Making Great Strides

How do we find Earth-like planets around Sun-like stars? One necessary step is to develop a coronagraph that works with a segmented mirror. Coronagraphs are instruments with masks that block the bright light of stars to reveal the much fainter light of planets orbiting them. Researchers working in the institute’s Russell B. Makidon Optics Lab have been working to improve their demonstration of a coronagraph with a segmented mirror for more than a decade. In recent years, their work has served to inform the concept for the next astrophysics flagship mission, the Habitable Worlds Observatory. 

This proposed space-based telescope will be the first specifically engineered to identify habitable, Earth-like planets next to stars like our Sun, and examine them for evidence of life. Here, we share the team’s major 2024 accomplishments — and a new initiative that will help them take another leap forward.

Maximizing Current Experiments

This year, the team completed a Strategic Astrophysics Technology grant from NASA, achieving and exceeding all of the project’s milestones on the lab’s testbed, the High-contrast Imager for Complex Aperture Telescopes (HiCAT). On the testbed is a tiny, model telescope mirror with 37 hexagonal segments — all of which can be tipped and tilted to form a single, focused mirror. That mirror gathers and funnels light from a bright laser (a stand-in for a distant star) into a coronagraph. The coronagraph creates a deep, dark zone around the star image. The darker the zone, the better the faint planets can be seen.

To simulate extremely tiny drifts, which all telescopes experience in space, the team introduced slight disturbances to the testbed that can disrupt the dark zone. In response, the team’s algorithms kick in to realign the mirror segments to recover and maintain the dark zone. 

This year’s experiments also focused on going from narrowband to broadband light. Narrowband is a specific wavelength of light. Broadband is a wider window of light, and each of these wavelengths interact with the coronagraph slightly differently, making it much more difficult to maintain a dark zone. Putting all of these variables together (light and drifts), the team achieved incredible stability and maintained a contrast ratio of 1 part in 40 million in narrowband light and 1 part in 17 million in broadband light. These results exceeded expectations, proving the team is pushing the HiCAT testbed to its limits.

Designing the Next Generation of Tests

Although the HiCAT testbed operates on extremely stable bedrock and has outperformed all of its objectives to date, it has known limitations. The testbed is in a room on Earth filled with air — but space is a vacuum. As is, HiCAT simply can’t match those conditions. Another facet is that its miniscule stand-in mirror is not sufficiently realistic to capture the complexities of a real space telescope. As a result, the team has started building the Active Segmented Surrogate for Integrated Systems Tests (ASSIST) telescope for its next generation of experiments.

ASSIST will be a new telescope stand-in with coarse and fine alignments, just like the smaller mirror on HiCAT. The team already has completed and tested a mirror with seven gold-coated segments in a vacuum chamber the size of a large watermelon on a table in the lab. In the upcoming year, it will be moved to a two-foot diameter by three-foot long vacuum chamber that is being refurbished by undergraduate students. This is tiny compared to the vacuum chambers that test full-scale space telescopes, but simulates the same space-like environment. Once complete, ASSIST could have other useful applications as a stand-in for other experiments beyond STScI to support the Habitable Worlds Observatory. The lab’s future in the coming years is certainly bright.