In the Crater Equation $$ D=0.07C_f\left(\frac{g_e}{g}\right)^{1/6}\left(E \frac{\rho_a}{\rho_t}\right)^{1/3.4} $$ where $\rho_a$ is the density of the impactor, $\rho_t$ that of the object, $g_e$ Earth's gravitational acceleration, $g$ the gravitational acceleration of the object, and $C_f$ the collapse factor.
In this equation, how is the $C_F$ calculated?
