The lightweight, ground-based telescope was 3D printed in about a third of the time that a traditionally manufactured telescope would have taken, for about a fifth of the cost. The researchers used a modular design with the 3D printed components as well as image-correction algorithms for the telescope’s optical design.
There are two ways to approach precision, says Sandia – to make every piece to exact tolerances for simple assembly, or to make less precise pieces and compensate with a highly precise assembly process. Machining results in parts with very precise dimensions, but it can’t handle the complex, unconventional geometries that 3D printing is capable of. 3D printing forms the part and the material at the same time, and research is still being conducted into how the properties of the materials are affected, and if those changes in properties matter.
“Can we design a system that doesn’t care if your material is not as good as you expected it to be? Can you design a system that doesn’t care that your parts aren’t as dimensionally accurate?” asked mechanical engineer Ted Winrow, who led the project. “If you make yourself insensitive to the things that additive’s not very good at, you take advantage of all its good things.”
As an example, a typical camera has a ledge that needs to be very precise, as its position determines where the lens sits. Sandia, when creating lenses for the telescope, created a straight cylinder with no ledges.
Winrow said that instead, “we hold the lens at a very precise position using very precise tooling. We hold the lens in the right spot and then we inject epoxy around it and lock it into place. We can make parts that are less precise as far as dimensions are concerned because of the epoxy in the process. It’s the tooling that’s precise.”
Sandia applied for a patent for a monolithic titanium flexure that is part of the telescope mirror mount. A flexure is a broad range of elements that are used like joints between rigid bodies. Bending the element produces the joint motion. It’s not possible to rigidly mount metal to glass because the two materials expand and contract at different rates as the temperature changes, causing the glass to deform or even crack.
A flexure acts like a spring, though it isn’t coil-shaped. Sandia’s design is cylindrical, about two inches long and 3/4 inch in diameter, with very thin flexure blades. Three flexure mounts attach to the mirrors with epoxy, relieving contraction and expansion stress where the mirrors attach to a carbon fiber backbone.
The precision mechanical design team worked with Sandia optical designer Jeff Hunt and algorithm authors Dennis Lee and Eric Shields on the project. According to Winrow, the lens design creates a raw image with distortions and other errors, but the software algorithms correct for those errors.
“The thought was you could have less precise optics and correct for it with software, essentially after the fact,” Winrow said. “Similar to how we designed the mechanical hardware to be insensitive to additive manufacturing shortfalls and take advantage of its benefits, Jeff optimized the optics of the system so the software maintained the image properties the algorithms could not have done as good a job correcting. You could get the same performance you could have if you spent three times as much money on better optics.”
The project has ended, but Sandia National Laboratories is still using the information that they got from it, according to Winrow.
“That was what the project was looking at, how these ways could make it faster and cheaper and just as good,” he said. “If you talk about things you can give up, things you can compensate for after the fact, it opens up realms on the design side.”
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