Filter tilt geometry. a) Projected on the pupil plane. b) Top view, projection on the horizontal plane. The pupil with diameter D = 1 is indicated by the circle in a) and by the two tick marks on the x axis in b). The focal length is f. In (x,y,z) space (z is the optical axis), a ray leaves the pupil plane at (x_p,y_p,0) and hits the detector in the focal plane at (x_f,y_f,F). The filter surface normal from (x_f,y_f,F) intersects with the pupil plane at an x-coordinate increased by an amount Δx given by Eq. (A.5). The angle θ is in a horizontal plane but α is in general not.