Steps toward 8m honeycomb mirrors iv: Some aspects of design and fabrication

A facility is to be built to make 8m diameter glass honeycomb mirror blanks by casting, in the same way that has been demonstrated with 1.8m blanks. The only major difference is that the larger furnace will he rotated on a turntable so as to preform the deep parabolic surface needed for f/2 mirrors. This paper explores the tolerances in glass homogeneity, thermal control and support of the blank necessary to meet the stringent imaging requirements of telescopes in the best ground based sites. In round numbers, homogenity in expansion coefficient of 10-8/°C and thermal equilibriation to 0.1°C are required. Laboratory measurements show that both can be met by a ventilated honeycomb of borosilicate or similar glass. Adequate resistance to wind pressure and buffeting can be achieved by an axial support that responds to pressure on the three defining points. The total time of about 6 weeks for melting, annealing and cooling of the blanks is set by the time constant of around 8 hours for the glass honeycomb to follow the furnace temperature.