Technology for eight-metre primary mirrors

The cost of an eight-metre telescope is still too high for most national observatories. Cost reductions must involve new technology for the primary mirror: its material and figuring, its mass, handling and the aluminising plant. Our scheme which addresses these problems uses some features of the Keck technology, but is simplified. Necessary R&D can be carried out at our facility in London. We consider a segmented mirror in which the segments are radially-cut sectors. The sectors for an eight-metre aperture will fit inside a four-metre aluminising plant (which already exists on some sites, and in any event is cheaper than an eight-metre plant). Zero-expansion material (glass-ceramic or fused silica) may be used. The model thickness is about ten centimetres. The proposed method of production is to figure the whole set assembled as a single mirror. No cutting takes place after figuring. We consider three such figuring methods including inverted polishing and a new 'stressed mirror figuring' approach. (These methods are partly applicable to monolithic mirrors). In one of these methods, the support system of our assembled mirror on the machine may perhaps be re-used on the telescope. We expect that the testing and figuring cycle can be controlled sufficiently accurately to provide mirror segments which are interchangeable between successive batches. The economies produced by this technology may also be effective for smaller telescopes.