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Mint Explainer: How James Webb shoots through cosmic dust

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But if you’re a professional photographer, the dust on your sensors will prevent light from reaching that specific area on your camera. As a result, the pixels in that region will appear as black spots in the photos. This is why professional photographers take a lot of care when shooting in dusty places. But even they can always clean the lens since the camera is in their hands or in their reach.

But what about huge telescopes like the James Webb Space Telescope (JWST) that are not on ground but orbiting in space to take pictures of the early universe?

Hubble, for instance, is in low-Earth orbit that is located approximately 375 miles (600 km) away from the Earth while Webb is being operated about 1 million miles (1.5 million km) away from the Earth. So how do these cameras see through the space dust, and how do these lenses remain clean despite being millions of miles away from Earth?

To begin with, JSWT uses infrared cameras to see through interstellar dust. This is similar to how firefighters use infrared cameras to see and rescue people through the smoke in a fire, or how forest officials use night-vision goggles to spot animals. Of course, this is done on an unimaginably exponential scale.

But what are infrared waves or infrared light?

Almost all TV remotes use near-infrared (near-, mid-, and far-infrared are terms) to allow you to switch channels. Near-infrared is that portion of radiation that is just beyond the visible spectrum. Scientists, for instance, can also study vegetation from space using reflected near-infrared radiation that can be sensed by satellites.

Infrared itself is part of the electromagnetic (EM) spectrum that the human eye cannot see but detect as heat. The EM spectrum comprises (in order of highest to lowest): gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. Gamma rays have the highest energies, the shortest wavelengths, and the highest frequencies in the EM spectrum. Radio waves rank lowest in the order.

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All electromagnetic radiation is basically light but we can only see a small portion of this radiation which we call visible light. Source: NASA

To be sure, all electromagnetic radiation is basically light but we can only see a small portion of this radiation which we call visible light. However, visible light (what we humans rely on) cannot detect very cool and faint objects in the universe such as planets, cool stars and nebulae. Hence, astronomers use infrared light because it penetrates dust clouds (stars and planets form inside those dust clouds) better than visible light.

Further, since infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption, they are used to reveal objects in the universe that cannot be seen in visible light using optical telescopes where the dense gas clouds block their light.

Moreover, objects that have a temperature similar to the Earth emit most of their radiation at mid-infrared wavelengths. These temperatures are also found in dusty regions, forming stars and planets. Hence, mid-infrared radiation allows scientists to see the glow of the star and planet formation taking place with the help of an infrared-optimized telescope.

Also, since the universe is expanding, we encounter redshifting which means that the light which is emitted as ultraviolet or visible light is shifted more and more to redder wavelengths, into the near- and mid-infrared part of the light spectrum.

Shedding more light on mirrors

As you may have realised by now, JWST uses infrared instruments to see past cosmic dust and help scientists study the origins of the universe and the formation of galaxies, stars and planets. That said, a telescope’s sensitivity (details it can see) is also directly related to the size of the mirror area that collects light from the objects being observed.

JSWT’s primary mirror is much larger than that of Hubble (2.7 times larger in diameter, or about six times larger in area) to allow it to gather more light. Its infrared instruments, too, have longer wavelength coverage and greatly improved sensitivity than Hubble, according to NASA.

JSWT's primary mirror is much larger than that of Hubble (2.7 times larger in diameter, or about six times larger in area). Source: NASA

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JSWT’s primary mirror is much larger than that of Hubble (2.7 times larger in diameter, or about six times larger in area). Source: NASA

JSWT’s mirrors (the 18 hexagonal-shaped mirror segments are made of beryllium which is light weight and stable at cold temperatures. The segments were designed to fit in the rocket and unfold in space) too are coated with gold to optimize them for infrared light since that latter absorbs blue light but reflects yellow and red visible light. The gold, too, is coated with a thin layer of amorphous SiO2, i.e. glass to protect it.

But infrared light posed another problem. The mirrors will need to be super cooled (around -240 degrees centrigrade) in deep space to avoid emitting an infrared glow (since warm objects emit infrared light) that could blind the faint infrared light from distant galaxies and mar the images.

And, of course, JSWT uses a high-frequency radio transmitter to transmit all this data at 28 MBps—about 57GB of data per day. An artificial intelligence(AI)-powered model dubbed Morpheus has been trained on UC Santa Cruz’s Lux supercomputer (that includes 28 GPU (graphic processing units) nodes with two NVIDIA V100 Tensor Core GPUs each). This machine will help scientists decode the full-colour images from the $10 billion space telescope (https://webb.nasa.gov/).

Finally, how does NASA keep the mirrors stable and clean in space?

Before launching on December 25, 2021, on an Ariane 5 rocket, the James Webb Space Telescope underwent months of commissioning where its mirrors were aligned, and its instruments were calibrated to its space environment besides ensuring that there was no contamination by the special wrapping provided in the mobile clean room (https://www.nasa.gov/feature/goddard/2018/nasas-webb-telescope-wrapped-in-a-mobile-clean-room). Webb’s mirrors, too, were polished to accuracies of less than one millionth of an inch. This is critical to get the sharpest images when the mirrors cool to -400°F (-240°C) in deep space.

Moreover, JSWT is being protected from external sources of light and heat (like the Sun, Earth, and Moon) as well as from heat emitted by the observatory itself with a five-layer, tennis court-sized sunshield made of a material called Kapton that was developed by DuPont in the late 1960s. NASA likens the sunshield to a parasol-providing shade that will keep the infrared cameras and instruments aboard very cold and out of the sun’s heat and light so that they can function properly.

Given the harsh conditions out in space, JSWT does not use conventional cameras that would rip apart in space or interfere with its extremely sensitive instruments. That’s one of the reasons why all JSWT’s systems—the Near Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor/Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS)—are contained in an Integrated Science Instrument Module (ISIM) and are also designed to survive micrometeoroid hits.

While NIRCam will be able to detect extremely distant objects, MIRI will help scientists confirm if these faint sources of light are indeed clusters of first-generation stars or second-generation ones that form later as a galaxy evolves. Since MIRI can see through even thicker clouds of dust than NIRCam, it will also detect molecules that are common on Earth such as water, carbon dioxide and methane.

NIRCam is equipped with coronagraphs that allow astronomers to take pictures of very faint objects around a central bright object, like stellar systems. They are expected to help astronomers determine the characteristics of planets orbiting nearby stars. You can find the technical details at SCIENCE INSTRUMENTS (stsci.edu).

MIRI has both a camera and spectrograph that sees light in the mid-infrared region of the electromagnetic spectrum as explained in the earlier paragraphs.

The sunshield will allow the telescope to cool down to a temperature below 50 Kelvin (-370°F, or -223°C) by passively radiating its heat into space. The near-infrared instruments (NIRCam, NIRSpec, FGS/NIRISS) will work at about 39 K (-389°F, -234°C) through a passive cooling system. The mid-infrared instrument (MIRI) will work at a temperature of 7 K (-447°F, -266°C), using a helium refrigerator, or cryocooler system. The sunshield also provides a thermally-stable environment which is essential to maintaining proper alignment of the primary mirror segments as the telescope changes its orientation to the Sun.

But before we wind up, it’s important to note that the full-coloured images that Webb delivered on July 12 this year were “synthetic (they’re known as “false colour” but are not fake images) because telescopes record images in black and white through coloured filters that are used to record specific wavelengths of light. Scientists combine several shots of the same field obtained at different wavelengths to give us those spectacular colourful images. We can only wait for more breathtaking images from JSWT.

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