  We describe a diagram containing the zero sets of the moment maps associated
to the diagonal U(1) and Sp(1) actions on the quaternionic projective space
HP^n. These sets are related both to focal sets of submanifolds and to
Sasakian-Einstein structures on induced Hopf bundles. As an application, we
construct a complex structure on the Stiefel manifolds V_2 (C^{n+1}) and V_4
(R^{n+1}), the one on the former manifold not being compatible with its known
hypercomplex structure.
