How will JWST's mirrors be aligned and how long will it last? Here are all the steps NASA has set

Credit image: NASA - A simulation of the defocused images used in Fine Phasing.
Credit image: NASA - A simulation of the defocused images used in Fine Phasing.

The James Webb Telescope is on track. It has already sent us the first image, but also a shelfie of his (you can see them here). And NASA has just revealed the next steps Webb will take until the mirrors are fully aligned. The mirrors are open at this time, completely, but they need to be slightly aligned/calibrated so that 18 images of the same kind are not transmitted from each mirror in different positions, but only one in one position.

NASA has already begun the process of aligning the mirrors, which according to the agency will take 3 months (approx.). Also, teams of engineers and scientists from Ball Aerospace, the Space Telescope Science Institute, and NASA's Goddard Space Flight Center began taking data from the Near Infrared Camera (NIRCam) tool for the alignment process. The tool helps a lot in terms of the position of certain components.

This whole 3-month process will take place in 7 different phases, the first of which has already begun. In order to be considered and to function, all mirrors must be aligned and matched with a fraction of the wavelength of light (approximately 50 nanometers). 

The steps will be as follows (source: NASA): 

1. Image Identification Segment 

"First, we need to align the telescope relative to the spacecraft. The spacecraft is capable of making extremely precise pointing moves, using "star trackers." Think of star trackers as a GPS for spacecraft. At first, the position of the spacecraft from the star trackers does not match the position of each of the mirror segments." - NASA

2. Alignment Segment 

"We begin by defocusing the segment images by moving the secondary mirror slightly. Mathematical analysis, called Phase Retrieval, is applied to the defocused images to determine the precise positioning errors of the segments. Adjustments of the segments then result in 18 well-corrected "telescopes." However, the segments still don't work together as a single mirror." - NASA

3. Image Stacking

"To put all of the light in a single place, each segment image must be stacked on top of one another. In the Image Stacking step, we move the individual segment images so that they fall precisely at the center of the field to produce one unified image. This process prepares the telescope for Coarse Phasing." - NASA

4. Coarse Phasing

"Conducted three times during the commissioning process, Coarse Phasing measures and corrects the vertical displacement (piston difference) of the mirror segments. Using a technology known as Dispersed Fringe Sensing, we use NIRCam to capture light spectra from 20 separate pairings of mirror segments. The spectrum will resemble a barber pole pattern with a slope (or angle) determined by the piston difference of the two segments in the pairing." - NASA

5. Fine Phasing

"Fine Phasing is also conducted three times, directly after each round of Coarse Phasing, and then routinely throughout Webb's lifespan. These operations measure and correct the remaining alignment errors using the same defocusing method applied during Segment Alignment. However, instead of using the secondary mirror, we use special optical elements inside the science instrument which introduce varying amounts of defocus for each image (-8, -4, +4, and +8 waves of defocus)." - NASA

6. Telescope Alignment Over Instrument Fields of View

"In this phase of the commissioning process, we make measurements at multiple locations, or field points, across each of the science instruments, as shown below. More variation in intensity indicates larger errors at that field point. An algorithm calculates the final corrections needed to achieve a well-aligned telescope across all science instruments." - NASA

7. Iterate Alignment for Final Correction

"After applying the Field of View correction, the key thing left to address is the removal of any small, residual positioning errors in the primary mirror segments. We measure and make corrections using the Fine Phasing process. We will do a final check of the image quality across each of the science instruments; once this is verified, the wavefront sensing and controls process will be complete." - NASA

After all these steps, when everything is complete and the mirrors are aligned, Webb will be able to send spectacular images. But this will only happen somewhere in the summer. We look forward to the spectacular images! 

You can follow the Webb journey here.

Article by: Andacs Robert Eugen, on 12 February 2022, at 11:26 am Los Angeles time

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