# How does the James Webb Space Telescope take images with individual mirror segments?

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?

• Wild guess; the other 17 are moved deeply out of focus, and/or tilted so that there are no bright objects in a small field of view somewhere on the sensor. There can be diffuse light but it will not interfere with seeing how sharply focused a target object is by one mirror.
– uhoh
Commented Jan 20, 2022 at 13:16
• I have work to do, so I can't take the time to provide an answer the question, at least not right now. This paper on the JWST wavefront sensing and control might help. Commented Jan 20, 2022 at 13:52
• My first thought was that first light from the JWST when looking at a guide star could have up to 18 distinct images of that star. An obvious brute force solution to make those 18 images converge is to tweak each segment individually and see what results. After tweaking all 18, tweak again so as to (one would hope) make all 18 images converge on the same point in the imager. Non-linearities mean this won't be perfect, but repeated applications should eventually converge. Commented Jan 20, 2022 at 13:58
• This apparently is exactly what is done in the coarse correction phase. The fine correction phase addresses phasing and astigmatism, and that's a bit beyond my depth of knowledge. Commented Jan 20, 2022 at 13:59
• A not-so-brute -force method is to take a picture with mirrors as they are. Leaving one mirror alone, move all 17 others, but each in a different direction/distance. A second picture will identify which image belongs to which mirror, and the distance/direction to adjust each so it aligns with the mirror which wasn't moved. Commented Jan 21, 2022 at 1:44