  In recent years, a great deal of emphasis has been placed on achieving the
diffraction limit with large aperture telescopes. For a well matched
focal-plane instrument, the diffraction limit provides the highest possible
angular resolution and sensitivity per pixel. But it offers another key
advantage as we now show. Conventionally, as the telescope aperture D grows,
the instrument size grows in proportion to D, and cost increases as D^2 or
faster. However, an instrument that operates at the diffraction limit can break
the trend of spiralling costs. In traditional instruments, the light must pass
through a succession of large lenses, mirrors and gratings, making it difficult
to conserve the integrity of such as small psf. An alternative approach, as we
now show, is to couple the diffraction limited beam directly into an integrated
photonic spectrograph operating in low-order modes.
