When the Hubble telescope retires in 2014 after almost 20 years of service, its successor, the James Webb Space Telescope, should be well on its way to sending NASA new images and data on objects farther away than ever before, both in terms of distance and time. The light Webb detects will have been traveling for up to 13 billion years, and at the speed of light, that adds up to a lot of miles. The far-off objects will also have been moving away from the Earth, according to the well-accepted Big-Bang Theory, so the light from them will be Doppler shifted to the IR spectrum. That’s why the Webb is designed to explore IR wavelengths.
The telescope is scheduled for launch in 2014, a three-month journey, and then a 10-year operational life. It consist of three sections: the telescope which gathers the light, the instrument module holding equipment that will examine the light, and the spacecraft which holds electrical, thermal, communications, and attitude and station keeping subsystems.
Webb’s primary mirror measures over 21 ft in diameter, too large to fit into the Ariane 5 launch vehicle. So NASA designed the mirror as 18 hexagonal segments, each 4.3ft across. A pair of three-segment wings fold over onto the 12-segment main section, leaving a compact package that fits in the launch rocket. When unfolded, it takes on a roughly circular shape. Circular mirrors focus incoming light into a more compact spot compared to oval and square mirrors. To focus the light properly, mirrors are ground to one of three different optical prescriptions, and three groups of six segments share a prescription.
The mirror will self-assemble about four days after launch. It will then take NASA engineers up to two months of tweaking to get the image quality of the telescope where they want it.
The Webb mirror, if made using the same technology as the Hubble’s mirror, would be too heavy to launch into Space. So NASA developed a new way to build strong yet light mirrors out of beryllium. The resulting mirrors are one-tenth the weight of the Hubble’s mirror on a unit area basis. Still, each segment tips the scales at 46 lb, so the entire mirror weighs 828 lb.
NASA uses beryllium because it is light, strong, and holds its shape despite exposure to cryogenic temperatures. To make mirror segments, technicians put powdered beryllium in a steel canister and press it flat. The resulting chunk of beryllium is cut in two to make a pair of blanks about 4-ft across. Various machining processes remove material until the mirror side is relatively smooth and the back side is crisscrossed by ribs 1/25th of an inch thick providing support. The blanks are shaped, smoothed, and polished. Workers then subject each segment to cryogenic temperatures (–364°F) and measure how they change in response to the cold using a laser interferometer. This data determines the final shape each mirror will have after its last surface polishing. The last step is putting a thin coating of gold over the mirrors to help them better reflect IR light.
Each mirror segment mounts on a hexapod connected to a stiff backplane. Each hexapod is controlled by six actuators for coarse and fine positioning and to give it six independent degrees of freedom: X and Y position, piston, tip, tilt, and clocking. An additional actuator on each mirror adjusts that mirror’s curvature. For clear, focused images, the actuators have to position segments to within 0.01 µm. A secondary mirror, the fine-steering mirror, also has dedicated actuators. A third mirror stays fixed in place.
To record IR signals collected by the mirror, NASA developed two types of low-noise, large-format IR detectors: 4-megapixel near infrared (NIR) mercury-cadmium-telluride arrays for 0.6 to 5-μm wavelengths, and 1-megapixel mid-IR silicon-arsenic detectors for 5 to 29 μm. To digitize all the image data, NASA developed a low-noise, cryogenic ASIC with low power dissipation and a 16-bit A/D converter with noise comparable to conventional warm electronics.
Webb’s backplane, the structure supporting the mirrors segments, must support the mass of the mirror as well 2,400-kg of optics and instruments. And because it supports the mirrors, it must remain motionless. It must also be thermally stable at temperatures colder than –400°F (–240°C). So engineers at Alliant Techsystems, Minneapolis, built the backplane out of graphite composites mated to titanium and Invar fittings and interfaces. (Invar is a nickel-steel alloy that changes little in volume due to thermal expansion.) Once on station, the backplane should move back and forth by no more than 32nm.
There are four major instruments on the Webb: the midinfrared instrument (MIRI) provided by the European Consortium with the European Space Agency (ESA), and NASA’s Jet Propulsion Laboratory; the near-infrared camera (NIRCam), developed by the Univ. of Arizona; the near-infrared spectrograph (NIRSpec), provided by ESA with components from NASA; and the fine-guidance-sensor tunable-filter imager (FGS-TFI), from the Canadian Space Agency.
MIRI is an imager/spectrograph that covers wavelengths from 5 to 27µm. It uses three arsenic-doped silicon-detector arrays for wide-field broadband imagery. It can also handle medium-resolution spectroscopy over a smaller field of view. The nominal operating temperature for the MIRI is 7°K. This level of cooling cannot be attained using the spacecraft’s passive cooling. Instead, a pulse-tube precooler gets MIRI down to 18°K, then a Joule-Thomson loop heat exchanger takes it down to 7°K.
NIRCam takes images with a large field of view and high angular resolutions in the 0.6 to 5-μm wavelengths using 10 mercury-cadmium-telluride arrays. These are analogous to CCDs found in ordinary digital cameras. NIRCam takes images and is part of the telescope’s optical compensation which NASA says is akin to instant Lasik vision correction for the telescope.
NIRSpec will reveal the temperature, mass, and chemical composition of objects the telescope sees. Interestingly, because the light from some objects may be so dim, the Webb must remain focused on them for hundreds of hours to collect enough light to form a spectrum. So to study thousands of galaxies during its mission, NIRSpec can watch 100 objects simultaneously in a 9 arc-min2 field of view, thanks to an array of over 62,000microshutters. Each microshutter measures 100 × 200microns and has a louvre that opens and closes in response to a magnetic field. Each cell is controlled individually, letting it be opened or closed to view or block a portions of the sky which might contain bright objects that would mask dimmer ones.
FGS-TFI is a broadband guide camera that will be used both to locate and stay focused on a guide star, a process used to check the focus of the 18 mirror segments, and for fine pointing. It operates in the 1 to 5-µm range and has two mercury-cadmium-telluride detectors. Its field of view is wide enough to provide a 95% probability of acquiring a guide star for any pointing direction.
To observe IR light from faint and distant objects, the telescope and its mirror must not be swamped by IR radiation from the Sun or the spacecraft itself. In fact, the telescope and its instruments must be under 50°K (–370°F). So a critical component of the spacecraft is a sunshield about the size of a tennis court (65.6 × 39.3ft). It consists of five layers of DuPont's Kapton, each one progressively cooler. Each Kapton layer has aluminum and doped-silicon coatings that reflect the sun’s heat back into space. A gap open to the vacuum of space separates each layer. NASA estimates the shield provides a sun protection factor (SPF) of 1 million. It will unfurl as the spacecraft travels to its station. The Webb spacecraft will, therefore, be positioned so that the Sun, Earth, and Moon are always on the same side of the spacecraft as the relatively warm electronics bus, and the telescope and its instruments will stay on the cooler, shady side of the shield.
The spacecraft portion also includes the spacecraft bus and its various subsystems that handle power distribution, communications, thermal management, and propulsion for station keeping.
The Webb telescope is a collaboration between NASA, the European Space Agency, and the Canadian Space Agency. The NASA Goddard Space Flight Center is managing development efforts, the prime contractor is Northrop Grumman, Los Angeles. The Space Telescope Science Institute will operate the Webb after launch.
So who was James Webb?
Webb was neither an engineer nor a scientist. Prior to joining NASA, he had served as director of the Bureau of the Budget and as Undersecretary of State in the Truman administration. He was also president and vice president of several private firms and served on the board of directors of the McDonnell Aircraft Co.