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.
High precision photoelastic and ultrasonic techniques for determining absolute and differential thermal expansion of titania-silica glasses.
Photoelastic analysis of a large fused sandwich seal, composed of two glasses with slight composition differences, permits reliable measurement of the thermal expansion difference to 0.0002 ppm/ degrees C. If the expansion of one of the glasses is known, the method becomes absolute. For the titania-silica system, it is shown that for any two compositions with small titania differences the expansion coefficient differential remains constant from -195 degrees C to 925 degrees C. A direct correlation between ultrasonic velocity and thermal expansion has been established for this glass system and has led to successful nondestructive measurements. With an experimentally defined relationship, the measurement of the ultrasonic velocities yields absolute or differential expansions. Excellent agreement with seal testing data is shown to exist with differential data taken by both methods.
Active profiling and polishing for efficient control of material removal from large precision surfaces with moderate asphericity
We present the development of an active fabrication technology for controlling material removal on large precision surfaces of moderate departure from a sphere. The underlying philosophy was established as an efficient solution to the challenging problem of fabricating secondary mirrors of up to nominally 2.5 m in diameter for modern 8 m class telescopes and beyond. The facility described comprises a CNC profiler, two contact profilometers, and a full size active polisher. The trial work-piece was a convex surface of 830 mm in diameter on a zero-expansion ceramic (`Cervit') blank – a 1/3 scale hyperbolic mirror for a proposed 2.5 m diameter f/7 secondary mirror for the 8 m Gemini telescope. Using software error-correction from profilometric metrology data, a factor of 2 improvement in generating the convex aspheric profile was achieved. An active loose-abrasive polishing process is also described, in which a full-size tool is configured to deliver variable, edgeless, sub-diameter removal-footprints. Real-time monitoring of process variables is described, and approximately 10% convergence in each polishing pass is reported.