Answer
$32cm$
Work Step by Step
We will assume a typical wavelength of about $550 * 10^{-9}m$. Using the given values of $L=400*10^{3}m$ and $D=0.85m$, we can find the required diameter of the telescope aperture to get a resolution of $85cm$.
$L=\frac{Dd}{1.22 \lambda}$ (36-17)
Solving for d, we obtain:
$d=\frac{L*1.22\lambda}{ D}$
$d=\frac{1.22*400*10^{3}m*550*10^{-9}m}{0.85m}$
$d=0.32m$
$d=32cm$
