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.
A method for the precise measurement of optical path-difference, especially in stressed glass
Abstract In connection with an investigation that involved the measurement of optical path-difference and thence birefringence it became necessary to have much higher accuracy than is ordinarily attained. After a careful study of the various methods available it appeared that the one which best meets the requirements of precision and simplicity is a method first elaborated by Friedel (1893), and based on an interesting property possessed by a fixed combination of (1) a polarizer, (2) the birefracting material, and (3) a quarter-wave plate, each placed in a definite orientation to the others. When plane parallel monochromatic light passes through the elements of this fixed combination in the order shown above, the emergent light is plane polarized and the azimuth is related in a simple manner to the path-difference of the specimen. Since this azimuth can be measured by means of an analyzer and graduated circle, the determination of path-difference thus reduces itself to the measurement of an angular displacement, which can be done easily and with high precision. The method was first made applicable to a petrographic microscope for the purpose of determining the path-difference at a given spot in small specimens. Later, there was constructed a separate polarimeter in which large specimens could be conveniently measured. This instrument has given all that was expected of it in facility and rapidity of taking readings, and for any path-difference has a sensitivity about one hundred times greater than that of the devices in common use. A resume of the general relations for elliptically polarized light as applied to the present method is given, and the magnitude of the errors that may arise is discussed in detail.