Why does the James Webb telescope show stars with 8 points?

We already have the first image of the universe provided by the James Webb Space Telescope (JWST), and it has eight points.

This is the deepest image taken of the early universe in the infrared range, obtained on June 7, 2022 after twelve and a half hours of exposure.

First image of the deep Universe taken by the James Webb Space Telescope in the infrared range (Galactic Cluster SMACS 0723).

In the foreground we see the galaxies of the SMACS 0723 cluster, located 4.2 billion light-years away. We also see distorted shapes of more distant (and hitherto unknown) galaxies just behind the cluster, whose light has been deflected by SMACS 0723’s gravitational lensing.

Faced with such an unprecedented image, we can ask ourselves the following questions: how is James Webb capable of obtaining such images? Why is an eight-pointed star image recorded when viewing an object such as a star?

We are going to answer these questions by looking in detail at the optics of this incredible space telescope.

The optical instruments of the James Webb

The formation of an image can be understood as a simple process in which the light that comes from an object is projected on a plane.

To make the object and the plane correspond, an optical system is necessary, which, in the case of the simplest telescopes, is made up of two elements: the eyepiece and the objective. Its purpose is to allow a correct focusing of the object.

In the case of the digital image (such as the ones we make with our mobiles) this light is captured by a sensor whose objective is to transform the light energy into a digital image. We generally distinguish between traditional charge-coupled device (CCD)-based sensors and metal-oxide-semiconductor (CMOS) sensors.

Photographic sensors for digital image CCD and CMOS
Photographic sensors for digital image CCD (left) and CMOS (right).

In this sense, the James Webb Space Telescope incorporates four key instruments based on optical sensors for the observation of the cosmos in the infrared:

1. MIRI (Mid Infrared Observation Instrument). It covers a wavelength range of 5 to 28 microns. It will allow the observation of distant galaxies and stars in formation.

2. NIRCam (camera for near infrared observation). This camera will allow the observation of the most distant objects in space in the spectrum range of 0.6 to 5 microns.

3. NIRSpec (Near Infrared Spectrometer). It is the only instrument that does not contain a camera and will be able to analyze the different wavelengths of very distant emission sources. You can observe 100 objects at the same time.

4. FGS/NIRISS (Near Infrared Imaging and Alignment Sensors). It will allow the telescope to be correctly aligned to obtain high quality images, especially the detection and characterization of exoplanets in the 0.8 to 5 micron range.

Graphic showing where the telescope's optical instruments are housed
JWST’s instruments are housed in a module behind the primary mirror, on the cool side of the telescope (protected by the huge tennis court-sized sunshield). NASA/STScl

The answer lies in diffraction

When the James Webb records the image of a star, the diffraction of light (due to the hexagonal geometry of the primary mirror of the telescope) is the cause of a typical pattern in the form of an “eight-pointed star”.

But what exactly does this optical phenomenon of diffraction consist of?

The definition is simple, although its mathematical treatment can be quite complex. Diffraction is deviation in the rectilinear propagation of waves (in our case, light waves) when they pass through an opening or the edges of an obstacle.

As a general example, in this animation you can see how water oscillations (coming from the right) are diffracted by a small opening, changing their propagation direction.

This phenomenon is more evident as the dimensions of the diffracting object are less than or equal to the wavelength of the oscillations.

Graph showing the phenomenon of diffraction
The phenomenon of diffraction is more evident as the size of the opening is smaller than the wavelength.

Initially observed and described in the 17th century by the Italian astronomer Francesco María Grimaldi, the diffraction of light is a clear manifestation of the wave theory of light waves defended, among others, by Christian Huygens, Thomas Young and Agustin Fresnel (as opposed to Isaac Newton’s corpuscular theory of light).

Many phenomena due to diffraction can be observed in everyday life.: if we look at a lamppost at night through a mosquito net (formed by a square mesh), we can see a kind of cross. When we illuminate a compact disc with white light, we appreciate a wide range of colors.

The typical diffraction cross when viewing a streetlight through a square mesh and diffraction produced by a compact disk due to its microscopic structure
Everyday phenomena due to diffraction: in the image on the left, the typical diffraction cross when a lamppost is observed through a square mesh. On the right, diffraction produced by a compact disc due to its microscopic structure.

Diffraction not only depends on the size of the diffracting aperture or obstacle, but also has a significant influence on its geometry. In the case of a reflector-type space telescope, the largest diffractive load is due to the primary mirror.

In those mirrors with circular geometry, the diffractive pattern consists of a series of concentric circles, the central one being the one with maximum intensity (also called “Airy disk”).

For square geometries, the diffraction image is formed by a cross. In the case at hand, the hexagonal geometry of the telescope’s primary mirror generates a six-pointed star diffraction image.

So what happens to the eight-pointed starry image recorded by the James Webb?

The key is in the primary mirror mounts (struts, in English) that also contribute to the diffraction of the telescope. As a consequence, two horizontal points appear crossing the 6 previously mentioned.

Graph showing the effect of the primary mirror supports on the JWST diffraction image
The effect of primary mirror supports on the diffraction image of the JWST.

For this reason, the stellar images recorded by its predecessor, the Hubble Space Telescope (with an almost circular primary mirror), present starry images with four points (taking into account their geometry and their supports) and not eight, like the James Webb.

Comparison of the same deep space zone between the JWST (8-point star images) and Hubble (4 points).
Comparison of the same deep space zone between the JWST (8-point star images) and Hubble (4 points).

Relevance of these new images

Looking into the deepest universe is equivalent to studying the oldest and most primitive universe, just when the first galaxies were forming.

It is not just the fact that looking at the image of the galaxy cluster SMACS 0723 we find new and unknown galaxies. We are entering the first moments of the universe.

It serves as data that the infrared light detected by the James Webb took 13 billion years to reach it (the age of the universe is about 13.7 billion years).

It is known that NASA scientists who had access to these first images were moved by the quality and beauty of them. It will be just a first step in the progress of observing the cosmos.

Without a doubt, the following captures by James Webb will continue to move and excite us, at least as much as the first.

*This article was originally published on The Conversation. You can read the original version here.

Oscar del Barco Novillo is an associate professor in the area of ​​Optics at the University of Murcia.

Francisco Javier Ávila Gómez is assistant professor, doctor in applied physics (optics area) at the University of Zaragoza.

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