The Wonder That Is Webb, Part 2

In Part 2 of this column, I talk a little more about the design of the James Webb Space Telescope (JWST) mirror, as well as the Canadian connection.

A telescope is all about collecting light. The more light you can collect, the fainter the object you can detect, which means the more distant the object. The amount of light collected also affects the resolution, or detail seen. So, the larger the telescope, the more light you can collect. The more light you can collect, the fainter the object and the more detail you can see.

On Earth, many telescopes use glass lenses to collect light. The bigger the lens, the more light you can collect. But using a large glass lens can be problematic. Glass is heavy, and the lens has to be ground to perfection to refract the light to the eyepiece (a much smaller lens) so you can see the object well. Sending glass lenses into space doesn’t work because a large glass lens is just too heavy. Heavy = more fuel. Fuel = $.

Most telescopes today use mirrors to reflect the light to the eyepiece (which is still a lens). The material used to make and coat mirrors is light, so telescope mirrors are easier to send into space, although a great deal of precision is still required to make the mirror surface reflect light perfectly.

As mentioned, we want a telescope mirror to be as large as possible to collect more light. The JWST mirror is made up of 18 hexagon mirrors positioned to approximate a circle as much as possible. Each hexagon is covered with an incredibly thin layer of gold, about one ten-thousandth the thickness of a human hair. The reason for the gold plating is because gold, being highly reflective, reflects infrared light well (the JWST collects infrared light).

The total amount of gold spread over the 18 hexagonal mirrors is about the same amount of gold that it takes to make five men’s wedding rings. Credit: NASA

But why hexagons? Why not just a round mirror, like the one that’s in the Hubble Space Telescope? The JWST mirror overall is 6.5 metres across, but the diameter of the rocket that carried the telescope into space is about 5.4 metres. So, to be able to build a large mirror to collect as much light as possible, the mirror has to be collapsible to fit in the rocket. The hexagon design allows for this.

The telescope is the size of a two-storey building, and the Sun shield is the size of a tennis court. So everything had to be folded up to fit inside the Ariane 5 rocket, which is only 5.4 metres in diameter. Credit:

The three images of a galaxy called M74 shown below illustrate how we can learn different things about an object when using different wavelengths of light. The image of M74 on the left was taken by the Hubble Space Telescope using visible light. The image of the same galaxy on the right was taken by the JWST using infrared light. The image of M74 in the middle is a combination of the Hubble and JWST images.

In the Hubble image (left), the bright pink spots are star-forming regions. In the JWST image (right), we can see more structure in the spirals. The blue and pink are star-forming regions. In the combination of the other two images (middle), we can see even more structure in the spirals. The spirals seem to resemble snakes. The reddish areas are hot dust. All three images show hot young stars at the galaxy’s centre. Credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST Team; acknowledgement: J. Schmidt

And what did Canada contribute? Canadian scientists and engineers built two important components of the JWST: the Fine Guidance Sensor and the Near-Infrared Imager and Slitless Spectrograph. The Fine Guidance Sensor helps the telescope figure out where it is pointing, locate the targets to observe and track them, and stay fixed on the targets. The Spectrograph helps identify the composition of the atmospheres of different solar system planets as well as exoplanets.

In return for these instruments, Canadian scientists will have guaranteed telescope time to have the JWST study exoplanets, galaxies, and more!