Coded Mask Instruments Key to HETE-2 Satellite’s Gamma-Ray Burst Discoveries

Coded optic mask foil
Dynamics Research Corp. (DRC)
Andover, MA

The High Energy Transient Explorer (HETE-2) satellite was launched into Earth orbit on October 9, 2000, and has been tracking and studying high-energy gamma-ray bursts (GRBs) from deep space. First discovered in 1967, GRBs had been difficult phenomena to observe, as they occur at random locations in the sky, last only a few seconds, and leave virtually no trace for ground-based observers. HETE-2’s ability to rapidly disseminate very precise positions of where the GRB was detected has allowed ground telescopes to catch and observe the event, leading to discoveries such as one that links GRBs with supernovas. Most of the satellite assembly occurred at Massachusetts Institute of Technology (MIT), but many subassemblies and critical components were manufactured by other institutions throughout the world.

The Soft X-ray Camera (SXC) is a key instrument on HETE- 2 that provides the accurate positions. A critical component for the SXC is the coded mask, which serves as the instrument’s optic element. Coded mask instruments are small and lightweight, and can be readily integrated into other systems with minimal impact. It operates on the principle that by casting well-defined shadows on an imaging device, one can determine the direction of the light source. In this case, the source of Xrays comes from the GRB, and the shadowing element is the mask. The mask must be made of gold and be strong enough to withstand the rigors of launch and the space environment.

DRC’s Metrigraphics Division developed a 5" square by 0.030-mm thick foil with thousands of micron-sized slits located in the data collection window of the satellite. The manufacturing process used to fabricate the foil was based on semiconductor- level photolithography, thin-film sputtered metal, and electrochemical metal deposition.

DRC and MIT have continued to develop and manufacture similar devices used for satellite-based data collection. Future GRB missions, as well as space missions tracking known X-ray sources and thereby serving as navigational aids, will rely on the same instruments.

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