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but the cost andweight were unacceptable. No suitable plastic primarymirrors
were available, and producing one required advancing the state of the art in
plastic mirror manufacturing. The segmented secondary mirror imposed a
significant optical alignment challenge during the assembly of the systems
and required the machining of an expensive faceted mounting structure. The
limitations associated with tracking system flexibility mandated that ORNL
develop a dedicated tracker controller that could work with varying system
geometries.
By 2004, the optical system had been redesigned and the segmented secondary
mirror replaced with a single elliptical mirror, the large core PMA waveguides
had been replaced with a single bundle of 127 PMMA 3-mm fibers, a molded
plastic primary mirror had been developed, custom tracker control electronics
and software were installed, and the services of an outside mechanical engineering
firm had been employed to refine the overall packaging. The new design, shown
in Fig. 5.14, was beginning to look more like a product than a prototype.
During the development of the second-generation HSL system, one of the
original inventors, Duncan Earl, began laying the groundwork for a spin-off
company, Sunlight Direct. That company licensed the technology from ORNL
and eventually produced further refinements to the system. One of the most
significant changes in the optical design was a novel means for reducing the
packing fraction losses in the fiber bundle. Sunlight Direct developed a pro-
prietary method for producing a hexagonal bundle with minimal packing losses
Fig. 5.14 Second-generation HSL system (2004)
