Large boule or spherically mounted telescopes are mechanically feasible and economical up to mirror diameters of at least 5 metres and focal ratios of two or less. An optical array consisting of nine such telescopes would provide a telescope of 15 metres equivalent aperture for all astronomical purposes, including direct imaging since it is now feasible to do this by aperture synthesis methods. The resolving power of such an array used in the interferometric mode would be equal to that obtained by the VLBA at radio wavelengths, and would thus provide information in the optical region not obtainable by monolithic or multiple mirror telescopes of the same equivalent aperture. A mechanical and optical design of a 5-metre individual boule-type telescope is presented with emphasis on the minimum weight structure of the outer shell. The major economy achieved by means of boule telescope design is due of course to the absence of the conventional dome and building, and a comparison is made with a conventional 5-metre telescope both in size and cost, and also with the cost of the 15-metre equivalent array and other feasible designs. It will be shown that the single boule-type telescope, at least up to 5 metres aperture, costs an order of magnitude less than conventional telescopes.