How Webb Stays in Focus

One of the most difficult challenges when assembling a telescope is aligning it to optical precision. If you don’t do it correctly, all your images will be fuzzy. This is particularly challenging when you assemble your telescope in space, as the James Webb Space Telescope (JWST) demonstrates.

Unlike the Hubble Space Telescope, the JWST doesn’t have a single primary mirror. To fit in the launch rocket, it had to be folded, then assembled after launch. For this reason and others, JWST’s primary reflector is a set of 18 hexagonal mirror segments. Each segment is only 1.3-meters wide, but when aligned properly, they act effectively as a single 6.5-meter mirror. It’s an effective way to build a larger space telescope, but it means the mirror assembly has to be focused in space.

To achieve this, each mirror segment has a set of actuators that can shift the segment along six axes of alignment. They are focused using a wavefront phase technique. Since light behaves as a wave, when two beams of light overlap, the waves create an interference pattern. When the mirrors are aligned properly, the waves of light from each mirror segment also align, creating a sharp focus.

The primary mirrors of Hubble and JWST compared. Credit: Wikipedia user Bobarino

For JWST, its Near Infrared Camera (NIRCam) is equipped with a wavefront camera. To align the mirrors, the JWST team points NIRCam at a star, then intentionally moves the mirrors out of alignment. This gives the star a blurred diffraction look. The team then positions the mirrors to focus the star, which brings them into alignment.

This was done to align the mirrors soon after JWST was launched. But due to vibrations and shifts in temperature, the mirror segments slowly drift out of alignment. Not by much, but enough that they need to be realigned occasionally. To keep things proper, the team typically does a wavefront error check every other day. There is also a small camera aimed at the mirror assembly, so the team can take a “selfie” to monitor the condition of the mirrors.

The JWST was designed to maintain a wavefront error of 150 nanometers, but the team has been able to maintain a 65 nanometer error. It’s an astonishingly tight alignment for a space telescope, which allows JWST to capture astounding images of the most distant galaxies in the observable universe.

You can learn more about this technique on the NASA Blog.

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