Abstract
Annular apertures are typical in modern reflective telescopes. They exhibit altered shapes of PSF and OTF if compared with perfect circular ones. As a consequence the optical power of such a telescope is also changed. This can be calculated with the use of classical optics theory. But the presence of atmospheric optical turbulence contributes its own effects. In this work the trend of the main performance characteristics through a broad spectral region (300 - 5000 nm) for different obscuration and atmospheric conditions is considered. It turns out that in certain conditions the performance (angular resolution, integral contrast and signal-to-noise ratio) of an obstructed telescope relative to a circular aperture case can be even worse than in the absence of turbulence. In particular, the relative imaging performance of an 8m telescope with high (ɛ=0.6) central obscuration as a typical example of modern large size survey telescopes is analyzed.
