NuStar is to be placed into a 375-mi.-high orbit inclined only 6 deg. from the equator. Hugging the equator minimizes the satellite's exposure to the South Atlantic Anomaly, where charged particles in the inner Van Allen radiation belt make their closest approach to the planet's surface.
The telescope will operate at 5-80 kiloelectron volts, well above the energy level of NASA's Chandra X-ray Observatory, which detects at between 0.1-10 keV. Launched in 1999, the 45-ft.-long Chandra is one of NASA's premier observatories, but its scale dwarfs NuStar. Chandra is the longest satellite ever orbited by a shuttle.
NuStar's 100-member astronomy team will work with observatories operating in different wavelengths. Chandra leads the partner list, but the European Space Agency's XMM-Newton mission—also launched in 1999—is expected to be tapped, as will NASA's infrared Spitzer Space Telescope, the Japan Aerospace Exploration Agency's Suzaku X-ray telescope and Hubble.
Data comparisons with NASA's Fermi Gamma-Ray Space Telescope, which spans the sky 16 times a day in extreme high-energy ranges that are far beyond NuStar's, will be useful in studying jets of light that stream from black holes with so much energy they affect an entire galaxy, says Harrison.
Unlike visible light, if X-rays strike telescope mirrors at high angles, they penetrate them. To reflect X-rays, the mirrors must be positioned at such slight angles that the rays merely graze off them. The telescopes use multiple mirrors, or “shells,” of different sizes, which are nested inside each other, to increase the telescope's collecting area. Each shell's surface has a slightly different angle but they all focus on the same spot on the focal plane.
NuStar's higher energy levels require smaller reflecting angles than previous telescopes, including Chandra. Technical advances have allowed the shells to become much thinner so they do not block light. Where Chandra uses four shells that are 0.8-1.2 in. thick, NuStar requires 133 that are 0.008 in. thick.
More than 9,000 individual mirrors were crafted at NASA's Goddard Space Flight Center in Greenbelt, Md., for the NuStar mission. They were coated in a vacuum chamber at the Danish Technical University Space Center with alternating layers of platinum and carbon or tungsten and silicon that are just a few atoms thick. Assembly took place at Columbia University's Nevis Laboratory in New York.
Crystals of cadmium-zinc-telluride allow NuStar's focal-plane detectors to stop the high-energy X-rays. Caltech printed the detectors in a 32 X 32-pixel pattern, and each one was connected to a readout chip designed to detect X-ray energy levels at better than one part in 60. Ironically, the telescope's sensitivity makes it vulnerable to stray high-energy photons and cosmic rays, so the system is programmed to detect and ignore background interference.
Because its X-rays graze off mirrors that are nearly parallel, the telescope needs a very long focal length between its twin mirror sets and its focal plane/detector. Both NuStar and Chandra's focal lengths are 33 ft. The difference is that the larger, heavier Chandra spacecraft's focal length is integrated inside the telescope. NuStar's low-cost solution is to rely on a collapsible composite mast that can fit inside a 3.3-ft.-tall canister when it is stowed for launch. Harrison quips that this is the “Tinker Toy” approach. The mast unfolds piece-by-piece once NuStar is in orbit. The buttoned-up NuStar is 3.7 ft. in diameter and 6.3 ft. tall. Once the mast is deployed, the observatory becomes 37.3 ft. long.
An adjustment mechanism on the mast will allow the mission operations center at the University of California-Berkeley's Space Sciences Lab to fine-tune NuStar's focal plane using X-ray reference sources.