An economical construction of large astronomical lightweight mirrors made of glass

The building of large flux collectors for infrared astronomy necessitates the construction of mirrors less precise than those of conventional telescopes (the optical quality is restricted to a value which takes into consideration, in particular, the dimensions of the detector that is used and of the collector itself). We are trying to make mirrors with new designs, light and less costly. With this aim, a prototype of hexagonal element, 98 cm across, of a primary mosaic of a projected collector of large diameter, was built, tested and polished. We give the description of the mirror and details of the making. The weight reduction of the mirror to 70 per cent and the optical quality obtained will allow the effective utilisation of this new mirror.