February 26, 2024

Next-generation space telescopes could use deformable mirrors to image Earth-sized worlds

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The Roman Space Telescope Coronagraph during static optics assembly at NASA’s Jet Propulsion Laboratory. Credit: Dr. Eduardo Bendek

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The Roman Space Telescope Coronagraph during static optics assembly at NASA’s Jet Propulsion Laboratory. Credit: Dr. Eduardo Bendek

Observing distant objects is no easy task, thanks to our planet’s thick, fluffy atmosphere. As light passes through the upper parts of our atmosphere, it is refracted and distorted, making it much more difficult to discern objects at cosmological distances (billions of light years away) and small objects in adjacent star systems, such as exoplanets.

For astronomers, there are only two ways to overcome this problem: sending telescopes into space or equipping telescopes with mirrors that can adjust to compensate for atmospheric distortion.

Since 1970, NASA and ESA have launched more than 90 space telescopes into orbit, and 29 of them are still active, so it’s safe to say we have everything under control.

But in the coming years, an increasing number of ground-based telescopes will incorporate adaptive optics (AOs) that will allow them to perform cutting-edge astronomy. This includes the study of exoplanets, which next-generation telescopes will be able to observe directly using coronagraphs and self-adjusting mirrors. This will allow astronomers to obtain spectra directly from their atmospheres and characterize them to see if they are habitable.

NASA is pursuing the development of adaptive optics through its Deformable Mirror Technology project, which is carried out at Caltech’s Jet Propulsion Laboratory and sponsored by the NASA Strategic Astrophysics Technology (SAT) Division of Astrophysics and the NASA Small Business Innovation Research programs ( SBIR).

The research is being led by JPL’s Dr. Eduardo Bendek and Adaptive Optics Associates (AOX) Program Manager Dr. Kevin King.

Direct imaging of exoplanets

The field of exoplanet studies has exploded in recent years, with 5,539 confirmed candidates in 4,129 systems and more than 10,000 awaiting confirmation. Finding habitable planets among these many candidates is crucial to solving one of the greatest mysteries of all time: are we alone in the universe?

Thanks to advances in instrumentation, advanced analytics, and data sharing, the field is transitioning from discovery to characterization. However, to date, most exoplanets have been discovered through indirect methods.

To do this effectively, scientists need to be able to observe exoplanets directly. This is known as the direct imaging method, where astronomers study light reflected directly from an exoplanet’s atmosphere and/or surface. This light is then analyzed with spectrometers to determine its chemical composition, allowing astronomers to narrow down habitability.

Unfortunately, it is very difficult to resolve smaller rocky planets that orbit closer to their parent stars – which is where Earth-like planets are expected to be found – due to the overwhelming brightness of their stars.

This is likely to change with cutting-edge telescopes like James Webb, as well as next-generation arrays like the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT). These ground-based arrays will combine 30-meter primary mirrors, advanced spectrometers and coronagraphs (instruments that block starlight). Deformable mirrors are an essential component of a coronagraph, as they can correct the telescope’s smallest imperfections and remove any remaining contamination from starlight.

This is essential, as a misalignment between the mirrors or a change in the shape of the mirror – i.e. leading to instability in the telescope’s optics – can result in a glow that obscures the detection of smaller rocky exoplanets. Furthermore, detecting an Earth-like planet requires extremely precise optical quality of 10s of picometers (pm) – approximately the size of a hydrogen atom. This requires very precise control of a telescope’s mirrors in real time, which can correct for any sources of interference.

Deformable mirrors

Deformable mirrors (DM) rely on precisely controlled pistol-like actuators to change the shape of a reflective mirror. For ground-based telescopes, DMs allow you to adjust the optical path of incoming light to correct for external disturbances (such as atmospheric turbulence) or optical misalignments or defects in the telescope.

For space telescopes, DMs do not need to correct for Earth’s atmosphere, but rather very small optical perturbations that occur as the space telescope and its instruments heat and cool in orbit.

Ground-based deformable mirrors have been tested and offer state-of-the-art performance, but further developments are needed for space-based DMs that will be used in future missions.

Two main DM actuator technologies are currently being developed for space missions: electrostrictive technology and electrostatically forced Micro-Electromechanical Systems (MEMS). For the first, actuators are mechanically connected to the DMs and contract to modify the mirror surface when voltages are applied. The latter consists of mirrored surfaces being deformed by an electrostatic force between an electrode and the mirror.

Several NASA-sponsored contract teams are advancing DM technology, including MEMS DMs manufactured by Boston Micromachines Corporation (BMC) and electrostrictive DMs manufactured by AOA Xinetics (AOX). Both BMC mirrors were tested under vacuum conditions and subjected to launch vibration testing, while the AOX mirrors were also vacuum tested and qualified for spaceflight.

While ground-based DMs have validated the technology – such as BMC’s coronagraph instrument at the Gemini Observatory – steps must be taken to develop DMs for future space telescopes.

Future observatories

NASA plans to demonstrate the effectiveness of DMs with a chronograph technology demonstrator that will launch aboard the Nancy Grace Roman Space Telescope (RST) in May 2027.

Lessons learned from this demonstration will help create an even more sophisticated system for the Habitable Worlds Observatory (HabEx). This proposed NASA mission will directly image planetary systems around Sun-like stars (scheduled to launch in 2035). HWO will require DMs with up to approximately 10,000 actuators, each of which will rely on high voltage connections – which will be a major design challenge.

HWO would also involve unprecedented wavefront control requirements, up to single-digit picometers and a stability of around 22h/hour. These requirements will drive not only the development of DM technology, but also the electronics that control it, since resolution and stability largely depend on the quality of the command signals sent by the controller. Ensuring this requires implementing filters to remove any electronic noise.

This work will be overseen by NASA’s Astrophysics Division, which is preparing a Technology Roadmap to further advance DM performance to enable HWO.

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