Alignment of the James Webb Space Telescope mirror

Abdul Ghani

The first images from JWST show the shape and orientation of the primary mirror and wildly scattered stars.

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(With a trick, the JWST can take a picture of its own primary mirror)Image by NASA

Every reflecting telescope must be adjusted before it is used for the first time. The James Webb Space Telescope consists of 19 mirrors that have to work together perfectly. After the telescope has arrived in the intended orbit and the heat shields have been deployed, this work begins and Nasa has provided the first images of the process.

To do this, the telescope was aimed at the star HD 84406 in the constellation Ursa Majoris (Great Bear) and the first images were taken with the near-infrared camera. Since the mirror elements are not yet aligned, each element of the primary mirror creates its own image of the star on the camera sensor. By wobbling the individual elements with the adjustment motors, it was determined which of these images of the star was generated by which mirror element.

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JWST's First Images (Alignment)Image by NASA

Each mirror element can be adjusted in 6 axes. So it can be moved sideways and vertically and rotated in 3 axes of rotation. In the fine adjustment, individual steps of 8 nanometers in length can be carried out. That is also necessary. As in any amateur telescope, the wavefront of the light from the mirror must arrive at the sensor with an accuracy of 50 nanometers. To do this, the mirror surface must correspond to the shape of an ideal parabolic mirror to within 25 nanometers, except that the mirror consists of 18 movable and deformable individual parts.

The stars are still out of focus

It can be clearly seen that some of the mirror elements do not yet provide punctiform, sharp images, but longitudinally distorted and partly blurred images. Unlike the Hubble Space Telescope, the thin mirror elements do not have a rigid shape but can be bent by additional motors.

In order to analyze the shape and decide on the next adjustment steps, slightly blurred images of the stars are taken in several steps. The interference patterns on the resulting images can be compared with the previously calculated ideal image and the necessary adjustments can be made. To create the blurred images, the small secondary mirror is moved, which is attached above the large primary mirror and reflects the light through the hole in the middle of the 18 mirror elements to the camera.

In order to get an overview of the general adjustment status of the entire mirror, a very blurred image can also be taken with a lens provided for this purpose in the beam path of the camera. Then every bright spot in the image becomes an image of the shape of the primary mirror. This allows the JWST to capture an image of the primary mirror without being able to point an external camera at the primary mirror.

JWST is not yet at operating temperature

When the shape of all mirror elements is adjusted and the elements are straightened relative to each other, they are sequentially combined into a single mirror with a single focus. According to the plan, it will take another six months until all adjustment steps have been completed and all measuring instruments have cooled down sufficiently for the scientific work.

At the moment the telescope would still be much too warm, even if it were perfectly adjusted. The heat means that the images still show strong sensor noise and hot pixels, i.e. permanently white pixels. This will change as the temperature decreases and eventually allow regular operation.

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Member Of Freelancers Union (USA), Freelance Writer!, and Digital Creator. Ghani Mengal is an enthusiast Freelance blogger and digital marketer. His content has been published and featured on many popular blogs, websites, and publications. Including TeelFeed, LifeHack.org, Data-Driven Investor, TextSniper, Scientific Publication The Predict, The Startup, The Ascent, Heart Affairs, Illumination, And The List goes on.

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