By Stefanie Schneider
As humanity develops, what lies beyond the Earth has always been an area of unknowns. On December 25th, 2021, NASA took a big step further into the unknown and launched their James Webb Space Telescope. This occurred on an Ariane 5 rocket launched from French Guiana. The program is international, involving NASA, the European Space Agency, and the Canadian Space Agency.
The Telescope’s Design
Building off of the discoveries made by the Hubble Telescope, the Webb telescope uses infrared light to see further through gas clouds that block visible light, allowing it to explore new aspects of our universe. As the largest telescope sent into space, it has large segmented mirrors to ensure that it can capture the necessary light, along with sun shields the size of tennis courts to prevent stray infrared light from obstructing the observation. In order to be able to launch a telescope of this size, engineers designed it to be folding, with the mirrors and sun shields having the ability to unfold once the telescope is released into space.
Infrared Technology
Webb uses infrared detectors as the basis for its ability to view space. The mirrors, after collecting the light, send it to be filtered and then focused on the detectors. The infrared detectors absorb light photons and convert them into electronic voltages, to allow them to be measured. The new design of the detectors for this telescope allows them to be more efficient with less noise and more durability.
Two different detector types are used by the telescope: mercury-cadmium-telluride (HgCdTe or H2RG) for near-infrared and arsenic doped silicon (Si:As) for mid-infrared. These each detect different wavelengths, near infrared being the shortest wavelength and mid infrared being longer (but not as long as far infrared). These detectors have the same basic structure with three main sections. The first is a thin semiconductor absorber layer and the second is a layer of indium which connects the absorber layer to the third layer, being a silicon readout integrated circuit to read the pixels with controlled outputs.
The detector begins its work when the semiconductor first absorbs a photon. This layer outputs mobile electron hole pairs, which are moved by electric fields to be collected and measured. Typically, multiple samples of the same pixel are used to look for signal spikes, indicating that a pixel has been disturbed by a cosmic ray. The data taken by the detectors are able to ‘see’ objects inside clouds that can’t otherwise be seen.
Wavefront Sensing and Control
Aligning the Webb mirror segments once launched is crucial to ensure appropriate light is collected. Webb uses Wavefront Sensing and Control (WFSC) to do so. This technology measures the position of each segment of the mirror and adjusts to the appropriate position. Once this process is completed for every segment, a single reflective surface is created. More specifically, WFSC is able to do so by first taking images of a star. The images are then processed with specific algorithms to identify the appropriate adjustments for each segment. The algorithms have gone through several simulations and demonstrations to refine the process, and the last demonstration use nine individual algorithms to collectively create the telescope.
Cooling Systems
Keeping the telescope’s equipment at appropriate temperatures is crucial to allowing it to function properly and efficiently. One aspect of this seen in the telescope’s design is the sunshield coating. The telescope must be kept below 50 Kelvin in order to be able to see the small infrared emissions from objects in space. The sun shields of the telescope are used as a form of “passive cooling” to block heat from both the Earth and the Sun from heating the telescope. Using five layers of Kapton and aluminum and doped-silicon coating, the shields are able to reflect the heat back to space.
The mid infrared detectors also have specific temperature requirements, needing to be kept at 7 Kelvin in order to detect thermal emissions. In order to ensure these conditions, “active cooling” is used in the form of a pulse-tube cryocooler. It basically functions as in “internal refrigerator” with parts located throughout the entire observatory and it circulates refrigerant (cooled by a precooler) to the detectors.
Discoveries
Since its launch, Webb has made several significant discoveries. For instance, it has identified star-forming regions that have even hinted at future creation of planets. This is the closest star-forming region to Earth and several of the stars are similar size or smaller than the Sun. It has also discovered smoke molecules in a galaxy 12.3 billion light years from Earth, which is the first time they have been found that far away. Webb has additionally spotted several exoplanets and is the only operating telescope with the ability to categorize the atmosphere of exoplanets the size of Earth. It has also captured imaging of several cosmic events, such as stars just before going supernova. Discoveries like these have already provided scientists with vast data to develop further or new understandings of the universe around us and will continue to prove a crucial tool for future discoveries.
Works Citated
Falk, Dan. “Seven Amazing Accomplishments The James Webb Telescope Achieved In Its First Year.” Smithsonian, Smithsonian Institution, 5 July 2023, www.smithsonianmag.com/science-nature/one-year-in-the-james-webb-space-telescope-has-allowed-for-stunning-discoveries-180982466/.
“NASA to Discuss Progress as Webb Telescope’s Mirrors Align.” NASA, NASA, 14 Mar. 2022, www.nasa.gov/news-release/nasa-to-discuss-progress-as-webb-telescopes-mirrors-align/.
“The Observatory.” Webb Space Telescope, webbtelescope.org/webb-science/the-observatory. Accessed 8 Oct. 2023.
“Webb Space Telescope.” NASA, NASA, webb.nasa.gov/index.html. Accessed 8 Oct. 2023.
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