Editorial Feature

The Infrared Sensors Defining the James Webb Space Telescope

The James Webb Space Telescope (JWST) was – after some delay – finally able to transmit images of deep space back to Earth in July, 2022. These pictures show the universe in unprecedented levels of detail, which is largely a result of the telescope’s cutting-edge design and implementation of infrared sensors.

A large pink, speckled galaxy resembling a wheel with a small, inner oval, with dusty blue in between on the right, with two smaller spiral galaxies about the same size to the left against a black background.

A large pink, speckled galaxy resembling a wheel with a small, inner oval, with dusty blue in between on the right, with two smaller spiral galaxies about the same size to the left against a black background. Image Credit: NASA, ESA, CSA, STScI

Webb’s first images, presented by US president Joe Biden in a press event that was viewed around the world, revealed some of the most distant and faintest objects we have ever seen.

NASA administrator Bill Nelson said:

Webb's First Deep Field is not only the first full-color image from the James Webb Space Telescope, it’s the deepest and sharpest infrared image of the distant universe, so far. This image covers a patch of sky approximately the size of a grain of sand held at arm’s length. It’s just a tiny sliver of the vast universe.

This accomplishment is the result of global cooperation, with the European Space Agency (ESA) and Canadian Space Agency (CSA) working in partnership with NASA to send Webb beyond Earth’s atmosphere.

The Webb project is decades in the making, and engineers have solved numerous unique and extreme challenges to make the space’s largest telescope work.

Highlights: First Images from the James Webb Space Telescope (Official NASA Video)

Video Credit: NASA/YouTube.com

Innovations in fields from thermal control systems to optics and detectors – and many more – combine on the JWST, and they are responsible for the captivating images that NASA is currently sending out.

These innovations enable Webb to meet a number of criteria that mean its images are truly out of this world:

  • It is very far away from Earth; it is not in Earth’s orbit but the Sun’s, closely following Earth to maintain the data link.
  • It is the largest telescope in space; its dish had to be folded up like a metal colander for launch and then the telescope self-assembled once it got into space.
  • It uses cutting-edge infrared and near-infrared sensing to detect the faintest objects we have ever seen.

Why Does Webb Use Infrared?

Webb uses a broad range of infrared light, and this is a major reason for its ability to see so far into space.

When galaxies are extremely distant from us, we can no longer detect them with the full range of electromagnetic radiation that they initially emit.

This is because the light that comes from has been stretched in its journey through time and space to reach us. The expanding universe causes the wavelengths to stretch so that they are longer, making the light shift toward redder frequencies.

This is known as cosmological redshift, and its discovery in the last century provided evidence for an expanding universe.

The broad range of infrared wavelengths that the Webb picks up means it is capable of seeing the farthest galaxies – the ones whose light has shifted most toward red and infrared.

Infrared Sensors in Space

There are two different kinds of detectors on board the James Webb Space Telescope. Mercury cadmium telluride (HgCdTe) or H2RG detectors sense energy in the near-infrared wavelengths (0.6 - 5.0 μm), while arsenic doped silicon (Si:As) detectors capture mid-infrared wavelengths (5.0 - 28.0 μm).

The near-infrared H2RG detectors in the telescope were manufactured by California, US-based company Teledyne Imaging Sensors. The mid-infrared detectors were made by another California company, Raytheon Vision Systems.

Each of the H2RG detectors has around 4 million pixels; each of the mid-infrared detectors has around 1 million pixels.

In space, Webb’s giant mirrors gather light and direct it toward the sensors. Filters first disperse the light into spectra, then focus it onto the detectors. Each instrument carries its own detectors, which absorb photons and convert them into electronic signals that can be measured.

The detectors need to be both extremely sensitive to detect the faint light that comes from the most distant galaxies, and also arrayed over large areas to survey the sky efficiently. The Webb arrays have less noise, a larger format, and more durability than any other infrared sensor array that has been produced.

The HgCdTe material in the H2RG detectors is tuned by varying the ratio of mercury and cadmium in the mix, making them sensitive to different wavelengths of light.

On Webb, two compositions of HgCdTe were applied so that each near infrared sensor was calibrated to perform over the two set wavelengths it would be used in (0.6 μm - 2.5 μm and 0.6 μm - 5.0 μm).

Each type of infrared detector on Webb has a similar sandwich-like configuration, composed of a thin semiconductor absorber layer, an indium layer that joins each pixel in the absorber layer to the readout, and a silicon readout integrated circuit or ROIC that reads out pixels.

The first images from the Webb mark the dawn of a new era in astronomy. As the infrared sensors in the space telescope keep finding new data in the night sky, our understanding of the universe will get deeper and further than ever before.

Continue reading: Unearthing an Ancient Universe; the James Webb Space Telescope

References and Further Reading

NASA (2022). Infrared Detectors. [Online] NASA. Available at: https://jwst.nasa.gov/content/about/innovations/infrared.html 

NASA (2022). NASA Reveals Webb Telescope’s First Images of Unseen Universe. [Online] NASA. Available at: https://www.nasa.gov/press-release/nasa-reveals-webb-telescope-s-first-images-of-unseen-universe 

NASA (2022). President Biden Reveals First Image from NASA’s Webb Telescope. [Online] NASA. Available at: https://www.nasa.gov/press-release/president-biden-reveals-first-image-from-nasa-s-webb-telescope 

NASA (2022). Webb Spinoffs. [Online] NASA. Available at: https://jwst.nasa.gov/resources/JWST_spinoffs_v122011.pdf 

Rauscher, B.J., et al (2014). New and Better Detectors for the JWST Near-Infrared Spectrograph. PASP. doi.org/10.1086/677681.

Rieke, G.H. (2007). Infrared detector arrays for astronomy. Annual Review of Astronomy and Astrophysics. doi.org/10.1146/annurev.astro.44.051905.092436.

Webb, S. (2022). A cosmic time machine: how the James Webb Space Telescope lets us see the first galaxies in the universe. [Online] The Conversation. Available at: https://theconversation.com/a-cosmic-time-machine-how-the-james-webb-space-telescope-lets-us-see-the-first-galaxies-in-the-universe-187015 

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Ben Pilkington

Written by

Ben Pilkington

Ben Pilkington is a freelance writer who is interested in society and technology. He enjoys learning how the latest scientific developments can affect us and imagining what will be possible in the future. Since completing graduate studies at Oxford University in 2016, Ben has reported on developments in computer software, the UK technology industry, digital rights and privacy, industrial automation, IoT, AI, additive manufacturing, sustainability, and clean technology.


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