for. They then went about actually manufacturing five prototypes, which was designed in an annular shape with the outer ring approximately 84mm an an inner one at 32mm. The printing was completed using a standard laser-bed powder fusion process commonly used to 3D print metals. In this case, the metal was AlSi10Mg, a typical alloy of aluminum used in the 3D printing.
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But since this was intended for use as a mirror, the print underwent further processing once it was initially laid down. Two underwent Hot Isostatic Pressing, which heats the print and then applies pressure to it in an effort to reduce its porosity and increase its surface uniformity. Chasing a smooth surface, the researchers subjecting four samples to single-point diamond turning, a post-processing technique that removes the top layer of material using a precise diamond-coated lathe.
After post-processing the samples were analyzed both internally and externally. To scan a samples internals the researchers used X-ray Computed Tomography, and found that there were small pores throughout the structure, and they seemed to track along the path of the laser used in the fusion process. There was a higher density of them around the perimeter, where the laser turns around.
Surface roughness is a key metric of mirrors used in telescopes, and all of the samples had less than 8nm of surface roughness, though those subjected to HIP were slightly rougher than the ones milled off with a diamond. The tradeoff was that the HIP process did in fact reduce porosity, making the material stronger internally. HIP processed mirrors also had a higher Total Integrated Scatter value, probably because of their higher surface roughness. Which also means they were bad for use in a telescope.
Details on CubeSat Optics from the 2021 CubeSat Developers Workshop. Credit – CubeSat YouTube Channel
There were also scratches scattered throughout the surface, seemingly caused by an alloy of titanium and vanadium. That would imply the feedstock aluminum alloy might have been inadvertently mixed with another metal powder.
Despite these setbacks, the process described in the paper represents a step forward for the development of CubeSat suitable, lightweight flexible mirrors. In the future, the researchers plan to add a chromium optical coating to the surface to try to improve the surface quality of the samples. They’ll also test the thermal flexibility of the system, testing what it would be like in an actual space environment. As the mirrors continue their path up the technology readiness levels, there will be an increasing demand from an increasing number of CubeSat manufacturers who will need inexpensive, robust, lightweight mirrors.
Learn More:
I. Aziz et al – Additive manufacturing in aluminium of a primary mirror for a CubeSat application: manufacture, testing and evaluation.
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