Designing the Next Generations of Telescopes
Staff at the institute’s optics lab are designing technologies that will support future large space-based observatories operating a million miles from Earth.
What will space-based telescopes that launch in the 2030s look like? Since astronomers will still be focused on studying Earth-like exoplanets, these observatories will need large, segmented mirrors that fold gracefully to fit into rockets for launch—and collect significantly more light than smaller, single-piece mirrors.
Directly spotting these elusive exoplanets is possible with coronagraphs, instruments that suppress the light of distant stars to reveal their orbiting exoplanets. In 2018, scientists and engineers at the institute’s Russell B. Makidon Optics Laboratory took a major step toward this goal by demonstrating proof of concept for a high-contrast coronagraph with a segmented mirror—a first worldwide.
The optics lab staff is currently testing how to align and fine-tune segmented mirrors through tiny deformations of each mirror segment. They are also designing prototypes of coronagraphs for different telescopes. Finally, they are continuing to calibrate their testbeds to become more and more stable to support these initiatives.
In 2018, the lab received a three-year grant to fund its activities with its partners: the Johns Hopkins University’s Department of Mechanical Engineering in Maryland; ONERA (the French Aerospace Lab); Laboratoire d’Astrophysique de Marseille in France; Observatoire de la Côte d’Azur in France; the University of Rochester in New York; NASA’s Goddard Space Flight Center in Maryland; NASA’s Exoplanet Exploration Program at the Jet Propulsion Lab in California; and Ball Aerospace in Colorado.
Orchestrating Segmented Mirrors
Compare the light moving through a telescope to the notes a violin section in an orchestra produces. If the first chair is a half a beat late, you will notice. They have to be perfectly in sync to make a big, cohesive sound. Space-based telescopes with segmented mirrors also have to act in coordination: Every one of the mirror segments leaves an imprint on light—and so does each gap between the mirrors. Each segment must be properly coordinated to organize the light into clear images—acting like a single piece of glass with a perfect concave shape.
In previous years, the team had demonstrated they could correct errors using deformable mirrors to “tune” light from a single mirror. In 2018, the team installed the segmented deformable mirrors and other optics to simulate a segmented telescope. With that device in place, the team was able to create a shadow 1 million times fainter than the laser “star” they observed through their coronagraph. This is no easy feat since that model has 37 mirror segments.
Building on Each Success
Every accomplishment the lab reports is preceded by another piece of foundational work. By the end of 2017, the optics lab staff had the basic infrastructure in place on the High-contrast Complex Aperture Telescope (HiCAT) testbed to enable basic coronagraph and wavefront control. In 2018, they built on these results by introducing the segmented aperture and demonstrating first light through the complete system.
Testing and running the hardware naturally includes a slew of automation. Recent advancements now allow the testbed to run at night, which also allows tests to run in a quieter environment, and does not require lab staff to be present.
The team also successfully implemented software—provided by their partners in France—known as the COronagraphic Focal-plane waveFront Estimation for Exoplanet detection (COFFEE for short). COFFEE measures distortions in the light as it moves along its path and helps to relay that information to the deformable mirrors to correct them.
The Makidon optics lab staff began assembling another wavefront sensor, similar to the one designed for the Wide Field Infrared Survey Telescope (WFIRST) and essential for detecting small changes in the mirror alignment over time. This device will play a central role in the lab’s work in the coming year.
Controlling for Vibrations
Another essential element of their work is mechanically improving the testbeds to become more and more stable to keep the camera as steady as possible over long periods. Take a moment to think about all the jitters in the air around you now.
Is the air conditioning on? Are people walking by? These movements cause vibrations in the air and ground, and each one hits the testbed. To improve stability, you start with the most obvious noise and keep identifying new sources until your system is as controlled and stable as it can be without being in a vacuum chamber.
While our team works to create stability at a small scale now, this is also a problem other teams that support future large telescopes will face. They will have a goal of restricting some motions to be smaller than an atom. One of the lab’s main 2018 achievements, done in close collaboration with Ball Aerospace, allowed the team to begin identifying the most important specific stability requirements to image Earth-like planets.
Overall, our scientists and engineers, in collaboration with their partners, made enormous progress this year by building a first proof of concept of a complete system. They will work to expand these tests significantly in 2019.