حل تشریحی سوال شماره 27 زبان انگلیسی - عمومی

The James Webb Space Telescope has an expected mass about half of Hubble Space Telescope's, but its primary mirror, a 6.5 m (21 ft) diameter gold-coated beryllium reflector will have a collecting area over six times as large, 25.4 m² (273 sq ft), using 18 hexagonal mirrors with 0.9 m² (9.7 sq ft) obscuration for the secondary support struts. JWST is designed primarily for 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: first, high-redshift objects have their visible emissions shifted into the infrared, second, cold objects such as debris disks and planets emit most strongly in the infrared, and finally, this band is difficult to study from the ground or by existing space telescopes such as Hubble. Ground-based telescopes must look through Earth's atmosphere, which is opaque in many infrared bands. 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 Ilubble cannot study these bands since their mirrors are insufficiently cool the Hubble mirror is maintained at about 15 °C (288 K; 59 °F) thus the telescope itself radiates strongly in the infrared bands. 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 km (340 mi) above Earth's surface, and the Moon is roughly 400,000 km (250,000 mi) from Earth. This distance made post-launch repair or upgrade of JWST hardware virtually impossible with the spaceships available during the telescope design and fabrication stage. Objects near this Lagrange point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance and with constant orientation of the single heatshield and the Bus toward the Earth and the Sun to block heat and light from the Sun and Earth and maintain communications. This arrangement will keep the temperature of the spacecraft below 50 K (-223 °C; -370 °F), necessary for infrared observations.