How Will The Mirrors Of The James Webb Space Telescope Align?

by Carson

Recently, the James Webb Space Telescope, also known as JWST, has started its three-month-long process of aligning its 18 mirrors to great precision so that they can function together as one single large mirror. Find out how the alignment process works in this article.

Why Do the Mirrors Need to Align?

The James Webb Space Telescope is a reflective telescope, which means it relies on mirrors to bounce light from space to its detectors, which then take the image. You may wonder that the mirror alignment is unnecessary in the first place because telescopes can carry the mirror in one piece. However, this does not work for the JWST because its mirror is made to be too large to fit in the fairings of the launch vehicle for the sensitivity of the telescope to be sufficient for the mission.

The Process of the Mirror Alignment

The alignment of the 18 mirrors of the James Webb Space Telescope takes so long to complete because the precision must be as high as no error within the nearest 50 nanometers or so. Therefore, the alignment process must rely on preliminary images that are expected to be blurry. In fact, in the first stage of the process, the JWST will take 18 images from each of its small telescopes. After that, the images are stacked on top of each other for the algorithms to examine how much the mirrors should move to unify the entire telescope.

However, even after one round of positional corrections, the 18 mirrors in JWST probably still act as 18 independent telescopes better than a unified, large one. Therefore, according to an article from NASA, the telescope requires three rounds of coarse phasing and fine phasing for its mirrors to be aligned so that the relative offset from the perfect position is smaller than the wavelength of visible light so that light can focus in one spot when it reaches the sensor. After they ensure that the image taken matches what they expect at the end of the process, the instrument commissioning process will begin.

Why Does the Mirror Alignment Process Take So Long?

Why does this mirror alignment process, which only includes minor adjustments, take a whopping three months to complete? Firstly, the JWST must operate at freezing temperatures. That means that each actuator couldn’t be fired for too long or move the mirrors too fast to prevent the mirrors from reaching a specific temperature.

Secondly, the process has to be done very cautiously due to the high precision necessary for the telescope to produce sharp images. Consequently, much preparation must be done before the adjustments for the actuators to be fired for the correct distance, and confirmation images must be received afterward for the mission operations team to prepare for the next phase. This applies to each of the 18 mirrors in a 7-step process.


To conclude, this article briefly mentioned how the 3-month process of aligning the mirrors of the James Webb Space Telescope work and why the series of slight modifications take so long. We hope that the mission of the JWST will be successful. If you want to learn more about this, please visit the webpages in the references below.

References and Credits

  1. Loren Grush. (2022, January 25). What’s next for NASA’s James Webb Space Telescope now that it’s reached its parking spot. Retrieved February 9, 2022, from
  2. Alise Fisher. (2022, February 3). Photons Incoming: Webb Team Begins Aligning the Telescope. Retrieved February 9, 2022, from
  3. Hannah Grace. (2022, February 4). James Webb Space Telescope Alignment Has Begun: Process Expected to Take 3 Months. Retrieved February 9, 2022, from
  4. Elizabeth Howell. (2022, January 18). Here’s how the James Webb Space Telescope is aligning its mirrors. Retrieved February 9, 2022, from

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