Comparison of two high-precision nondestructive measurement methods for evaluating thermal expansion differences in the 8.3-m ultralow-expansion Subaru primary mirror blank.
The building blocks for fabricating the 8.3-m Subaru primary mirror blank are ultralow-expansin solid hexagonal units that were subjected to rigorous ultrasonic examination to establish thermal expansion characterization as reported by Hagy [Appl. Opt. 12, 1440 (1973)]. Following assembly of the mirror by fusion and fine annealing, photoelastic analyses at hex-to-hex seals were used to calculate thermal expansion differences. These differences are found to be in excellent agreement with the ultrasonically established differences.
ULE - Zero expansion, low density, and dimensionally stable material for lightweight optical systems
The zero expansion titania-silica binary glass ULE™ offers an optimum combination of thermal, mechanical, and optical properties which make it an ideal material for precision optical structures and lightweight telescope mirrors. Its near-zero thermal expansion over the ambient operating temperature range helps preserve optical figure; the absence of hysteresis in its thermal expansion curve ensures dimensional stability of the mirror in extreme environments; its low density affords high specific stiffness thereby reducing elastic deformation of lightweight structures; its high fatigue resistance permits higher long-term stress without compromising mechanical reliability; and its excellent optical and birefringence properties facilitate inspection and quality assurance in terms of thermal expansion homogeneity, defect level, and residual strain. This paper reviews the key physical properties of ULE glass and the various fabrication techniques available for making lightweight mirrors. Special emphasis is given to ULE's mechanical behavior which controls the long-term reliability of mirror blanks during fabrication, shipping, installation, and operation. It describes the 8-meter class mirror blank manufacturing process and the stress/time histories to which such blanks are exposed. These parameters, together with strength and fatigue data, were used to optimize surface finish for adequate strength and to evaluate an 8-meter blank support system to ensure long-term reliability of the blank during transportation. The paper concludes with recent advances and development programs in support of the manufacture of precision optical mirror blanks.
Control of reflective cracking
The first is 'fatigue' cracking of the layer. This type of cracking is the result of the application of loads on the pavement and is controlled by appropriate thickness design which 'ill not be discussed in detail here. Thickness design includes not only the design strength and thickness of the stabilised, but other design elements such as delaying reflection cracking, or limiting stresses in the stabilised layer, by adopting a minimum thickness of overlying material. This may include an unbound layer utilising the 'upside down' pavement approach. The actual thickness required of the overlying material varies but for example PIARC (Ref.1) recently reported when the thickness of coated materials exceeds 20 cm the emergence of shrinkage cracks at the surface is generally uncommon; when it is 15 cm thick, the cracks emerge at the surface, but do not shorten the life of the surface course, especially if they are sealed. This experience has been gained 'with legal axle loads of 130 kN, considerably greater than the Australian legal axle load.