The James Webb Space Telescope has detected its first photons thanks to NIRCam

Good news for the James Webb Space Telescope which began to detect the first photons. The news arrived in the evening in Italy and follows that of a few days ago where the Nasa announced the start of scientific instrument cooling operations to start the observation phases in the coming months. What happened yesterday is one of many “key moments” for the mission of JWST extension which will allow us in the future to investigate different mysteries of the Universe.

As is known, engineers in these days (weeks and months) will be busy aligning the mirrors of the Large Space Telescope to enable it to capture clear images of phenomena even far distant in space (and time). The initial target is the star called HD84406, which is located in the constellation Ursa Major, and which will serve as an alignment reference. Here is the latest news.

The first photons detected by the James Webb Space Telescope

As reported by official blog of the mission, the alignment process which should last at least three months began this week. Like all operations of this type, we proceed in small steps and one of them was the detection of first photons speak NIR camera (Near Infrared Camera) LED JWST extension on the day of February 2, 2022.

nircam jwst

The alignment phases consisted of a simulation on a 1/6 scale model of the telescope which made it possible to start testing the alignment algorithm and then to apply it to the James Webb Space Telescope which is actually in space. As mentioned, these images will not be “spectacular” but useful precisely for the purpose of obtaining the definitive images that we will see this summer.

The alignment of the 18 segments of the primary mirror, made of beryllium and coated with a very thin layer of gold (and a protective layer of silicon oxide), will have an accuracy of 50 nm with operations taking place at 1 .5 million km from the earth. Complex, long but necessary operations. Indeed, remember that the segments can only move one at a time and that they can maintain the chosen position even without power to the actuators. The sequence of operations includes:

  • identification of the image captured by the segment
  • segment alignment
  • image stacking
  • coarse alignment
  • precise alignment
  • alignment with the field of vision of the instruments
  • final fixes

The Phases of JWST Mirror Alignment

First you will need to align James Webb Space Telescopethanks to the star identification system present in the lower area, to the lens (in this case HD84406). At this point, multiple images of the area will be captured and stitched together. Since the mirrors are not yet aligned, the engineers see 18 slightly different images. We will then move on to analyzing these images to understand which segment captured which image. This will give you an idea of ​​how the mirrors should be moved.

jwst alignment

A single star appears repeated 18 times due to mirror misalignment

The actual alignment process will then begin. Macroscopic errors will be corrected here and the secondary mirror will be moved slightly using the defocus. To these images (again “believed”) Phase Retrieval will be applied to understand the degree of error in segment positioning. At this point they will still be obtained “18 stars” (instead of just one) but with better definition.


This ranges from 18 stars to a single star centered in the field of view

We will then move on to the next phase. Here you will have the superposition of the different images produced by moving the segments so that the “target star” is centered in the field of view. Despite now HD84406 appears as a single star, there is a need for more precision in the alignment. In this case the vertical alignment of the segments is modified by exploiting the spectra acquired from the NIR camera by pairs of mirrored segments.

webb jwst

Final alignment before final image quality check

Then there will be the precision alignment of the segments. This will be done once during this phase, then gradually routine during the life of James Webb Space Telescope. As you can guess, we are almost at the end of the process and here we are correcting the less gross alignment errors. Also in this case we use the defocus to determine the degree of correction to apply.

At this point, the mirrors will provide a NIR camera an accurate picture. But the JWST extension takes advantage of several tools that will need to be aligned (NIRSpec, FGS, NIRISS and MIRI). It will therefore be necessary to provide different acquisitions for each scientific instrument in order to guarantee that the alignment concerns the entire field of vision. Only the last step is missing. Here the smallest errors are corrected and there will be the final check on quality fromimage produced by space telescope. As explained, some steps may have to be repeated several times, but this will not be a problem because it is already planned also because the Road map it’s flexible. There are about three months until the first real image of the JWST extension but the wait is long.

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