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.
Determining absolute thermal expansion of titania-silica glasses: a refined ultrasonic method.
Specific test conditions for determining ultrasonic velocities in titania-silica glasses are shown to improve the experimentally established linear correlation between velocity and thermal expansion. Finer surface finish, normalized thermal history (annealing), and controlled temperature for the test specimen improve the relationship. A statistical analysis shows that 5-35 degrees C average expansion coefficients can be accurately predicted from ultrasonic velocities with a standard deviation of 1 ppb/ degrees C. Excellent agreement is established between the ultrasonic predicted expansion and test results supplied by the University of Arizona Optical Sciences Center.
Viscous Flow in Glass‐to‐Metal Seals
Birefringence measurements were made on glass-to-metal seals with simple geometries during cooling at various rates. Thermal contraction data were also obtained on the metal and on the glass while cooling at the same rates. Using these data, the viscosity of the glass during cooling at a constant rate was calculated. The viscosity deviates markedly from the equilibrium curve and is a function of cooling rate in this temperature region. Viscous flow in the seal is measurable as much as 125°C. below the strain point.