At last, the brand spanking new space telescope, JWST, has captured its first photons of light. Traveling through its entirety, reflecting from the primary mirror to the secondary to the tertiary and then finally into the Near Infrared Camera (NIRCam) instrument, the light comes from a star in the constellation of Ursa Major, the great bear.
This is the beginning of the alignment process that will last the next three months. The images collected so far do not have much scientific value but they have great technical value. They will be used by the team of engineers and scientists to slowly align the mirrors so that the telescope is perfectly focused and ready to peer back to when the first stars and galaxies started to form.
“This milestone marks the first of many steps to capture images that are at first unfocused and use them to slowly fine-tune the telescope. This is the very beginning of the process, but so far the initial results match expectations and simulations,” the team wrote in a blog post.
The process requires an incredible level of precision. The 18 hexagonal segments that make up the primary mirror need to match each other to about 50 nanometers. As NASA explained, if JWST was the size of the US, each hexagonal segment would be the size of Texas (695,662 square kilometers), and the offset on the height of those segments ought to be less than 3.8 centimeters (1.5 inches). This is a staggering level of precision and the team has been running simulations and algorithms on a one-sixth scale model of the space telescope.
There are several steps the process must go through before we can expect the first real images from the telescope this summer. The first is to align the telescope relative to the spacecraft and that has been done by pointing the telescope (and the individual mirrors) to star HD 84406, which is bright and isolated. The unfocused image produced should have 18 pictures of the stars in a somewhat random position, and in step two, the team will align them and adjust the secondary mirror too.
The third phase, the image stacking phase, will allow the segments to start acting as a single unified mirror, producing a single image of the star. Steps four and five will be run three times in pairs, first a coarse phasing and then a fine phasing that will delicately bring the segmentS to higher and higher alignment, which will be checked by the NIRcam.
After correcting the field of view and any other final corrections, the telescope’s optics should be ready to start working and JWST can cast its incredible eye onto the universe.