The huge 6.5-meter mirror will have five times the light-collecting area of Hubble, making it far more sensitive and allowing far higher resolution when carrying out infrared observations. Despite the vast mirror and huge heat shield (the diamond-shaped structure beneath the mirror), JWST will weigh only about half as much as Hubble, as the telescope’s optics will not be encased by a large metal structure.
JWST is not an exact replacement for Hubble. Rather, it is designed to answer many of the questions that Hubble raised. To this end, it has been designed to study the most distant galaxies in the universe by observing the cosmos in infrared light, which is technically very difficult to do from within Earth’s atmosphere.
The JWST will be far larger than Hubble, with a primary mirror 6.5 meters across compared to Hubble’s 2.4-meter mirror. The mirror is so big that it will be built of segments that will unfold, like an origami flower, once the spacecraft has reached its final orbit.
The huge mirror is necessary for two reasons. First, infrared light has a longer wavelength than the visible and ultraviolet light that Hubble specializes in, and to get the same sharpness that Hubble has accustomed us to, an infrared telescope needs a much larger mirror. The second reason is more exciting: one of JWST’s main scientific objectives is to study very distant and very faint galaxies, such as those Hubble has seen in the Ultra Deep Field. To capture more light and obtain brighter images of faint objects, you need a bigger mirror.
JWST’s mirror will be made of beryllium (unlike Hubble’s, which is made of glass) plated in gold (Hubble’s is plated with aluminum), giving it a distinctive color as well as helping it reflect as much infrared light as possible. Here, six of the eighteen segments that will unfold to form JWST’s primary mirror are being tested at Marshall Space Flight Center in the USA.
NASA, MSFC, and David Higginbotham
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Observations of the very distant cosmos are just at the limit of Hubble’s capability, leading to much debate among scientists about some of their findings. When you’re dealing with such a tiny, faint fleck of light, results can only ever be tentative, and may have to be withdrawn if new observations contradict them.
JWST will change this. Because it combines a 6.5-meter mirror (which collects around five times as much light as Hubble’s) with highly sensitive instruments, the new telescope’s observations of distant galaxies and quasars will be much better than Hubble’s,and quasars will be vastly improved over Hubble’s, offering astronomers the clarity and certainty they need.