Good heavens: Space telescopes may have steadier view
UW-Madison research may help large-aperture space telescope engineers ensure that future multimillion-dollar instruments provide a clear view of objects in space and on Earth.
Telescope operators need tremendous resolution for viewing small objects or objects at great distances, says Daniel Kammer, a professor of engineering physics. “In order to attain that much precision, you’re starting to look at very small deformations in that optical surface,” he says.
On-ground physical testing and mathematical modeling are key indicators of how these telescopes will perform in space. Particularly important is how they respond to and adjust for vibration, he says. “If you’re worried about being able to predict very small deformations, that means you’ve got to worry about very high-frequency vibrations,” he says.
Researchers compare physical vibration tests with mathematical predictions. If the outcomes match, then they can further simulate how the telescope will operate under a range of load conditions. However, while such test-validated mathematical modeling is standard for even complex spacecraft, it’s not accurate enough for a finely tuned space telescope.
The current technique centers around characterizing a structure’s mode shapes — the shapes in which it naturally vibrates — and frequencies, or rates at which it naturally vibrates. For a spacecraft, that might mean a range of frequencies between zero and 50 hertz. Because a large-aperture telescope is sensitive to even small vibrations, the frequency range widens to about 300 hertz and it’s nearly impossible to differentiate one mode shape from the next. “Before, I had nice techniques for discerning one from another, but now I don’t,” says Kammer. “All the techniques that used to work great just don’t work anymore.”
Researchers have tried to push the old technology to help them deal with the problem, yet have made little progress. “It just doesn’t quite get you to where you need to go,” he says.
As an alternative, Kammer is developing a new model-verification technique based on the structure’s frequency response: If researchers exert a force on a structure at a certain frequency, it responds at that frequency. “I can measure that in a test,” says Kammer. “And I can predict that using my mathematical model.”
Funded by the Air Force Office of Scientific Research, Kammer also is establishing metrics, or measures of how accurate the analysis prediction is in relation to the physical tests.
Ultimately, says Kammer, the Air Force can use the tools he develops to learn how well a space telescope will function in the heavens — before it’s launched — and to develop systems that help manage vibration. “You want to be able to develop a control system on board that is going to be able to sense that very high-frequency vibration, and kill it,” he says. “And in order to be able to do that, you’ve got to have accurate mathematical models of that structure, because that’s built into the control system.”