Less than 36 months from its launch, the James Webb Space Telescope (JWST), a NASA, European Space Agency (ESA), and Canadian Space Agency (CSA) collaboration touted as “Hubble’s successor,” continues to take shape right on schedule. Yesterday, NASA announced that the telescope’s Integrated Science Instrument Module (ISIM), described as the “scientific heart” of JWST, has completed its last round of essential cryogenic tests. In addition, the telescope has been deemed “officially optically complete.” NASA announced that JWST received the last mirrors in its optical path on Sunday, March 6th. Engineers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have been working around the clock to ensure JWST is ready to fly in October 2018, even working right through this winter’s severe snowstorms.
ISIM Completes Third And Final Round of Cryogenic Tests
NASA announced that the ISIM was removed from the Space Environment Simulator (SES), a large, 40-foot-tall thermal vacuum chamber located at Goddard. These tests are critical, as JWST will operate at the second Lagrangian point (L2), located one million miles away from Earth. Here, the telescope will be exposed to temperatures as frigid as -387 degrees Fahrenheit (40 degrees Kelvin). NASA underscored that this temperature is 260 degrees lower than any recorded on our planet, in any location.
The elephant-sized ISIM has already undergone two rounds of cryogenic testing in a bid to see how the components will respond to the beyond-bone-chilling temperatures it will encounter at L2. The telescope cannot be serviced once it is in space (unlike the Hubble Space Telescope, which functions in low Earth orbit), so engineers must ensure it is 100 percent ready to work prior to launch.
Begoña Vila, NASA’s Cryogenic Test Lead for the ISIM at NASA Goddard, emphasized the crucial nature of these tests: “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 test results were reported as positive. Jamie Dunn, NASA’s ISIM Manager for JWST at Goddard, enthused, “This is the culmination of a lot of hard work by a lot of people who have been working for many, many years. This final test was phenomenal, everything is working spectacularly well.”
The ISIM also recently underwent electromagnetic interference testing, key in discovering how these influences will affect the spacecraft. A previous AmericaSpace article discussed these tests at length: “The ISIM, described as the “brain” of JWST, underwent a series of tests where it not only was exposed to electromagnetic emissions, but also emitted these signals in a bid to gauge how these forces will affect the rest of the spacecraft. While 10 days were allotted for these tests, which occurred in an ‘anechoic’ (‘no echo’) chamber, engineers were able to complete the tests in 8.5 days, ahead of schedule. These tests are critical because the ISIM contains the Webb’s essential scientific instruments, which include its Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec), Mid-Infrared Instrument (MIRI), and Fine Guidance Sensor/Near InfraRed Imager and Slitless Spectrograph (FGS/NIRISS).”
JWST Receives Final Flight Mirrors, Is “Officially Optically Complete”
NASA also announced that shortly after completion of the ISIM cryogenic testing, the JWST received its final mirrors in its optical path. This news comes a little more than a month after the telescope’s large primary mirror was completely assembled at NASA Goddard. Optical completeness is a huge milestone for JWST, underscored Lee Feinberg, the Webb’s Optical Telescope Element Manager at NASA Goddard: “We can now say ‘we have a telescope’…it’s a huge milestone many years in the making.”
The JWST possesses an innovative, unique optical system design; it doesn’t resemble a “classic” telescope. Many “Webb followers” are familiar with the appearance of its primary mirror: 18 distinctive hexagonal mirror segments consisting of beryllium, coated in gold for a better infrared reflective quality (JWST will function primarily in infrared). However, JWST has several other mirrors: a secondary convex mirror, a tertiary concave mirror, and the fine steering mirror, described by NASA as “a moveable turning flat mirror.”
The space agency described how the concave tertiary mirror and fine steering mirror function as the sophisticated optical system’s “anchor”: “These two mirrors are located inside the Aft Optics Subsystem (AOS), which is a phone booth-sized beryllium structure surrounded in black covering. It is located right in the center of the primary mirror, and about half of it sticks up above the primary mirror. After incoming light hits the expansive primary mirror, it is directed onto the small circular secondary mirror, which reflects it back in the direction of the primary mirror and into the AOS. Inside the back end of the AOS is the tertiary mirror, where light bounces forward to the fine steering mirror at the ‘front’ of the AOS, which then reflects it out the back of the AOS to a focus behind the primary mirror for the scientific instruments.”
This system will allow JWST to literally function as a “time machine,” looking back at the faint emissions signaling the beginnings of our Universe.
What’s Next For The Webb?
Next up, the ISIM will be integrated into the telescope’s structure. Bill Ochs, the Webb telescope Project Manager at NASA Goddard, related, “The completion of these major milestones represent huge achievements for NASA and our industry, European, Canadian, and academic partners. ISIM and the telescope only reached these milestones because of the passion, dedication, and imagination of an outstanding group of individuals. The next major step is assembling the instrument module and the telescope together to complete the entire ‘cold’ section of the Webb observatory.”
NASA also recently released a list of other milestones that must occur prior to JWST’s launch. NASA Goddard is responsible for metrology, vibration, and acoustic tests of the telescope and its instruments. NASA’s Johnson Space Center in Houston, Texas is responsible for the optical test of the telescope and its instruments in Chamber A. The telescope’s main contractor, Northrop Grumman, will assemble the spacecraft, finish and integrate a sunshield that will further control temperature extremes, will finish general assembly, make observatory-level tests, and will ultimately transport the spacecraft to French Guiana. If all goes as planned, space watchers can expect to see JWST climb into space aboard an Ariane 5 launch vehicle from Europe’s Spaceport on an early morning in October 2018.Missions » James Webb Telescope »