Through a process called wavefront sensing and control, the JWST’s NIRCam instrument measures any imperfections in the alignment of the mirrors that might prevent them from working as a single mirror. One hundred and thirty-two actuators and tiny mechanical motors help to achieve a single perfect focus, allowing the mirror segments to align as one. The alignment process must be repeatable, as the mirror needs to be realigned each time the telescope turns and points at a different object in space.
Any heat generated by JWST’s systems could interfere with the faint signals captured by the telescope. A thermal control subsystem maintains the crafts bus operating temperature, ensuring the observatory is always at the proper temperature. Working like the world’s most effective refrigerator, the cryocooler pumps warmth-absorbing gas through the mid-infrared detector (MIRI). The instrument must be kept at –447° Fahrenheit (7 kelvins)—colder than JWST’s counterparts—to see farther into the infrared.
The Webb observatory’s real sensing magic comes from a unison of three main elements—the size of the mirror, the infrared detector, and the filter wheel. The bigger the mirror, the more energy reflected back to the detector—and JWST has the largest mirror ever put in orbit.
The process is simple, in theory, but complex in its details. The mirror collects light and directs it to the various science instrument that filter or spectroscopically disperse it before focusing the light onto the detectors. The detectors are where light is absorbed and converted into electronic voltages to be measured and later analyzed.
The infrared detectors are exotic semiconductor devices made of unusual materials with highly unusual properties. That’s one of the reasons they are cooled to near absolute zero. The MIRI detector, for example, is a charge-coupled device with an unprecedented 1024 × 1024 pixels made of arsenic doped silicon. Each pixel sensor records a voltage based on how much light strikes it.