H1 H2 H3 \[ \cos \alpha = \frac{d}{r} \sin^2 \theta + \cos \theta \sqrt{1 - \frac{d^2}{r^2} \sin^2 \theta}\text{.} \] \[ \cos \alpha = \frac{\sin^2 \theta}{\sin \theta_{\max}} + \cos \theta \sqrt{1 - \frac{\sin^2 \theta}{\sin^2 \theta_{\max}}}\text{.} \]