On Mauna Kea Mountain in Hawaii, USA, there are numerous telescopes scattered everywhere. Among these telescopes, the two telescopes named "Keck" stand out. If you have the opportunity to see their primary mirrors, it might completely change your impression of telescopes—their primary mirrors are not a single complete reflecting mirror, but are composed of 36 hexagonal small mirrors, which look like a honeycomb. When these small mirrors are combined, their power is just as strong as that of a large mirror.
This technology is called "mosaic mirrors". Why use mosaic mirror technology? Because the larger the aperture of a telescope, the more light from distant celestial bodies it can receive, allowing us to observe more distant and dim objects, as well as more detailed celestial features. However, when the aperture reaches about 8 meters, larger mirrors encounter unexpected problems. The main challenge is that existing infrastructure cannot meet the requirements of large-aperture mirrors. For example, how to transport a 10-meter-aperture mirror? Vehicles are not large enough, and roads, bridges, and tunnels are not wide enough. Moreover, manufacturing larger mirrors is more difficult and costly.
People came up with a solution: break the problem down into smaller parts and use multiple mirrors for assembly, which reduces manufacturing costs and avoids infrastructure constraints. However, mosaic mirrors face a series of challenges. These small mirrors must accurately collect and focus the light—this is called "focusing". Only after focusing can all the small mirrors collect light effectively. However, to make these small mirrors work together to achieve the clarity of a large mirror, in addition to focusing, the phase error of the reflected light from each small mirror must be very small, which is called "phase matching". This extremely difficult technology was achieved by the Keck telescopes, which allowed them to capture clear photos of celestial objects with fine details.