The Incredible Expanding Telescope
With a mirror that towers over two stories high and a wide and a sprawling sunshield, Webb is the biggest observatory ever sent into orbit. Imagine digging a tennis court out of the ground, loading it onto a rocket and blasting it to a location four times farther away than the Moon, and you have NASA’s design conundrum.
The bigger the telescope, the better its vision. The smaller the telescope, the easier it is to get into space. Webb's designers have combined the best of both worlds — a telescope that starts out small and expands upon arrival.
In spring 2019, NASA will load the six-ton Webb telescope onto an Ariane 5 rocket, which is just a little smaller than the space shuttle was. The telescope will be intricately configured and compacted to nestle inside the shroud of the rocket, its solar panel and mirror tower retracted, its primary mirror, sunshield, secondary mirror, communications antennae, and momentum flap folded up.
The Journey Begins
Half an hour after launch, Webb will separate from the rocket and begin its month-long solo journey to its new home, the Second Sun-Earth Lagrange Point.
As Webb travels, its parts will extend and unfold. Its five-layer sunshield will slowly unfurl, stretched taut by cables attached to motors. The delicate layers, each about as thin as plastic wrap, are reinforced with ribs that prevent tearing. The fragility of the layers works in their favor — they are more likely to form the occasional small hole than tear apart. And since the sunshield has so many layers, the next layer (or the next) can stop any light that penetrates from such a tiny fissure.
Because the telescope is so far away from Earth, it lacks the protection of our planet's magnetic field. This means it will be bombarded by high-energy cosmic rays, which can interfere with signals, or even build up enough of an electrical charge to cause small lightning strikes on the telescope. The sunshield's coatings of aluminum and a grid of conducting strips on the sunshield layers control the electricity and prevent voltage arcs from damaging the telescope.
To fit on the rocket within its six-and-a-half-ton weight limit, yet maintain the size necessary to perform its observations, the Webb telescope was deliberately designed to have as little mass as possible, making its structure somewhat flexible. This flexibility makes the telescope susceptible to vibrations whenever it moves — even the tiny movements that keep it pointed.
Vibrations must be minimized. Just as a shaky camera results in blurry photographs, even small tremors will interfere with the telescope's image quality. In fact, telescopes have to be held more still than cameras, because their exposures last so long.
Eye in the Sky
The most important part of Webb's design is, of course, its giant mirror. Telescopes work by using their large mirrors to collect more light than the human eye can detect on its own. The more light a telescope mirror collects, the sharper the image and the deeper the telescope can see into space.
Though mirror size is vital for all telescopes, it's particularly so for infrared telescopes. The clarity of a telescope's images, or "resolution," is determined by the size of the mirror versus the size of the wavelength of light. Since infrared has such a long wavelength, a larger mirror is needed to produce a quality image.
The biggest infrared telescope in space now is the compact Spitzer Space Telescope, which has a 2.7-foot (0.8-meter) mirror. The Webb telescope's primary mirror is 21.3 feet (6.49 m) across, allowing it to see in infrared as clearly as the Hubble Space Telescope sees in visible light.
Webb's mirror is made up of 18 hexagonal segments. The segments are six-sided because that configuration allows the segments to come closest to forming a circle, the best shape for a telescope mirror. The mirror, folded inside the rocket, opens after the telescope deploys.
At first, the segments will be looking at different points in the sky. On the ground, Webb's operators will go through a painstaking process of aligning the segments so they all focus on the same point.
Each segment is attached to six legs, allowing the mirrors to tilt, twist, and shift to face the correct direction and position. In addition, a pressure pad in the center of each segment can be moved like a piston, allowing Webb's operators to warp the very shape of the segment to perfectly match the others. Once the operators are done, the segments will operate as a single, unified mirror.
Coasting among the stars, following our planet in its path around the Sun, the Webb telescope will be invisible to us on Earth, too far away to be seen with the naked eye. The giant telescope will make its presence known by the impact it has on the way we view the universe, and the horizons it opens on its 10-year journey through the cosmos.