NASA engineers will focus the James Webb Space Telescope from afar once it reaches space — and a new video from the agency shows how.
The James Webb Space Telescope, scheduled to launch in mid-2019, is set to be the Hubble Space Telescope's successor. Its optical range overlaps with that famous telescope, although the James Webb will focus more on near-infrared wavelengths of light. Its main mirror, which is more than six times the size of Hubble's, consists of 18 hexagonal segments that allow it to fold for launch. This means the fine adjustments needed to bring the array into focus will have to happen after the telescope is already in space.
The process is similar to laser corrective surgery, according to the statement that accompanied the video. Eye surgeons use a laser to measure the imperfections in the patient's cornea, the surface of the eye. In the same manner, engineers will use light from a distant star to determine the imperfections in the positioning of the James Webb's mirror segments, NASA officials said in the statement.
The James Webb will make its first adjustment by splitting the starlight it sees into its component spectrums. "The light from each segment will interfere with adjacent segments, and if the segments are not aligned to better than a wavelength of light, that interference shows up like barber pole patterns," Lee Feinberg, the mission's optical telescope element manager, said in the statement. The engineers will slowly bring the array into alignment using 126 small motors, called actuators: seven for each mirror segment.
To make finer corrections, the team will, again, direct the telescope at a bright star, and use the out-of-focus image to guide the final adjustments. Then the team aligns the secondary mirror to the primary, the tertiary mirror to the first two, and finally the scientific instruments to all three. The entire process is expected to take months, according to NASA. The engineers will need to check the mirrors' positioning every few days, just like a patient who underwent vision correction surgery would schedule regular visits to their eye doctor.