Design and fabrication of a single crystal silicon (SCSi) telescope: a success story
2007
Silicon components such as mirrors and infrared lenses have been manufactured for many years, primarily from polycrystalline
silicon (poly). There are inherent advantages that Single Crystal Silicon, (SCSi), has over poly, such as strength and dimensional
stability, that make it more suitable for telescopes. However, there are challenges in the design of an all-SCSi telescope. SCSi is
brittle and has low tensile strength compared to its compressive strength. These properties therefore dictate designs that
minimize tensile stresses and eliminate direct mechanical attachments. McCarter has accepted these challenges and has designed
and is fabricating a lightweight telescope that can replace one of beryllium at substantial savings of cost and schedule.
The challenge of direct attachment has been solved with the use of bonded threaded inserts of low expansion metal. Bonding has
been studied extensively as described in a companion paper, but the proprietary frit-bonding technique developed by Frank
Anthony proved to be the most predictable, stable, and reliable. This technique is also used to fabricate complex components
from an assembly of simpler parts.
To minimize tensile stresses, the mechanical design had to be modified from the original without changing the optical
prescription. This has been successfully accomplished through a "design for manufacturing" approach teaming designers, the
stress analyst and manufacturing personnel. This approach has provided a design that is being produced at lower risk, lower cost
and with higher predicted reliability with no loss in performance.
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