TH-TMO 2005 Problem 2

Let $\vartriangle ABC$ be an acute triangle, and let $A'$ and $B'$ be the feet of altitudes from $A$ to $BC$ and from $B$ to $CA$ , respectively; the altitudes intersect at $H$ . If $BH$ is equal to the circumradius of $\vartriangle ABC$ , find $\frac{A'B}{AB}$ .