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How the James Webb Space Telescope Will Work

The Instruments: Sight Beyond Sight
Webb's Near-Infrared Camera hangs out in a clean room at the Lockheed Martin Advanced Technology Center on Feb. 12, 2014. It's safe to say that a working NIRCam will take in some seriously awesome cosmic sights.
Image courtesy Lockheed Martin

Although it sees somewhat into the visual range (red and gold light), Webb is fundamentally a large infrared telescope (see sidebar).

Its primary imager, the Near-InfraRed Camera (NIRCam), senses in the 0.6-5.0 micron range (near-infrared). It will detect infrared light from the earliest stars and galaxies being born, take a census of nearby galaxies and spot objects swinging through the Kuiper Belt -- the expanse of icy objects orbiting beyond Neptune that contains Pluto and other dwarf planets. It will also aid with correcting Webb's telescopic vision as needed.

NIRCam comes equipped with a coronagraph, which will enable the camera to observe the wispy halo surrounding bright stars by blocking their blinding light -- an essential tool for spotting exoplanets.

The Near InfraRed Spectrograph (NIRSpec) operates in the same wavelength range as NIRCam. Like other spectrographs, it analyzes the physical characteristics of objects such as stars by splitting light their light into a spectrum, the pattern of which varies according to the target's temperature, mass and chemical makeup.

NIRSpec will study thousands of ancient galaxies with radiation so faint that a single spectrograph will require hundreds of hours to make. To aid in this daunting task, the spectrograph equips a remarkable gadget: a grid of 62,000 individual shutters, each measuring roughly 100 by 200 microns (the width of a few human hairs) and capable of opening and closing to block out the light of brighter stars. Thanks to this microshutter array, NIRSpec will become the first space-based spectrograph capable of observing 100 different objects at a time.

The Fine Guidance Sensor / Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS) is actually two sensors packaged together. The NIRISS incorporates four modes, each associated with a different wavelength range. These vary from slitless spectroscopy, which creates a spectrum via a prism and grating combination called a grism, to aperture-masking interferometry, which uses a mask to create interference patterns that help distinguish exoplanetary light from background star shine [source: STSI].

The FGS is a sensitive, unblinking camera that takes navigational pictures and feeds them to the attitude control system to keep the telescope pointed in the right direction.

The final Webb instrument extends its range beyond near-infrared and into the mid-infrared, handy for picking up redshifted objects, as well as planets, comets, asteroids, starlight-heated dust and protoplanetary disks. Both a camera and a spectrograph, this Mid-InfraRed Instrument (MIRI) covers the widest wavelength range, from 5-28 microns. Its wide-field broadband camera will snap more of the kinds of images that made Hubble famous.

But infrared observation is essential to understanding the universe. Dust and gas can block the visible light of stars in stellar nurseries, but infrared passes through. Moreover, as the universe expands and galaxies move apart, their light "stretches out" and becomes redshifted, sliding toward longer EM wavelengths such as infrared. The farther away the galaxy, the faster it recedes and the more redshifted its light -- hence, the value of a telescope like Webb.

Infrared spectra also can provide a wealth of information on exoplanet atmospheres -- and whether they contain molecular ingredients associated with life. On Earth, we call water vapor, methane and carbon dioxide "greenhouse gases" because they absorb thermal infrared (aka heat). Because this tendency holds true everywhere, scientists can use Webb to detect such substances in the atmospheres of distant worlds by looking for telltale absorption patterns in their spectroscopic readings.

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