Engineers for the James Webb Space Telescope are in the middle of an intricate, three-month-long process of aligning the telescope's 18 separate mirror segments to work together as one giant, high-precision 6.5-meter (21.3-foot) primary telescope mirror.
The process of phasing began in February and includes seven different steps.
Engineers are on step number two, an indication of how precise and detailed the procedure is. The entire process has never been performed under all of the in-space conditions that the telescope is currently experiencing. Engineers can do a simulation of the cold and vacuum of space, but they can't do zero G.
In an interview last month, Lee Feinberg, the Optical Telescope Element Manager for the mission, explained that the testbed telescope is 1/6th the size. The telescope was put through its paces in the vacuum chamber at Johnson Space Center. We were able to show aspects of the process, but not the whole thing. When the telescope is in space, is the first time we do the entire process.
The telescope team announced on February 12 that the first photons of starlight had traveled through the entire telescope and were detected by the NIRCam instrument. Scientists and engineers are using unfocused images from the telescope to fine- tune it. Initial results match expectations, according to the team.
I like to think that we have 18 mirrors that are all doing their own thing, singing their own tune in whatever key they are in.
The 18 primary mirror segments need to match each other by 50 nanometers. Each segment has a series of small mechanical motors attached to the back that can be used to align the segments, and an additional small mechanical motor at the center of each segment that adjusts its curve.
The team moved each of the 18 primary mirror segments to bring 18 unfocused copies of a single star into a planned hexagonal formation. That alignment is shown in the lead image. The first movements of the mirror segments can be as small as a millimeter, and the final movements can be tens of nanometers.
The last steps.
2. Phase Retrieval uses mathematical analysis to determine the precise positioning errors of the segments. The segments still don't work together as a single mirror at his current stage.
3. The images are stacked on top of each other. The individual segment images are moved so that they fall at the center of the field to create a unified image. The light is put in one spot on the detector.
4. The segments are still acting as small telescopes. Coarse phasing is the process of lining the segments up with each other with an accuracy smaller than the wavelength of the light. The light spectrum from the mirror segments will be captured by NIRCam. This will be done at least three times.
5. Engineers will use the same defocusing method used during step 2, segment alignment, to measure and correct alignment errors after each round of Coarse Phasing. Instead of using the secondary mirror, they will use special optical elements inside the science instrument which will change the amount of defocus for each image.
6. Telescope alignment over the instrument fields of view is done to make sure alignment works for the rest of the instruments. Engineers will use various fields of view and make adjustments to achieve a well-aligned telescope across all science instruments.
7. Iterate Alignment for Final Correction removes any small positioning errors in the primary mirror segments and checks the image quality across each of the science instruments.
The process is flexible and modular to allow for iteration. We will be ready to commission the instruments after roughly three months of aligning the telescope.
We're used to seeing images from other space telescopes, but none of them will be taken during the alignment procedures. The first light images from the fully function JWST will be visible by June or early July.
The image array is an early alignment image with dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror.
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