Wondering About the 6 Rays Coming out of JWST's Test Image? Here's why They Happen

At the Space Telescope Science Institute (STSI) in Baltimore, Maryland, NASA engineers are busy aligning the mirrors and instruments on the James Webb Space Telescope (JWST). In the meantime, the mission team has provided us with another glimpse of what this observatory – a successor to the venerable Hubble Space Telescope – will see once it is fully operational. The latest teaser is a “telescope alignment evaluation image” of a distant star that looks red and spiked!

This milestone represents the completion of the fifth phase of preparation, known as “fine phasing,” where the mission controllers adjusted each of Webb’s primary mirror segments to produce a unified image using only the Near-Infrared Camera (NIRCam). This image was focused on a bright star at the center of JWST’s alignment. This star is known as 2MASS J17554042+6551277 and is located about 2,000 light-years from Earth.

The sensitivity of Webb’s optics and NIRCam (and a red filter that optimized the visual contrasts) meant that the galaxies and stars in the background were also visible. But whereas the background stars and galaxies are billions of years away (and a bit distorted), the foreground star is spiked in appearance. These are known as diffraction spikes (or a “spider”), which refer to artifacts created by a telescope’s secondary mirror or aperture.

Image of the alignment evaluation star, called 2MASS J17554042+6551277. Credit: NASA/STScI

According to Dr. Christopher S. Baird, an Assistant Professor of Physics at West Texas A&M University:

“Certain telescopes have a large primary mirror that focuses the incoming beam of light onto a secondary mirror or a sensor that is held over the primary mirror. The secondary mirror diverts the light out of the telescope so it can be seen or further processed. Or, alternately, a sensor held above the primary mirror converts the image to an electrical signal that is delivered to a computer.”

The key to diffraction spikes, writes Baird, is that the secondary mirror (or sensor) is held in place over the primary mirror by support rods (aka. struts or vanes), which obstruct the incoming light. As starlight enters the telescope and heads towards the primary mirror, some of it skims past the support rods and is slightly deflected. This diffraction ultimately shifts light in the final image, forming a “spider” that conforms to the position of the support rods (not the original image).

“For stars and other bright point sources of light, this shifted light pattern takes the form of radial spikes,” adds Baird. “When the support rods of a telescope’s secondary mirror are built in a nice, symmetrical cross pattern, the diffraction spikes in the image of the star takes on the same cross pattern.”

One look at JWST’s secondary mirror shows that it does not conform to a crossed or six-sided “spider” diffraction. However, diffraction can also be caused by the edge of a telescope’s aperture, which incoming light must also pass through. Since the apertures of most telescopes and cameras are circular, they typically create diffraction rings rather than spikes that are generally very faint – and known as an “Airy pattern.”

Webb’s primary mirror intercepts red and infrared light traveling through space and reflects it onto a smaller secondary mirror. Credits: IMAGE: STScI, Andi James (STScI)

As Baird explained, diffraction spikes can also be caused by hexagonal-shaped apertures, which is consistent with James Webb’s mirror segments:

“If the aperture is not circular but has some other shape, then both rings and spikes can result from just the aperture. Such polygonal apertures also cause diffraction spikes. Diffraction spikes seen in images taken by lens-based cameras are therefore not caused by support rods but by the non-circular aperture. In contrast, telescopes usually have circular apertures and therefore create images with diffraction spikes caused by the support rods.”

This is common with segmented primary mirrors, which are common for ground-based observatories. Examples include the Keck Telescopes, the Gran Telescopio Canarias (GTC), the Hobby-Eberly Telescope (HET), the Southern African Large Telescope (SALT), and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in China. With its 6.5-meter (21-foot 4-inch) primary mirror (made up of 18 hexagonal beryllium mirror segments), Webb is the first space telescope to utilize such a design.

Although there are months to go before Webb commences science operations and delivers new views of the cosmos, this image represents a major milestone. It signals the completion of Phase 5 and that Webb’s primary imager and its optical system are functioning as well as can be expected. As Webb’s deputy optical telescope element manager, Ritva Keski-Kuha, indicated in a recent NASA press release, it has bolstered the mission team’s confidence in the telescope.

“We have fully aligned and focused the telescope on a star, and the performance is beating specifications,” she said. “We are excited about what this means for science. We now know we have built the right telescope.” Over the next six weeks, the team will proceed through the remaining alignment steps before conducting the final science instrument preparations.

The team is currently in the sixth phase of preparation, where they will conduct measurements at multiple field points and extend the alignment to the rest of the instruments – the Near-Infrared Spectrograph (NIRSpec), Mid-Infrared Instrument (MIRI), and Near InfraRed Imager and Slitless Spectrograph (NIRISS). For this phase, an algorithm will evaluate the performance of each instrument and then calculate the final corrections needed to achieve a well-aligned telescope across all science instruments.

Following this, Webb’s final alignment step will begin, and the team will adjust any small, residual positioning errors in the mirror segments. Said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate (SMD) in Washington D.C.:

“More than 20 years ago, the Webb team set out to build the most powerful telescope that anyone has ever put in space and came up with an audacious optical design to meet demanding science goals. Today we can say that design is going to deliver.”

The team is on track to conclude all aspects of the Optical Telescope Element (OTE) alignment by early May before moving on to the final two months of science instrument preparations (Phase 7). Preparations are expected to wrap up by this summer, at which point Webb’s first full-resolution imagery and science data will be released. So get ready for more breathtaking images like this one!

Speaking of images, check out the #JWSTArt Submissions Slideshow, which features art inspired by the JWST.

Further Reading: NASA, West Texas A&M University