Aperture shapes and the effectiveness of ground-based large and extremely large telescopes

Aperture shapes in modern large and forthcoming extremely large telescopes (ELTs), with effective light-gathering sizes more than D ∼ 10 m, differ significantly from the desirable circular one. They deliver specific point spread functions, which may also differ notably from that of the fine structure of the classical Airy pattern. The optical power of such a telescope can be changed notably compared with a circular aperture with the same area. The presence of atmospheric optical turbulence complicates the effect additionally and makes it seeing- and wavelength-dependent. So, what is the impact of a non-circular pupil on telescope exploitation? It concerns the efficiency, which is an important point, especially for instruments of such a class. In this research an attempt is made to assess the values of these changes in the context of the Keck, HDRT, GMT, TMT and ELT telescopes. Relative performance characteristics (integral contrast and signal-to-noise ratio, S/N) of the telescopes, working in the seeing-limited regime, under a range of plausible turbulence conditions, for a wide (from UV to mid-IR) spectral region are obtained. The partial role of central obscuration is assessed. The effect of adaptive optics implementation in this context is also analysed. It is shown that, for instance, maximal S/N degradation due to the non-circularity of the pupil shape can be as much as $\sim 6~{{\ \rm per\ cent}}$ (TMT) to $30~{{\ \rm per\ cent}}$ (HDRT), depending on the telescope and observational mode. The numbers are comparable with or may even substantially exceed the losses that could be caused by the other parameters (e.g. residual wave-front error, optical transmittance) relevant to the quality of the optical system.