(Near InfraRed Camera) is an infrared imager which will have a spectral coverage ranging from the edge of the visible (0.6 micrometers) through the near infrared (5 micrometers). NIRCam will also serve as the observatory's wavefront sensor, which is required for wavefront sensing and control activities. NIRCam was built by a team led by the University of Arizona , California.
NIRSPEC
( InfraRed Spectrograph) will also perform spectroscopy over the same wavelength range. It was built by the European Space Agency at ESTEC in Noordwijk, Netherlands. The leading development team includes members from(Near Airbus Defence and Space, Ottobrunn and Friedrichshafen, Germany, and the Goddard Space Flight Center; with Pierre Ferruit (École normale supérieure de Lyon) as NIRSpec project scientist. The NIRSpec design provides three observing modes: a low-resolution mode using a prism, an R~1000 multi-object mode, and an R~2700 integral field unit or long-slit spectroscopy mode. Switching of the modes is done by operating a wavelength preselection mechanism called the Filter Wheel Assembly, and selecting a corresponding dispersive element (prism or grating) using the Grating Wheel Assembly mechanism. Both mechanisms are based on the successful ISOPHOT wheel mechanisms of the Infrared Space Observatory.
MIRI
(Mid-InfraRed Instrument) will measure the mid-to-long-infrared wavelength range from 5 to 27 micrometers. It contains both a mid-infrared camera and an imaging spectrometer. MIRI was developed as a collaboration between NASA and a consortium of European countries, and is led by George Rieke (University of Arizona) and Gillian Wright (UK Astronomy Technology Centre, Edinburgh, Scotland, part of the Science and Technology Facilities Council (STFC). MIRI features similar wheel mechanisms as NIRSpec which are also developed and built by Carl Zeiss Optronics GmbH under contract from the Max Planck Institute for Astronomy, Heidelberg, Germany. The completed Optical Bench Assembly of MIRI was delivered to Goddard Space Flight Center in mid-2012 for eventual integration into the ISIM. The temperature of the MIRI must not exceed 6 Kelvin (K): a helium gas mechanical cooler sited on the warm side of the environmental shield provides this cooling.
FGS
(Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph), led by the Canadian Space Agency under project scientist John Hutchings (Herzberg Institute of Astrophysics, National Research Council (Canada)), is used to stabilize the line-of-sight of the observatory during science observations. Measurements by the FGS are used both to control the overall orientation of the spacecraft and to drive the fine steering mirror for image stabilization. The Canadian Space Agency is also providing a Near Infrared Imager and Slitless Spectrograph (NIRISS) module for astronomical imaging and spectroscopy in the 0.8 to 5 micrometer wavelength range, led by principal investigator René Doyon at the Université de Montréal.[43] Because the NIRISS is physically mounted together with the FGS, they are often referred to as a single unit; however, they serve entirely different purposes, with one being a scientific instrument and the other being a part of the observatory's support infrastructure
ORBIT
The JWST will be located near the second Lagrange point (L2) of the Earth-Sun system, which is 1,500,000 kilometres (930,000 mi) from Earth, directly opposite to the Sun. Normally an object circling the Sun farther out than Earth would take longer than one year to complete its orbit, but near the L2 point the combined gravitational pull of the Earth and the Sun allow a spacecraft to orbit the Sun in the same time it takes the Earth. The telescope will circle about the L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius of approximately 800,000 kilometres (500,000 mi), and take about half a year to complete. Since L2 is just an equilibrium point with no gravitational pull, a halo orbit is not an orbit in the usual sense: the spacecraft is actually in orbit around the Sun, and the halo orbit can be thought of as controlled drifting to remain in the vicinity of the L2 point. This requires some station-keeping: around 2–4 m/s per year from the total budget of 150 m/s. Two sets of thrusters constitute the observatory's propulsion system
Features
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 Earth's 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 (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 kilometres (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 made post-launch repair or upgrade of the JWST hardware virtually impossible with the spaceships available during the telescope design and fabrication stage. SpaceX says its new Starship has the ability to deliver satellites and space telescopes even larger than the James Webb and is designed to reach Mars orbit. 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 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 (−223.2 °C; −369.7 °F), necessary for infrared observations.
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