Image processing at wavelengths outside the visible range of the spectrum can produce spectacular results, such as this iconic James Webb Space Telescope image of the so-called cosmic cliffs. Gas and dust in the field of view surround NGC 3324 near the Carina Nebula. The filters used in this image range from 0.9 to 4.7 microns; all are infrared, outside the range of human vision, cutting off around 0.7 microns. Credit: NASA, ESA, CSA, STScI
When it comes to appearances, the universe is a complicated place. Visible light occupies only a small portion of the electromagnetic (EM) spectrum. To study the entire cosmos, scientists must look beyond visible light using specialized instruments, including radio telescopes and X-ray telescopes. And the James Webb Space Telescope (JWST) detects infrared (IR) radiation, piercing through dusty veils that block visible light.
Researchers and developers tasked with processing this data face a unique challenge: what color is the light that humans can't see? It takes a dazzling combination 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 reproduce color photos from raw data, including some of Hubble’s most famous images, which combine the telescope’s optical capabilities with its ultraviolet (UV) and infrared data. Like any other camera, this processing creates images from a series of zeros and ones that would otherwise make no sense.
“The term ‘representative color’ is more accurate,” says Joe DePasquale of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, where JWST and its images are managed. As one of only two space image developers at STScI, DePasquale is working to dispel any public skepticism generated by the term “false color.”
“It’s a delicate combination of art and science, leaning more towards science,” he explains. “We work with scientists to make aesthetic judgements and highlight key scientific features without changing the data. The goal is to show what the telescope observes in the best possible way.”
There is nothing false about false color
The process of representational coloration involves assigning colors based on relationships that follow patterns of human perception of visible light. Since the JWST detects light in near- and mid-infrared bands, that range of wavelengths must be shifted (or mapped, as image processors call it) into visible color space.
“The color mapping is pretty well established,” says Alyssa Pagan, another STScI space imaging developer. Typically, this remapping follows the same order as the visible part of the spectrum, “making the shorter 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 a more complete color representation.”
JWST's different filters (29 in the mid-infrared and 10 in the near-infrared) reveal different aspects of an object's structure. A nebula may have filaments of dust overlaid with clouds of warm gas, each accentuated by different wavelengths. Adjustments are made to highlight these features.
“You have to do everything you can to eliminate or enhance it,” Pagan says. “Although scientists can use six filters to get an image, you don’t have to use all of them. That can neutralize overlapping features, like mixing too much paint and getting a brown color. You have to be careful, but you want to get the widest range of colors possible.”
Spanning the spectrum
In addition to JWST, astronomers employ a wide array of observatories to observe the rest of the electromagnetic 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 supernova explosions. (A key observatory in this fleet, the Chandra X-ray Observatory, was recently retired by a NASA committee with no replacement on the horizon, to the dismay and protest of scientists.)
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While the same basic rules apply to interpreting data from this wide array of telescopes, combining their data across the entire electromagnetic spectrum is a challenge. But the results – known as multiwavelength images – can be spectacular and revealing.
Pagan compares the process to trying to match a normal photograph of a human arm with X-rays of the bones inside. “They don’t match very well, but you can assign one color to the X-ray and another to the skin to see what’s really going on. It’s like plotting a graph with five different colors to show different numerical information.”
Visualizations for the public eye
In the spring of 2022, in the run-up to releasing the first science images from JWST, STScI gave itself a short window of time to select and reproduce some of the most spectacular targets the telescope had imaged to date. “The team met every day for six weeks,” DePasquale says. “We had scientists, writers, graphic designers, and then Alisa and me.”
The resulting images became instantly iconic. Pagan worked on the Cosmic Cliffs, the team's nickname for 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 colorization work, or its impact, for granted. “It’s a difficult task to interpret what we get from a telescope and create the images that shape the public’s perception of the universe.”
If you'd like to try your own hand at representative coloration, STScI makes the JWST data and software available. The mission websiteFor more information, see Warren Keller's article in our September issue on how to process your own JWST images.
