DESIGN ASPECTS OF FUTURE VERY LARGE TELESCOPES (HONEYCOMB MIRRORS).

Abstract

Research has been carried out on three major difficulties in designing efficient and economic telescopes with 8m f/1 lightweight mirrors. These problems are polishing f/1 aspherics, thermal distortion of borosilicate glass mirror and mirror seeing. Viable solutions to all three have been developed. Solving the fundamental problems allows future very large telescopes to use such mirrors as the basic elements in the design which will reduce the cost. Accurate mirror figure together with good pointing stability given by the short focal length will enable the telescope to form images as sharp as that permitted by nature on the ground. A new technology of polishing f/1 aspherics with a computer controlled stressed lap will give very accurate figure because the lap is changed accurately to adapt the desired figure. Design parameters and performance specifications for a 0.6m aluminum stressed lap for polishing a spun cast 1.8m f/1 borosilicate glass honeycomb mirror have been developed. These can be readily scaled up for polishing 8m f/1 mirrors. Stressed lap polishing also requires accurate material removal over the entire mirror surface. An optimization algorithm using the theory of material wear has been developed to search for the polishing strokes suited for uniform or other desired removal rates. Direct casting of lightweight mirrors requires that the glass be borosilicate. The figure distortion caused by the expansion of borosilicate glass requires the mirror be isothermal to less than 0.1°C for image degradation not to exceed 0.1 arcsecond. The problem of thermal interference by air and the environment has been investigated. A method of injecting well controlled air into the cells that forces the mirror to be isothermal to within 0.1°C has been discovered. Mirror seeing caused by temperature difference between the mirror and ambient air can degrade the telescope performance, but can be reduced by careful thermal design. A simple theoretical thermal model is used to select the glass thickness of a honeycomb structure mirror. Under air ventilation thermal control, the mirror responds to changing air temperature in less than an hour, reducing mirror seeing also to 0.1 arcsecond for telescopes at good seeing sites.