In the Hubble Space Telescope, light rays from a distant celestial object (1) first passes through an aperture window of diameter 3m (2) incident upon, and reflected by a primary mirror of diameter 2.4m (3) reflected by a secondary mirror of diameter 0.3m to form the final image. For the angular resolution limit of the HST (assuming diffraction limited), the diameter of the primary mirror is taken to be the diameter of the aperture. What I was wondering is why diffraction effects due to the secondary mirror are not taken into account? More generally, I think this concerns what happens to an Airy disk when I reflect it about, to which I'm not quite sure (does it just scale in size but retain the same angular resolution? - if so, does this not mean that there are no diffraction effects due to the secondary mirror?). Would really appreciate input on this. Thanks in advance.