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Illustration of a rocky exoplanet in space

Full Steam Ahead

Our staff and contributors leveraged STScI's Russell B. Makidon Optics Laboratory’s fully functional testbeds to execute several successful projects to help future space-based telescopes spot distant planets.

Every 10 years, astronomers around the world contribute their ideas for the next great telescopes, including flagship space-based observatories. "Pathways to Discovery in Astronomy and Astrophysics for the 2020s," the decadal survey on astronomy and astrophysics, was released in November 2021. Instead of one large space-based telescope, the committee recommended a fleet of observatories that observes light across the electromagnetic spectrum. The first facility in this lineup is a large space telescope that collects infrared, visible and ultraviolet light, and captures high-contrast images and data known as spectra. This observatory will rely on ultra-stable optics that can achieve very high contrast imaging—technology that the institute's Russell B. Makidon Optics Lab is helping to pioneer.

Since its founding in 2013, our lab's staff members have worked to advance technologies for future generations of segmented space telescopes in the areas of optical mirror alignment, wavefront sensing and control, and coronagraphy to capture images of distant worlds. Despite the ongoing pandemic, their remote setup, accessible 24/7, allowed them to make significant progress on a number of projects in 2021.

Maintaining a Telescope's Imaging Stability

Two charts show a central orange circle encircled by yellow and purple. The purple circle at left is lighter than the purple circle at right.
Each image shows an artificial star that is blocked by a coronagraph at the center. The starlight has been almost canceled out in the circular purple/black area, where potential planets could be identified. The image at left shows a coronagraph image produced in the lab in November 2020. One year later, the staff significantly improved the contrast—the purple area at right is far darker than at left.

Staff and contributing graduate students located around the world took full advantage of the lab's now fully functional testbeds, leading to major progress on a number of projects. First up: controlling for the extremely tiny drifts telescopes experience in space. One graduate student wrote an algorithm designed to stabilize a telescope's coronagraph, an instrument that needs an incredibly stable environment to block a star's light and create a dark zone to spot potential planets orbiting it. The project introduces random disturbances to a miniature testbed telescope for 10 hours, watching as the algorithm sensed the drifts and continued to stabilize the instrument's dark zone. This year, the project achieved an incredible stability, maintaining a contrast ratio of three parts in 10 million in the presence of these tiny drifts.

The team also spent time tackling the same issue, drifts and instability, with an adaptive optics system installed on one of the lab's testbeds. After upgrading to full a closed-loop control system, they successfully sensed the disturbances in the air on the testbed and corrected for them to achieve similar contrast ratios of three parts in 10 million. The next step is to run these approaches in parallel, work that will begin in 2022.

Another member of the team was granted time to use the W. M. Keck Observatory in Hawaii to test a similar algorithm on a ground-based telescope. Although it's impossible to achieve the same clarity found in space due to the Earth's atmosphere, the algorithm consistently reduced the brightness of image artifacts, which occur due small misalignments inside the observatory, by a factor of four or five.

Aligning Mirror Segments to Act as One

Next, the team turned to the challenges introduced by segmented mirrors on a telescope. Once in space, each segment must align precisely to view the same distant galaxy or star and return a single, unified image. Another graduate student worked to develop mathematical models to predict what the tolerances are for each mirror segment to align and remain stable, particularly for observations that exceed several hours. This year, the first theoretical model was developed and proven to accurately predict what the tolerances are. The segments at the center have very little room for error, while those along the edges have slightly more. 

Computers sent into space have to be far simpler than those supported and maintained on the ground. Another graduate student wrote an entirely new algorithm designed to use less memory and demonstrated it in the lab, a first. The replacement? Far more complicated math—but the theoretical and experimental outcomes proved to be the same. A telescope's coronagraph can now achieve the same dark zone with the new, memory-light algorithm that can work in space. 

With each project, the optics lab team is gaining significant momentum: achieving darker dark zones to spot planets more quickly and becoming more sensitive to small disturbances to ensure long, stable telescope observations are possible. Our staff, which includes the next generation of professionals—the graduate students who are actively completing groundbreaking projects—are well positioned to make substantial contributions to all future space-based telescopes.