Abstract: Modern large telescopes are complex systems whose optical performances not only depend on their own optical properties but also on external factors such as atmospheric turbulence and local air conditions. Because angular resolution is mainly limited by these external factors, a simple geometrical approach is inadequate and efficiency criteria based on overall throughput and signal-to- noise ratio are preferable. The idea is to establish functional performance criteria which cover all aspects linked to the telescope efficiency. The result is that the user is provided with a realistic estimation of the final performance, while the manufacturer is given maximum freedom to re-arrange his own error budget according to his particular experience. Additionally, it reduces the risk of overspecification, with evident consequences for costs and leadtime. !11
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