William R. Powell

Primary mirror blank fabrication of SUBARU Telescope

The Japan National Large Telescope (SUBARU) under construction adopted Corning ULE glass for its primary mirror, which is made from 44 hexagonal blank segments (hexes) sealed together. The optimized hexes placement pattern was determined using the simulated annealing method so as to minimize the thermal deformation due to the inhomogeneity of the coefficient of thermal expansion of the hexes. The sealing process has been successfully completed and now the flat-to-flat blank is under the grinding phase to be followed by the sag-down phase for meniscus forming. The present paper summarizes the hexes placement pattern optimization method and various analysis behind its actual application, including current status of the fabrication.

P‐184L: Late‐News Poster: Transient Thermal Stress Modeling of the Laser Frit Sealing Process

The frit laser sealing process has been modeled through the finite element method. Transient thermal stress results have been used to explain experimental observations, and to provide further processing recommendations and design guidelines. The results explained why high CTE glass is hard to seal. A simple and practical solution is demonstrated to reduce the transient thermal stress by elevating ambient temperature during high CTE glass sealing.

Safe handling of 8-m mirror blanks during manufacture: equipment designs based on finite element analyses of stresses

The manufacturing of 8-meter mirror blanks by Cornings hex seal process requires several handling steps, including lifting and turnover of both plano and contoured configurations. Special equipment was designed and built to lift and turn these 35-ton, 27-foot diameter pieces of glass. Finite Element Analysis was used to evaluate the lifting and turnover equipment designs and the handling processes. The analyses predicted the stress and distortion in the glass. The results showed that the equipment designs and processes would be safe; that is they would not stress the glass beyond the safe design stress limits.

Thermal analysis on hex-placement patterns of the Gemini primary mirrors

The photoacoustic tow-beam phase lag method was described for studying the thermal diffusivity of metal and superconductor materials in this paper. A beam of laser modulated from an argon ion laser is divided into the two beams, which illuminate respectively front and rear surface of the sample. The phase photoacoustic signal difference (phase lag) can be measured by lock-in amplifier. The phase lag is related to thermal diffusivity of sample. We measured the metal samples such as lead, copper, aluminium and stainless-steel in room temperature, and the results are very agreeable with the other method. We also have studied the temperature dependence of the thermal diffusivity of the high-temperature superconductor Bi1.5Pb0.5Sr2Ca2Cu3O. The experimental results are presented.

Mechanical reliability of 8-m class ULE glass telescope blanks during manufacturing

The manufacture of 8m class ULETM glass telescope mirror blanks involves multiple steps each of which can induce both stresses and flaws. To ensure the mechanical reliability of such large mirror blanks, both during and post-manufacturing, extreme care is required in the various process steps (including handling, grinding, finishing, and transportation) so as to limit the stress and flaw severity to below the threshold level for the specific surface finish. This paper examines the critical stress/time histories the blanks experience during manufacturing and their ability to initiate slow crack growth from grinding flaws. Two different surface finishes, namely 120 grit and 270/325 grit, with and without acid etching are characterized with respect to strength, flaw, and fatigue behavior. These data show that the 270/325 grit finish with acid etching is appropriate for the sagging step which imposes a static stress of 650 psi in the center region of the mirror blank over a two-week sag period. Similarly, the 120 grit surface finish is adequate for grinding, packaging, and transporting steps which impose a static stress of 435 psi in the support pad region for a three month period. These predictions, based on power law fatigue model, were verified by conducting static fatigue tests (at appropriate stress levels) on a large sample of 6 inch diameter ULETM discs (with appropriate surface finish) at 25 degrees C and 100% RH. A comparison of strength distribution of the discs, before versus after static fatigue tests, showed no changes in strength indicating absence of slow crack growth at stresses and environment representative of manufacturing process. Such a verification is imperative for selecting appropriate surface finishes for the mirror blanks to ensure their mechanical reliability during manufacturing.

Analysis of thermal stability of fused optical structure

Large glass mirrors made by fusing smaller pieces together are susceptible to distortions during manufacture and temperature changes due to differences in thermal expansion within the mirror. A method of predicting the size and shape of this distortion using the finite element method of computer simulation is described. The method is verified by comparison to a set of measurements on glass samples.