Effects of Small Oscillations on the Effective Area

We analyze the effective area of the Simbol‐X mirrors as a function of the off‐axis angle for small oscillations. A reduction is expected due to: 1) geometrical effects, because some of the photons miss the secondary mirror surface; 2) reflectivity effects, caused by the variation of the coating reflectivity with the incidence angle. The former are related to the length of the two mirror surfaces, and can be reduced by making the secondary mirror longer. The second ones are energy‐dependent, and strongly related to the characteristics of the reflecting coating. These effects are analyzed by means of ray‐tracing simulations in order to optimize the mirror and coating design, aiming to improve the effective area stability.