The Decadal Survey Testbed: Demonstrating Coronagraphs to Search for Life in the Universe
This blog post originated in the 2018 Science Mission Directorate Science and Technology Report.
Decadal Survey Testbed (DST)
This advanced testbed enables the laboratory demonstration of coronagraph instruments capable of directly imaging Earth-like exoplanets and searching for signs of life from a future space observatory.
Earth-like planets orbiting other stars can be 10 billion times dimmer than their host star. To detect these planets and to search for spectroscopic signatures of life in their atmospheres, a space telescope must be used, and the light from the host star must be blocked so the planet comes into view. For the first time, a next-generation testbed is now available for demonstrating coronagraph instruments capable of imaging these planets.
The Astrophysics Division’s Exoplanet Exploration Program (ExEP) Office has commissioned a testbed to demonstrate new coronagraph technology for imaging nearby Earth-like planets with a future space telescope. The Decadal Survey Testbed (DST) is a new optical bench built from the ground up with extreme thermal and vibrational stability in a vacuum chamber with a coronagraph instrument.
The search for life in the universe is one of NASA’s loftiest goals. Thanks in part to NASA’s Kepler mission, we know that planets orbit nearly every star. Determining evidence for life on the abundant planets in the galaxy will involve measuring visible-band and near-infrared spectra of the planets to look for the presence of biosignature gases like oxygen, carbon dioxide, water, and possibly methane. NASA is now conceiving future space missions capable of spectral characterization of distant planets. These include LUVOIR, Habitable Exoplanet Observatory (HabEx), and Origins concept studies commissioned by NASA.
An Earth-like planet is 10 billion times dimmer than a Sun-like star, and the planet and star are very close to each other in angular distance on the sky. An essential step in directly measuring a distant planet is to perform starlight suppression, reducing the light from the host star enough that the planet can come into view. One technique to perform this suppression uses a coronagraph instrument on a space telescope. This instrument uses a series of intricate masks to block light from the star while allowing light from the orbiting planet to pass through to a camera. While coronagraphs have been in operation since the 1930s for observing our Sun and have been used for astronomy on the Hubble Space Telescope, an advanced stellar coronagraph capable of studying a distant Earth-like planet has not yet been built.
Achieving a factor of 10 billion in contrast is a huge challenge, mainly because any imperfection in the optics can diffract light around the masks. Future space telescopes could have segment gaps and secondary mirror support structures that also diffract the incoming starlight in a way that is challenging for a coronagraph to handle. Vibrations on a spacecraft and heating and cooling of the space telescope can introduce disturbances in the optical path that a coronagraph must handle and compensate dynamically. Most of this correction is done using deformable mirrors: flat, flexible mirrors that have a grid of tiny pistons behind the reflective surface that can reshape it to a small fraction of the wavelength of light, to the picometer scale.
The Coronagraph Instrument (CGI) technology demonstrator on NASA’s Wide Field Infrared Survey Telescope (WFIRST), scheduled for the late 2020s, is making essential progress in maturing exoplanet imaging technology for space, but CGI is not designed to achieve the factor of 10 billion improvement in contrast needed to find Earth-like planets. Beyond CGI, coronagraph designers have developed advanced masking techniques and deformable mirror technology that, in principle, can achieve the factor of 10 billion in contrast. But NASA needs a way to demonstrate that this technology works in a space-like environment.
The DST meets this need with a new advanced testbed that eliminates disturbances from the laboratory and local environment as much as possible. At its foundation, the DST consists of a carbon-fiber optical table that is highly stable to thermal fluctuations and includes active temperature control on all optics. The bench is mounted on vacuum-compatible vibration isolators to reduce sensitivity to seismic and laboratory vibrations. Every mirror in the system is designed to be thermally stable, and a fast tip/tilt steering mirror will actively suppress residual mechanical vibrations. Two deformable mirrors allow the optical imperfections to be corrected. The entire testbed is inserted into a 12-m long vacuum chamber at JPL, eliminating effects of the atmosphere and better simulating the space environment.
The team at JPL assembled and aligned the DST, finally inserting the bench into the vacuum chamber in the summer of 2018. The initial commissioning tests include demonstrating the performance of a Lyot coronagraph. As of February 2019, the team has shown that the testbed is sensitive enough to detect a planet 1x10-10 times as bright as the star at high significance. In practice, the choice of observation strategies and data post-processing will allow for further improvement of this detection limit. Later in 2019 and 2020, the team will add additional complexity by simulating the effects of a segmented primary mirror and aim for the same extreme starlight suppression.
The DST is available to investigators funded via NASA’s SAT program to demonstrate new coronagraph ideas. As the HabEx and LUVOIR teams develop scientific and technical plans for future direct imaging missions, this new testbed will help NASA advance technology capable of discovering life around distant planets.
Astrophysics Division’s ExEP Office
Drs. Keith Patterson & Byoung-Joon Seo, NASA JPL
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