This February, NASA will launch NuSTAR (nuclear-spectroscopic telescope array) for a two-year mission exploring space using an advanced X-ray telescope. It will be the first orbiting telescope capable of focusing on and viewing this high-energy portion of the electromagnetic spectrum.
Previous X-ray telescopes, such as Chandra and the X-ray Multi-Mirror Mission (XMM), were limited in that they could not focus. Instead they used specially constructed apertures to image X-ray signals, and these structures had intrinsically high background noise and limited sensitivity. The two earlier X-ray telescopes were also limited to looking at low-energy X-rays (15 keV and lower). NuSTAR will be able to handle X-rays with up to 79 keV. According to NASA, NuSTAR will also have 10 to 100 times the sensitivity and spatial and spectral resolution of previous X-ray telescopes sent into space.
SMEX: More bang for NASA’s bucks
Since the first SMEX mission in 1992, the missions have come to be called Explorer missions. Here are some of those past missions: 1992 – Sampex (Solar Anomalous and Magnetospheric Particle Explorer): detected solar energy particles, anomalous cosmic rays, and galactic cosmic rays throughout a solar cycle.
1997 – Trace (Transition Region and Coronal Explorer): explored the 3D magnetic structures on the visible surface of the Sun, as well as the geometry and dynamics of the sun’s upper atmosphere.
2003 – Galex (Galaxy Evolution Explorer): an orbiting space telescope that observes galaxies in UV light. Since its launch, it has surveyed tens of thousands of galaxies across 9 billion years of time.
To keep costs down on the NuSTAR mission, the spacecraft will be shot into space aboard a Pegasus launch vehicle, so it must fit in that rocket vehicle’s cargo bay. This limits NuSTAR to a 2-m-long, 1-m-diameter envelope. But this raises a problem: A focusing X-ray telescope needs focal lengths in the 10-m (33-ft) range. Both Chandra and XMM, for example, measure 10-m long and weigh about 9,000 lb. But it took the Space Shuttle to put Chandra in space and an Arianne 5 to take the XMM into orbit.
NASA engineers’ solution is an extendable mast, a scaled-down version of the 60-m mast used on a Shuttle-based radar topography mission. NuSTAR’s 10-m version, which collapses for storage, should provide a relatively stiff, stable, and reliable platform for the optics or focusing lenses, putting the necessary separation between the optics and detectors to focus the telescope and get clear images.
To ensure the optics and detectors are aligned, an adjustment mechanism is used when the mast is first deployed after the spacecraft is in orbit. Then to compensate for small, but inevitable, motions at the optics end of the mast, a pair of lasers will send light beams to three sensors mounted on the detector end of the telescope. Real-time measurements from the lasers will be used to correct X-ray images, which would otherwise be blurred.
NuSTAR will carry two Wolter-1 mirrors that will point at the same small patch of space but will reflect X-rays from space onto their own individual semiconductor-based detectors. Each cylindrical, slightly tapered mirror consists of two sections, each made up of concentric mirrors laid down layer by layer, starting with the innermost layer. The many layers are held together by epoxy, which is relatively transparent to X-rays. Graphite spacers maintain proper spacing during construction. The upper layer through which X-rays pass first consists of parabola-shaped mirrors. The lower section consists of hyperbola-shaped mirrors. Incoming X‑rays hit the first then second section of mirrors at shallow angles, then they exit the mirrors and converge on a detector chip. These types of optics are called grazing-incidence optics. (See Light path through Wolter-1 mirror illustration.)
The shallow angles of incidence and reflection prevent incoming X‑rays from penetrating or being absorbed by mirrors. But the shallow angles also limit the amount of X-ray light a single layer of the mirror can collect. That’s why there are 130 layers nestled tightly together.
Earlier X-ray telescopes used mirrors made of highly polished, dense materials such as platinum, iridium, and gold. But they were only designed to handle low-energy X-rays. When those mirrors are used for high-energy X-rays, their reflective efficiency falls off. So NASA is coating NuSTAR mirrors with depth-graded multilayers to prevent this.
These multilayers consist of thin coatings of alternating materials. Each layer can have up to 200 coatings. For the best reflectivity, the alternating materials must have significantly different densities. NASA uses two pairs of alternating layers, Pt/SiC and W/Si. The denser layers are platinum and tungsten, while silicon and silicon carbide make up those that are less dense.
NuSTAR will also map areas of the sky already charted by other space telescopes, including the Hubble. Adding NuSTAR’s high-energy X-rays to the visible and infrared images from the earlier telescopes will provide a fuller understanding of what is happening in space. For example, the telescope will peer into the center of the Milky Way where many X-ray sources have been discovered, including one called Sagittarius A with 4 million times the mass of our Sun.
NuSTAR will not ignore phenomenon in our own backyard, so to speak. It will be taking images of the Sun and its corona, the Sun’s outer atmosphere, that could tell scientists more about the particle-acceleration processes taking place in supernovae remnants, black holes, and high-temperature astrophysical plasmas. In addition, it will be looking toward the Sun in search of escaping axions, or at least signs of their passing. These hypothetical elementary particles were postulated in 1977 to account for a charge-parity problem in the Standard Model of particle physics.
Using platinum, however, puts a restriction on the telescope because platinum begins to absorb rather than reflect X-rays with energies above 79 keV.
Each Wolter-1 mirror measures 1.5-ft long, has a maximum diameter of 15 in., a focal length of 33 ft, and weighs about 70 lb. Each of the 130 layers of mirror starts out as thin, flexible glass sheets which get formed over precise quartz mandrels at NASA’s Goddard Space Flight Center in Maryland. The mirrors receive their multilayer coatings at the Danish Technical University in Copenhagen and are then assembled into a Wolter-1 mirror at Columbia University in New York.
The incoming X-rays from each Wolter-1 mirror are focused on a 32 × 32-pixel cadmium-zinc-telluride detector. The detectors convert X‑rays into electrons, which are digitally recorded by ASICs (developed at the California Institute of Technology), and sent to Earth. There, scientists combine the recordings from both sets of mirrors and detectors into a single image.
The detectors are surrounded by shields made of crystallized cesium-iodine. These shields detect high-energy photons and cosmic rays which cross the focal plane but are not coming from the direction the NuSTAR telescope is pointed. This lets the software identify and subtract these extraneous signals.