JWST sees light invisible to the eye. These scientists decide how to color it

Processing images in wavelengths outside the visible range of the spectrum can lead to spectacular results — like this iconic James Webb Space Telescope view of the so-called Cosmic Cliffs. The gas and dust in the field of view surrounds NGC 3324 near the Carina Nebula. The filters used in this image range from 0.9 micron to 4.7 microns; all are the infrared, outside of human vision, which cuts off around 0.7 micron. Credit: NASA, ESA, CSA, STScI

When it comes to appearances, the universe is a tricky place. Visible light occupies merely a sliver of the electromagnetic (EM) spectrum. To study the cosmos in its entirety, scientists must peer beyond visible light using specialized instruments, including radio telescopes and X-ray telescopes. And the James Webb Space Telescope (JWST) senses infrared (IR) radiation, piercing through dusty veils that block visible light.

The researchers and developers who are tasked with processing these data are presented with a unique challenge: What color is light that humans cannot see? It takes a dazzling mix of art and science to answer this question and bring these cosmic scenes to life.

The process of deciphering data from non-optical telescopes is often called false colorization, but the word “false” does it a disservice. The technique has been used for decades to render color photos from raw data, including in some of Hubble’s most famous images, which combine the telescope’s optical capabilities with its ultraviolet (UV) and IR data. Just like any other camera, this processing creates pictures from otherwise meaningless series of zeroes and ones.

“Representative color is a more accurate term,” says Joe DePasquale of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, where JWST and its imagery are managed. As one of only two space visuals developers at STScI, DePasquale is passionate about dispelling any public skepticism bred by the term “false colors.”

“It is a delicate blend of art and science, leaning more toward the science,” he explains. “We work with scientists to make aesthetic judgments and highlight key scientific features without changing the data. The whole point is to show what the telescope observes in the best way possible.”

Nothing false about false color

The process of representative colorization involves assigning colors based on relationships patterned after human perceptions of visible light. Since JWST detects light in near- and mid-IR bands, that range of wavelengths must be shifted — or mapped, as image processors call it — into the visible color space.

“Assigning colors is pretty well established,” says Alyssa Pagan, STScI’s other space visuals developer. Typically, this remapping follows the same order as the visible portion of the spectrum, “making the shortest wavelengths bluer, and the longer wavelengths redder,” she explains. “But there’s a lot of flexibility. We can make the middle wavelengths slightly more cyan or green just to get fuller color representation.”

JWST’s different filters — 29 in the mid-IR and 10 in the near-IR — reveal different aspects of an object’s structure. A nebula may have filaments of dust overlapping with clouds of hot gas, each of which are accentuated by different wavelengths. Adjustments are made to highlight these features.

“You do your best to either subtract or enhance,” says Pagan. “Even though scientists might use six filters to get an image, you don’t necessarily want to use all of them. That can neutralize features that overlap, like mixing too much paint and getting brown. You have to be careful, but you want to get the biggest color gamut you can.”

Spanning the spectrum

Besides JWST, astronomers employ a broad array of observatories to see the rest of the EM spectrum. Starting with low-energy radio waves and moving up to microwaves, then IR, UV, X-rays, and finally gamma rays, these telescopes reveal the unimaginable, including the cosmic microwave background left over from the Big Bang, supermassive black holes, the internal structures of nebulae, and exploding supernovae. (One key observatory in this fleet, the Chandra X-ray Observatory, was recently tagged for retirement by a NASA committee with no substitute on the horizon — to the dismay and protests of scientists.)

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Although the same basic rules apply in interpreting data from this diverse range of telescopes, combining their data across the EM spectrum is challenging. But the results — known as multi-wavelength images — can be spectacular and revealing.

Pagan compares the process to trying to combine a normal photo of a human arm with X-rays of the bones inside. “They don’t match up very well, but you can assign one color for your X-ray and another for your skin to see what’s really happening. It’s like plotting a graph with five different colors to show different numerical information.”

Visualizations for the public eye

Back in spring 2022, in the leadup to the unveiling of JWST’s first scientific images, STScI gave itself a short window of time to select and render some of the most spectacular targets the telescope had imaged up to that point. “The team met daily for six weeks,” DePasquale says. “We had scientists, writers, graphic designers, and then Alisa and myself.”

The resulting images became instantly iconic. Pagan worked on the Cosmic Cliffs — the nickname the team gave to a billowing wall of gas and dust surrounding the star cluster NGC 3324, near the Carina Nebula. DePasquale tackled the Tarantula Nebula (30 Doradus), a massive stellar nursery in the Large Magellanic Cloud, among others.

DePasquale says he doesn’t take the task of colorization — or its impact — for granted. “It’s a tall order interpreting what we get from a telescope and creating the imagery that shapes the public’s perception of the universe.”

If you want to try your own hand at representative colorization, STScI makes JWST data and software available on the mission’s website. For more information, check out Warren Keller’s story in our September issue on how to process your own JWST images.