Webb will be the premier observatory of the next decade, serving thousands of astronomers worldwide, just like the current Great Observatories (Hubble, Chandra, and Spitzer). It will study four main science themes:
The observations needed to accomplish these goals require a large telescope that can study the Universe in infrared light. Hence, Webb will have a 6.5 meter diameter mirror and its instruments will be able to cover the range from 0.6 to 28.5 micrometers (or "microns"; 1 micron is 1 x 10-6 meters). This gives Webb sensitivity to wavelengths of light ranging from the red part of the visible spectrum deep into the infrared.
Additionally, since infrared radiation is basically heat, instruments on infrared telescopes need to be very cold to avoid being swamped by the heat radiating from the telescope itself. The instruments must be at temperatures just a few tens of degrees above Absolute Zero. To aid in this goal, Webb will reside far from the Earth at the L2 region.
There will be four science instruments on Webb:
NIRCam is a near-infrared camera that will be the primary imager on Webb.
NIRSpec is a near-infrared multi-object spectrograph in the wavelength range of 0.6 to 5 microns.
MIRI is a mid-infrared instrument and will provide imaging and spectroscopy at wavelengths of 5 to 28.5 microns.
FGS is the fine guidance sensor, a sensitive camera that provides dedicated, mission-critical support for the observatory's attitude control system.
One Mirror Segment in the Lab (Courtesy: Ball Aerospace)
Several innovative new technologies have been developed for this next generation space telescope to enable the ground-breaking discoveries Webb is expected to make. Two of these are discussed below:
A light-weight, segmented mirror that can be folded up inside the launch rocket and unfold after launch.
To accomplish its science goals, Webb requires a large mirror. But a regular mirror would weigh too much to be launched efficiently! And if the mirror were assembled completely and fully opened on the ground, there would be no way to fit it into a rocket. Engineers solved this problem by using Beryllium, a very light-weight material, to construct the mirror. They also came up with a design that allows the segmented mirror to fold, like the leaves of a drop-leaf table. Therefore, Webb's primary mirror can be 6.5 meters in diameter which is over 2.5 times larger than the diameter of the Hubble Space Telescope's primary mirror, but will weigh roughly half as much. The telescope will be launched with the mirror segments folded up and they will unfold and be set into place when the telescope is in space.
Comparison of JWST and HST mirrors (Image credit: NASA)
In order to be able to study large numbers of distant objects at one time, scientists need to be able to block out the light of brighter nearby objects. The microshutters were developed to help solve this problem. Microshutters are tiny cells that measure 100 by 200 microns, or about the width of three to six human hairs. The microshutter cells have lids that open and close when a magnetic field is applied. Each cell can be controlled individually, allowing it to be opened or closed to view or block a portion of the sky. It is this adjustability that allows the instrument to do spectroscopy on so many objects simultaneously.
On the left is an array of microshutters, about the size of a postage stamp. On the right is a closeup view of the microshutters themselves. (Image credit: NASA)
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