Small Engineering Firm in NM Designs Key TMT systems

Flash back to May 25, 2005: it is not even one month since Eric Ponslet, long time Director of R&D for HYTEC Inc., a small engineering firm located in Los Alamos, NM, returned from a two-year sabbatical roaming the more remote areas of North America in a converted 1963 Greyhound coach, in search of rock, ice, and mountain-climbing challenges. Another type of challenge now awaits. Today, Eric is sitting in the conference room of the Center for Adaptive Optics at UC Santa Cruz, listening to a group of scientists and engineers describe their plan for the largest optical telescope ever built. TMT has just hired HYTEC to develop conceptual and preliminary designs for the primary mirror segment support mechanisms, and this is the kick-off meeting for that activity.

HYTEC is not unfamiliar with this type of work. About 10 years ago, the company designed and built the ultra-stable support platforms for the LIGO experiment, now in operation. More recently, it designed stable platforms for a variety of space instruments, including the twin STEREO sun-mapping spacecrafts (scheduled for Launch this September), and the Gamma Ray Large Area Space Telescope (GLAST), to be launched in the fall of 2007. High precision, stable structures and mechanisms are a core area of expertise for HYTEC’s engineers.

Today’s challenge is of a somewhat different nature. Each one of TMT’s numerous primary mirror segments, thin hexagonal pieces of glass 1.2 meters across by 45 millimeters thick, must be independently supported by systems of flexures and load-spreading mechanisms, in a way that maintains the shape of the reflective surface within about 10 nanometers of perfection while the telescope changes orientation in Earth’s gravity field, the local temperature fluctuates, and aerodynamic forces push on the segments due to airflow inside the telescope dome. The system also includes a set of 18 actuators that make it possible to alter the shape of the reflecting surface to compensate for other disturbances. This alone would not be so unusual in the world of scientific instruments. What makes the TMT segment support assembly unique, however, is the sheer number of identical systems that will need to be produced: 738 assemblies, plus 123 spares. This puts the system somewhere between mass-produced commercial instruments and one-of-kind scientific instrumentation. TMT is also cost-capped, which puts tremendous downward pressure on the cost of every subsystem, including segment support assemblies. Manufacturing techniques that are rarely used in the world of scientific experiments must be considered to keep the cost within the budget: only a few thousand dollars per assembly.

In the last 15 months, HYTEC has completed the conceptual design of the support assemblies. During that initial phase, various types of supports and assembly concepts were studied and compared for their effectiveness at minimizing deformations of the mirror segments. The results are a conceptual layout for the system and its major components, and numerical simulations of expected performance. The design project is now entering its preliminary phase, in which individual components will receive more attention, materials and fabrication approaches will be finalized, and mechanical details will be defined. Component prototypes will soon be fabricated and tested, and a first complete prototype assembly is planned for the end of next summer. In the middle of all this, Eric and his wife Lucie continue pursuing their passion for climbing, through weekly trips to climbing areas and mountain ranges throughout the Southwest. Accounts and pictures of some of their climbing can be found on their website.