Later today, US president Joe Biden will unveil the first full-colour image captured by the James Webb Space Telescope (JWST) – with a batch of additional pictures set for release tomorrow. We know the first image is called “Webb’s First Deep Field” and will provide us with one of the deepest and highest-resolution images of the infrared universe yet captured.
, which is a star-forming region, and a distant group of galaxies known as Stephan’s Quintet.
For , the lead JWST image processor at the Space Telescope Science Institute – the project’s US base in Baltimore, Maryland – the release of the pictures will be a moment of “relief” and “gratitude” after months of what he describes as, at times, emotional work.
Advertisement
“I can say, without revealing any details, that the first image that I worked on was one of the first ones that we got,” says DePasquale. “I got really deep into the details. Then, at one point, I took a step back and I pulled myself out of the pixel level and looked at the image as a whole. It was a very overwhelming, kind of moving, experience,” he says. “[I was] literally sitting at my desk looking at the very first real-colour image from Webb knowing that I’m the first person in the world to ever have seen this. That moment for me was just amazing.”
The images DePasquale and his colleague, Alyssa Pagan, have been processing were beamed back across the 1.5 million kilometres of space between Earth and the telescope as a stream of ones and zeroes. They were then formed into an image file before being stored within an archive at the Space Telescope Science Institute. But these raw images look very different from the pictures we will see tomorrow.
For starters, the raw data has an enormous dynamic range. This means that much of the detail is contained in the dark regions of the picture, says DePasquale. “When you first open that image it essentially just looks like a blank screen, just black,” he says. Image-processing software is used that essentially brightens the picture to reveal the hidden subtleties within it.
The resulting images are black and white, however, as the detectors in JWST’s instruments only create monochromatic data. To create colour views, the team had to map different filtered wavelengths of infrared light, captured in monochrome by the telescope, onto three colours – red, green and blue. By combining the resulting three images, whose bright and dark areas now represent the contribution of each of those assigned hues, a final full-colour picture emerges.
Hubble Space Telescope teams did a similar thing in the early years of its mission, resulting in the famous “Hubble palette” of ochre, gold and teal that made images like its famous “ picture so recognisable. The JWST image processors will follow Hubble’s lead by mapping the longest wavelength infrared data to red and shorter wavelength imagery onto green and then blue.
As for the colours that might make up a “JWST palette”, that really depends on which of JWST’s instruments is being used, says DePasquale. “ has produced images that have more earthy brown and deep blue hues in them, depending on the object. , seeing in mid-infrared, sees the sky very differently and produces some very interesting colours leaning more towards blues and purples,” he says.
One striking aspect of the new JWST imagery, visible even in the already released monochrome snapshots, are the six-pointed stars bisected by a thin line. “That’s very unique to Webb and I think at some point that’s going to become an iconic indicator [of a JWST image],” says DePasquale. The spiky motif is what’s known as a diffraction pattern and it is something that arises from a characteristic of the telescope called the point spread function.
This point spread function is the way in which the JWST optical system “imprints” itself on the light that it captures of a point source, like a bright star, explains DePasquale. “It’s very highly dependent on the construction of the observatory,” he says. Hubble’s internal optics, for example, bent and interacted with light from point sources in such a way as to produce images of stars that had four lines sticking out of them.
“Webb, because it has hexagonal mirrors, imprints a completely different-looking point spread function. We have the interaction of the light with the edges of the mirrors and there are 18 of them, so they all contribute in some small way to the final look,” says DePasquale. “The support struts that hold up the secondary mirror also imprint part of the pattern.”
Aside from the telescope’s optical idiosyncrasies, what then should we look out for in the new images when they are revealed? DePasquale says it is the sharpness of the pictures, for one thing. “Webb, with its precision and its resolution, is able to bring out a level of detail that we have never been able to see in the infrared universe,” he adds.
Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every Friday
