Instrumentation: Deployable Mast
Focusing X-ray telescopes require long focal lengths. In other words, the optics, which are effectively lenses, must be separated by several meters from the detectors, which act as the film upon which X-ray images are recorded. Previous X-ray missions have accommodated these long focal lengths by launching fully deployed on large rockets. For example, the Chandra X-ray Observatory and the XMM-Newton satellite are two low-energy X-ray missions which launched in 1999. Both are 10 m (33 feet) long and weigh approximately 4000 kg (9000 lb). Chandra, built by NASA, launched on the Space Shuttle Columbia and XMM-Newton, built by the European Space Agency (ESA), launched on an Arianne 5 rocket. These are two of the largest lift capacity rockets currently available.
Though the NuSTAR focal length is similar to that of Chandra and XMM-Newton, NuSTAR is a Small Explorer mission and must fit in the low-cost Pegasus launch vehicle: at launch, NuSTAR must be no more than two meters long and one meter in diameter. Therefore, NuSTAR uses a unique deployable mast, or boom, that extends the optics after the payload is in orbit.
The articulated mast, being built by ATK-Goleta, is low risk, low weight, compact, has significant flight heritage, and provides a stiff, stable, and reliable stucture on which the optics are mounted. It is based on a design used to establish a 60-meter separation between the two antennae of the the Shuttle Radar Topography Mission (SRTM) which flew on the Space Shuttle Endeavor in February 2000 and made high-resolution elevation (topographic) maps of most of our planet.
In order to assure that the NuSTAR optics are well-aligned with the detectors, an adjustment mechanism will be deployed on the mast. This mechanism will be used once at the beginning of the mission to align the telescope. In order to measure deflections of the mast, NuSTAR uses a laser metrology system consisting of two lasers on the optics end that are pointed at three light-sensing detectors at the detector end of the telescope. Measurements from the laser metrology system will be used to correct the X-ray images, which would otherwise be blurred by the motion of the mast.