The next steps

R. C. Bingham. Are we ready to use large metal mirrors? First, Larry Barr, I would like you to imagine that you are summoned urgently to Kitt Peak. The director says: "Larry, Australian astronomers predict that a comet will collide with the Earth. President Clinton commands us immediately to begin construction of a four-metre optical telescope to observe it. You are our consultant — should we use a metal mirror?" L. Barr. You really want an answer! I do think that a four-metre class metal mirror makes a lot of sense for the reasons discussed in the last two days. One aspect though is that there has not been much movement in putting metal mirrors into telescopes in the last 20 years. I hear the discussion of the technical issues, active supports, but the reality is, if you are the first or second customer it's not cheaper, yet, so the rationale of the sixties is not the driving force. What could drive you is a technical advantage — as we just heard from Phillipe Dierickx, mirror seeing should become a non-issue if we use thermal control. But I would give some thought to how much time I would be willing to spend trying to convince reluctant astronomers to adopt a solution which I think is perfectly valid but they don't. Bingham. We'll come back to those points. Arne Ardeberg, I have a hypothetical situation for you also. One day you are due to meet your Prime Minister, and as on any other day, you have an idea for a 25-metre telescope. You raise the issue. The Prime Minister says: "Is the technology for this giant mirror really in place?" Would you convince him to support your project by mentioning metal mirrors? A. Ardeberg. Even without the comet? In our project, the primary mirror is as simple as possible, leaving all the sophistication to the much smaller following mirrors. One implication is that the primary mirror segments are completely passive. Certainly metal mirror material offers many advantages handling, attachment, weight, cost — but I would need the guarantee that two-metre segments in this material could be used reliably in a passive mode, as are conventional mirrors. Given that guarantee, I would transform it into a minister-understandable answer and say: "Yes, metal". Bingham. Thank you. Phillipe Dierickx, your hypothetical situation is that the comet has landed on Garching. You, however, are in La Silla. Thus you take over the VLT project, and your first task is to decide on the fifth, spare, mirror. Would you choose to buy an eight-metre aluminium mirror? P. Dierickx. What is the magnitude of the comet? Bingham. It destroys Garching but not TELAS. Dierickx. Given the active supports of the VLT primary mirrors, and if it's the spare, I would