NASA Engineers Complete First ‘Center of Curvature’ Test on James Webb Space Telescope

Engineers conducting a white light inspection of the James Webb Space Telescope, currently located in the clean room at NASA's Goddard Space Flight Center, Greenbelt, Maryland. Photo NASA/Chris Gunn
Engineers conducting a white light inspection of the James Webb Space Telescope, currently located in the clean room at NASA’s Goddard Space Flight Center in Greenbelt, Md. Photo Credit: NASA/Chris Gunn

The James Webb Space Telescope (JWST) has completed another significant milestone toward becoming the most powerful space telescope ever built: the finished primary mirror just underwent an optical measurement test called the Center of Curvature test. In essence, this is a “before” and “after” measurement of the mirror, both before the telescope undergoes more rigorous mechanical testing which could affect the mirror’s capabilities, and then again after.

According to Ritva Keski-Kuha, the test lead and NASA’s Deputy Telescope Manager for JWST at NASA’s Goddard Space Flight Center in Greenbelt, Md.: “This is the only test of the entire mirror where we can use the same equipment during a before and after test. This test will show if there are any changes or damages to the optical system.”

The mechanical tests are essential before the telescope is launched, scheduled for October 2018, since the telescope will experience violent sound and vibration environments inside the rocket. The shape or alignment of the mirror could be affected or even adversely affect its performance once JWST is in space.

The same optical measurement is then taken after the mechanical tests for comparison, to ensure that the telescope can survive the launch and function properly in orbit.

The five layers of the sunshield, which will protect the telescope in space. Photo Credit: Northrop Grumman
The five layers of the sunshield, which will protect the telescope in space. Photo Credit: Northrop Grumman

The optical measurement tests consist of using an inferometer to measure the shape of JWST’s primary mirror with incredible precision. The optics in the mirror need to be extremely accurate, even more than waves of visible light which are less than a thousandth of a millimeter long. By using wavelengths of light to make very tiny measurements, engineers can avoid physical contact with the mirror, reducing the chances of any physical damage occurring such as scratches. The inferometer records and measures the tiny ripple patterns which result from different beams of light mixing and their waves combining or “interfering” with each other.

More specifically, the Center of Curvature test measures the shape of the primary mirror by comparing the light reflected off it to a computer-generated hologram depicting what the exact shape should be; the inferometer compares the two with astounding precision.

“Interferometry using a computer-generated hologram is a classic modern optical test used to measure mirrors,” said Keski-Kuha.

“We have spent the last four years preparing for this test,” said David Chaney, who is JWST’s primary mirror metrology lead at Goddard. “The challenges of this test include the large size of the primary mirror, the long radius of curvature, and the background noise. Our test is so sensitive we can measure the vibrations of the mirrors due to people talking in the room.”

After engineers make sure that the mirrors are perfectly aligned in the first Center of Curvature test, the launch environmental tests will follow. Then the Center of Curvature test will be repeated and compared to the first test to ensure that the mirrors remain aligned.

The primary mirror actually consists of 18 smaller hexagonal mirrors, making it kind of look like a giant puzzle piece. These mirrors will allow JWST to see deeper into space (and thus further back in time) than ever before, to when the first stars and galaxies were forming. Infrared sensitivity will help astronomers compare them to today’s largest galaxies.

Last month, a sunshield, consisting of five sunshield membrane layers, was completed on the telescope. This sunshield, designed by Northrop Grumman in Redondo Beach, Calif., will prevent background heat from the Sun from interfering with the telescope’s infrared sensors. Each of the five layers is as thin as a human hair and the entire sunshield is the size of a tennis court. The layers in the sunshield can reduce temperatures by approximately 570 degrees Fahrenheit, and each successive layer, made of kapton, is cooler than the one below. The final layer was delivered to Northrop Grumman Corporation’s Space Park facility on Sept. 29, 2016. Protecting the telescope when it is in space is of course just as important as during the launch.

Another view of the completed primary mirror of JWST. Photo Credit: NASA/Chris Gunn
Another view of the completed primary mirror of JWST, consisting of 18 smaller hexagonal mirrors. Photo Credit: NASA/Chris Gunn

“The completed sunshield membranes are the culmination of years of collaborative effort by the NeXolve, Northrop Grumman and NASA team,” said James Cooper, JWST Sunshield manager at Goddard. “All five layers are beautifully executed and exceed their requirements. This is another big milestone for the Webb telescope project.”

The sunshield and the rest of the telescope will fold origami-style into the Ariane 5 rocket for launch.

“The groundbreaking sunshield design will assist in providing the imaging of the formation of stars and galaxies more than 13.5 billion years ago,” said Jim Flynn, Webb sunshield manager at Northrop Grumman Aerospace Systems. “The delivery of this final flight sunshield membrane is a significant milestone as we prepare for 2018 launch.”

As Greg Laue, sunshield program manager at NeXolve, also noted, “The five tennis court-sized sunshield membranes took more than three years to complete and represents a decade of design, development and manufacturing.”

