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 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.
“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.”
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.”
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