Robots Helped Build the James Webb Telescope by Placing Its Mirror Segments
In January 2022, NASA's James Webb Space Telescope began preparing for a three-month process of aligning the 18 hexagonal mirror segments that make up the telescope's mirror. The process starts by moving each segment up by just 12.5 mm.
The James Webb Space Telescope is the largest telescope ever launched into space. With all of its mirror segments deployed, it is the size of a tennis court.
"First, engineers commanded the actuators - 126 devices that will move and shape the primary mirror segments, and six devices that will position the secondary mirror - to make sure they all work properly after launch. The team also commanded the actuators that direct Webb's fine steering mirror through small movements, confirming they work properly. The fine steering mirror is critical for the image stabilization process," NASA said.
Ground commands began instructing the primary and secondary mirror segments to move out of their launch-stowed configuration, away from the dampers that held them tightly and kept them from rattling during vibration. These movements will take at least ten days, after which engineers can begin the three-month process of aligning the segments so they work as a single mirror.
NASA engineers had to place each hexagonal piece with extreme precision. To do this, they used a robotic arm that can move in six directions. The arm was controlled by a special hexapod built by PI USA. The hexapod was mounted at the end of the manipulator to ensure that each part was in the correct place.
“For the combined mirror segments to function as a single mirror, they must be positioned within a few millimeters of each other, with accuracy to a fraction of a millimeter. A human operator cannot place the mirrors that precisely, so we developed a robotic system for assembly”
Placing the pieces required two teams of engineers working at the same time. The first task was focused on controlling the manipulator and maneuvering around the telescope structure.
“Getting there will require some patience: the computer-controlled mirror actuators are designed for extremely small movements, measured in nanometers. Each mirror can be moved with incredible precision, with adjustments as small as 10 nanometers (or about 1/10,000 the width of a human hair). Now we are using the same actuators instead of moving a centimeter. So these initial deployments are certainly the largest movements of Webb's mirror actuators ever made in space”
Another team worked on laser measurements while the segments were being placed, bolted and glued.
“Instead of a measuring tape, a laser is used to measure distance very precisely. Based on these measurements, a coordinate system is used to place each of the primary mirror segments. Engineers can move the mirror into its exact position on the telescope structure with accuracy down to the thickness of a sheet of paper”
The James Webb Space Telescope was launched in December 2021. It is designed to remain in space for at least five and a half years. However, ideally NASA hopes the telescope will operate for 10 years.
The telescope was built on and will extend the work of the Hubble Space Telescope. It will orbit about a million miles from Earth to observe some of the first galaxies formed in the universe.
“The mirror control system is designed to control only one actuator at a time. This approach is both simpler (in terms of control electronics complexity) and safer (since computers and sensors can closely monitor the operation of each individual actuator). In addition, to limit the amount of heat entering the very cold telescope mirrors from the drive motors, each actuator can run only for a short period of time. Thus, these large 12.5-millimeter movements for each segment are broken into many short movements, carried out one actuator at a time. Scripts sent from the mission operations center will manage this process under human supervision, slowly and steadily moving one actuator at a time, alternating between segments. At full speed, it takes about a day to shift all the segments by just 1 millimeter. That is roughly the speed at which grass grows!”
According to her, this may not be the most exciting period of commissioning the telescope, but that is fine. There is no rushing. On the days when the mirrors slowly unfold, they also continue to slowly cool, radiating heat into the cold of space.
The instruments are cooling gradually and in a carefully controlled manner, and the telescope continues to move smoothly outward toward L2. Slowly and steadily, as all these gradual processes bring scientists a little closer each day to the final goal: aligning the mirror.
