Strehl number degradation by large-scale systematic surface deviations

Experience shows that optical and radio reflector surfaces may have systematic residual deviations to which the statistical analysis is not necessarily applicable. For large-scale systematic deviations we derive a general expression for calculating the degraded on-axis intensity, i.e., the Strehl number. For this type of deviation we introduce a quasi-rms value, which has the same optical significance as the rms value of random surface deviations. Numerical examples of realistic cases are presented.