It says here (and elsewhere) that during mirror alignment the James Webb Space Telescope will take $18$ “individual images” of a star, “one from each mirror segment”. I’m wondering how this is possible. Can they direct light from one mirror segment at a time onto the sensors, and if so, how? Or do they mean that the data from the sensors is analyzed into $18$ separate images, and if so, how?
Starting with this quote,
After launch and deployment, the primary mirror segments, secondary, and science instruments will be misaligned relative to each other by up to several millimeters. An iterative process using several types of wavefront sensing and control will bring these mirrors into alignment within tens of nanometers. The large dynamic range (millimeters to nanometers) means that several distinct stages and types of sensing are necessary. This commissioning process is necessarily iterative, due to finite sensing precision and also to mechanism uncertainties inherent to the coarse stage actuator design. As a result, Optical Telescope Element (OTE) commissioning will be iterative at both small scales (a given step may need to be performed several times to converge) and at much larger scales (mechanism uncertainties will likely require looping back to repeat entire sections of the commissioning plan).
My interpretation is that they will move one segment at a time and observe the change in image PSF as well as changes in the measurements made by the local wavefront measurement tools. Due to the magic of matrix math, after every mirror segment has been moved a few times, a maximum a posteriori estimate of the optimum position for all segments can be made.
I am reasonably sure this is the approach being taken, rather than hard-drive each segment to a FOV which excludes all other segments. [background: I'm a former employee of AOA-Xinetics and was involved in the design of many wavefront sensors, including the system provided for JWST alignment]