From Webb: “NASA Seeks Picometer Accuracy” 

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July 19, 2016
Lori Keesey
NASA’s Goddard Space Flight Center

Team Develops New Tool to Assure Ultra-Stable Space Telescopes

Finding and characterizing dozens of Earth-like planets will require a super-stable space telescope whose optical components move or distort no more than a few picometers — a measurement smaller than the size of an atom. It also will require next-generation tools with which to assure that level of stability.

With NASA funding, a team of scientists at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has begun working with an Arizona-based company to develop a highly sophisticated laboratory tool — a high-speed interferometer — capable of assuring picometer-level stability, a feat not yet accomplished.

At a Goddard cleanroom, technicians unveil the James Webb Observatory’s segmented mirror in preparation for an alignment test this summer. The tool used to determine the segments’ alignment has inspired Goddard technologists to create another that offers picometer accuracy for next-generation observatories. Credits: NASA/Chris Gunn

New Tool to Assure Picometer-Level Stability

To help NASA reach this next level of precision, Saif and his Goddard colleague, Lee Feinberg, have begun working with 4-D Technology, of Tucson, Arizona, to develop the instrument.

Like all interferometers, the instrument would split light and then recombine it to measure tiny changes, including motion. With this tool, technicians would measure distortions in mirror segments, mounts, and other supporting telescope structure primarily during thermal, vibration, and other types of environmental testing.

Displacements and movement occur when materials used to build the optics shrink or expand due to wildly fluctuating temperatures, such as those experienced when traveling from Earth to the frigidity of space or when exposed to fierce launch forces more than six-and-a-half times the force of gravity.

If optics must conform to a specific prescription to carry out a challenging mission, even nearly imperceptible, atomic-size movements caused by thermal and dynamic changes could affect their ability to gather and focus enough light to distinguish a planet’s light from that of its parent star — to say nothing of scrutinizing that light to discern different atmospheric chemical signatures, Saif said.

Leveraging Instrument Developed for Webb Testing

The effort leverages a similar instrument that 4-D Technology created to test the optics of the Webb Observatory, which will be the most powerful observatory ever built once it launches in October 2018. From its orbit 930,000 miles from Earth, it will study every phase in the history of our universe, from the first luminous glows after the Big Bang to the evolution of our own solar system. Among many other firsts, Webb will carry a 21-foot primary mirror made of 18 separate ultra-lightweight beryllium segments that unfold and adjust to shape after launch.

To carry out its job, the Webb Observatory also must be highly stable. However, the movement of its materials is measured in nanometers — the unit of measure that scientists use to determine the size of atoms and molecules.

“What we did was measure the surface of each mirror after each environmental test to see if we could see changes,” Saif said. “I started questioning, what if something behind the mirror moves. Just measuring the surface isn’t enough.”

To assure nanometer-level stability — 4-D Technology worked with the Webb Observatory team at Goddard to develop a dynamic laser interferometer that instantaneously measured displacements in the mirror segments as well as those in their mounts and other structural components, despite vibration, noise, or air turbulence.

“The high-speed interferometer actually enables you to do nanometer dynamics for large structures,” Saif said. “This is absolutely new. The instrument is four orders of magnitude more sensitive than other measurement tools and it measures the full surface of the mirrors.” That instrument now is used in laboratories, manufacturing areas, clean rooms, and environmental-testing chambers operated by the project’s major contractors.

LUVOIR-Type Mission Ups the Ante

However, a next-generation LUVOIR-type mission will demand even greater stability, and consequently an instrument capable of quickly measuring picometer displacements, which are two orders of a magnitude smaller than an atom. Although it is possible to calculate picometer movements with existing tools, the physics are non-linear and the resulting calculations might not accurately reflect what actually is going on, Saif said.

“Every subsystem needs to be designed on a picometer level and then tested at picometers,” Saif explained. “You need to measure what you’re interested in and the instrument needs to calculate these motions quickly so that you can understand the dynamics.”

The team is developing the tool with $1.65 million in funding from NASA’s Cosmic Origins Strategic Astrophysics Technology program. It expects to complete the work in four years.

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The James Webb Space Telescope will be a large infrared telescope with a 6.5-meter primary mirror. Launch is planned for later in the decade.

Webb telescope will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

Webb telescope was formerly known as the “Next Generation Space Telescope” (NGST); it was renamed in Sept. 2002 after a former NASA administrator, James Webb.

Webb is an international collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center is managing the development effort. The main industrial partner is Northrop Grumman; the Space Telescope Science Institute will operate Webb after launch.

Several innovative technologies have been developed for Webb. These include a folding, segmented primary mirror, adjusted to shape after launch; ultra-lightweight beryllium optics; detectors able to record extremely weak signals, microshutters that enable programmable object selection for the spectrograph; and a cryocooler for cooling the mid-IR detectors to 7K.

There will be four science instruments on Webb: the Near InfraRed Camera (NIRCam), the Near InfraRed Spectrograph (NIRspec), the Mid-InfraRed Instrument (MIRI), and the Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS). Webb’s instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range. It will be sensitive to light from 0.6 to 28 micrometers in wavelength.
Webb has four main science themes: The End of the Dark Ages: First Light and Reionization, The Assembly of Galaxies, The Birth of Stars and Protoplanetary Systems, and Planetary Systems and the Origins of Life.

Launch is scheduled for later in the decade on an Ariane 5 rocket. The launch will be from Arianespace’s ELA-3 launch complex at European Spaceport located near Kourou, French Guiana. Webb will be located at the second Lagrange point, about a million miles from the Earth.

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