The JWST has an expected mass about half of Hubble Space Telescope's, but its primary mirror, a 6.5 meter diameter gold-coated beryllium reflector will have a collecting area over six times as large, 25.4 square metres (273 sq ft), using 18 hexagon mirrors with 0.9 square metres (9.7 sq ft) obscuration for the secondary support struts.

The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument.
The design emphasizes the near to mid-infrared for three main reasons: high-redshift objects have their visible emissions shifted into the infrared, cold objects such as debris disks and planets emit most strongly in the infrared, and this band is difficult to study from the ground or by existing space telescopes such as Hubble.
Ground-based telescopes must look through the atmosphere, which is opaque in many infrared bands (see figure of atmospheric absorption).
Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis.
Existing space telescopes such as Hubble cannot study these bands since their mirrors are insufficiently cool (the Hubble mirror is maintained at about 15 °C or 288 K) thus the telescope itself radiates strongly in the infrared bands.

The JWST will operate near the Earth–Sun L₂ (Lagrange) point, approximately 1,500,000 km (930,000 mi) beyond Earth's orbit.
By way of comparison, Hubble orbits 550 kilometres (340 mi) above Earth's surface, and the Moon is roughly 400,000 kilometres (250,000 mi) from Earth.
This distance makes post-launch repair or upgrade of the JWST hardware virtually impossible.
Objects near this point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance and use a single sunshield to block heat and light from the Sun and Earth.
This arrangement will keep the temperature of the spacecraft below 50 K (−220 °C; −370 °F), necessary for infrared observations.
The JWST has an expected mass about half of Hubble Space Telescope's, but its primary mirror, a 6.5 meter diameter gold-coated beryllium reflector will have a collecting area over six times as large, 25.4 square metres (273 sq ft), using 18 hexagon mirrors with 0.9 square metres (9.7 sq ft) obscuration for the secondary support struts.
The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. The design emphasizes the near to mid-infrared for three main reasons: high-redshift objects have their visible emissions shifted into the infrared, cold objects such as debris disks and planets emit most strongly in the infrared, and this band is difficult to study from the ground or by existing space telescopes such as Hubble. Ground-based telescopes must look through the atmosphere, which is opaque in many infrared bands (see figure of atmospheric absorption). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis. Existing space telescopes such as Hubble cannot study these bands since their mirrors are insufficiently cool (the Hubble mirror is maintained at about 15 °C or 288 K) thus the telescope itself radiates strongly in the infrared bands.
The JWST will operate near the Earth–Sun L₂ (Lagrange) point, approximately 1,500,000 km (930,000 mi) beyond Earth's orbit. By way of comparison, Hubble orbits 550 kilometres (340 mi) above Earth's surface, and the Moon is roughly 400,000 kilometres (250,000 mi) from Earth. This distance makes post-launch repair or upgrade of the JWST hardware virtually impossible. Objects near this point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance and use a single sunshield to block heat and light from the Sun and Earth. This arrangement will keep the temperature of the spacecraft below 50 K (−220 °C; −370 °F), necessary for infrared observations.
James Webb Space Telescope