The James Webb Space Telescope delivers impressive images that bring incredible resolution to the infrared universe. But displaying faint nebulae or distant galaxies has an interesting side effect. The stars, which are generally closer and certainly brighter, appear to have eight spikes or tips, six large and two small. This effect is so iconic that it is even possible to use it to confirm at a glance that the photo was taken by JWST.
The reason for these sharp protrusions lies in the architecture of the telescope, and any modern observatory has to deal with this fact. Telescopes for advanced astronomy no longer use lenses to magnify distant features of the universe and instead use mirrors. That is, light is first reflected from the primary mirror to the secondary mirror. This secondary mirror is located just in front of the primary mirror, and its position and the way it is held can create strange patterns.
Looking at JWST, the secondary mirror of our newest space telescope is circular and held by three struts. The struts cause a phenomenon known as diffraction. This is the way waves are affected by an obstacle in their path. The three supports of the secondary mirror produce diffraction bumps of light from the bright object. Each strut produces two spikes at a 90 degree angle to the strut, meaning there should be six spikes in total. But if you count the spikes visible, you’ll see that there are definitely eight spikes in the image, six large and two small.
This is because JWST has another unique feature. Its primary mirror is segmented and its diffraction characteristics are also made up of 18 hexagonal segments that make up it. In fact, it is the shape of the mirrors that contributes most to the sharp protrusions seen. Since the edges of the mirrors form the most prominent sharp protrusions, the protrusions created by these regular hexagonal mirrors are equally spaced.
Why aren’t there more protrusions?
The reason there aren’t 12 protrusions here is that the team aligned four of the mirror’s prongs with the four prongs of the struts, reducing the number of protrusions to just eight in total. When you pay attention to the spikes, you’ll see that their tips are often very faint and may even look like a dashed line.
This is another effect that follows diffraction, commonly known as interference. Here, light waves cancel or strengthen each other depending on how they overlap.
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