As reported earlier this year, the Integrated Science Instrument Module (ISIM), known as the “scientific heart” of JWST, completed its last round of essential cryogenic tests.

According to Begoña Vila, NASA’s Cryogenic Test Lead for the ISIM at Goddard: “We needed to test these instruments against the cold because one of the more difficult things on this project is that we are operating at very cold temperatures. We needed to make sure everything moves and behaves the way we expect them to in space. Everything has to be very precisely aligned for the cameras to take their measurements at those cold temperatures [for] which they are optimized.”

Diagram showing different parts of JWST. Image Credit: NASA
Diagram showing different parts of JWST. Image Credit: NASA
Some of the JWST team members outside a full-scale model of the telescope at Goddard Space Flight Center. Photo Credit: NASA
Some of the JWST team members outside a full-scale model of the telescope at Goddard Space Flight Center. Photo Credit: NASA

The JWST mission, often dubbed as the successor to the Hubble Space Telescope, will be an exciting one, allowing astronomers to learn even more about distant galaxies and exoplanets. JWST will look at distant exoplanets and the dust clouds where new stars and planets are being born, as well as search for the molecular building blocks of life. It will be able to directly image some larger exoplanets orbiting brighter stars by using coronagraphs, and it will also be able to study the atmospheres of those exoplanets. The telescope is named after a former NASA administrator, James Webb.

“The James Webb Space Telescope will be the premier astronomical observatory of the next decade,” said John Grunsfeld, astronaut and associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. “This first-mirror installation milestone symbolizes all the new and specialized technology that was developed to enable the observatory to study the first stars and galaxies, examine the formation stellar systems and planetary formation, provide answers to the evolution of our own Solar System, and make the next big steps in the search for life beyond Earth on exoplanets.”

The James Webb Space Telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency. More information is available at two NASA websites, here and here.


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  1. Thank you Paul Scott Anderson for the interesting update on the James Webb Space Telescope which will be in an L2 orbit beyond the Moon.

    “Spacecraft at Sun–Earth L2

    Spacecraft at the Sun–Earth L2 point are in a Lissajous orbit until decommissioned, when they are sent into a heliocentric graveyard orbit.

    1 October 2001 – October 2010—Wilkinson Microwave Anisotropy Probe[22]
    July 2009 – 29 April 2013—Herschel Space Telescope[23]
    3 July 2009 – 21 October 2013—Planck Space Observatory
    25 August 2011 – April 2012—Chang'e 2,[24][25] from where it travelled to 4179 Toutatis and then into deep space
    January 2014 – 2018—Gaia Space Observatory
    2018 — James Webb Space Telescope will use a halo orbit
    2020 — Euclid Space Telescope
    2024 — Wide Field Infrared Survey Telescope (WFIRST) will use a halo orbit
    2028 — Advanced Telescope for High Energy Astrophysics (ATHENA) will use a halo orbit"

    And, “‘Lunar Far-Side Communication Satellites’ Earth–Moon L2 NASA Proposed in 1968 for communications on the far side of the Moon during the Apollo program, mainly to enable an Apollo landing on the far side—neither the satellites nor the landing were ever realized.[43]”

    From: ‘Lagrangian point’ Wikipedia

    Note also:

    “The JWST will operate near the Earth-Sun L2 (Lagrange) point, approximately 1,500,000 km (930,000 mi) beyond the Earth. By way of comparison, Hubble orbits 340 miles (550 km) above the earth’s surface, and the Moon is roughly 250,000 miles (400,000 km) from Earth. This distance makes post-launch repair or upgrade of the JWST hardware virtually impossible.”

    From: ‘James Webb Space Telescope’ Wikipedia

    However, perhaps “post-launch repair or upgrade of the JWST hardware” and various other spacecraft in Sun–Earth L2 orbits may become much more doable if quiet “negotiations” are implemented.

    “During the latest round of negotiations in Houston last month, the ISS partners narrowed down the list of potential modules that would comprise their periodically visited habitat. According to the provisional plan, four key pieces made the cut for the first phase of the assembly, which is penciled in to take place from 2023 to 2028 in lunar orbit: The spartan outpost will include the U.S.-European space tug, a Canadian robot arm, a pair of habitation modules from Europe and Japan, and an airlock module from Russia.”

    And, “This hardware would hitchhike on NASA’s giant SLS rocket, along with the Orion crew vehicle at the top of each booster.”

    From: ‘Why NASA May Ferry the First Cosmonaut to the Moon
    The big space agencies are planning their next project together—a human outpost that orbits the moon.’ By Anatoly Zak Nov 7, 2016

    ‘Food’ for thought.

  2. It is not clear whether the mirror will subjected to launch liftoff and maxQ acoustic environments while in the folded launch configuration after the baseline center of curvature test and before the final center of curvature test. Basic tenants of “test-like-you-fly” dictate that the mirror should be in the flight configuration that applies (folded) when the significant mechanical acoustic environments will be present (liftoff and maxQ).

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