2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/58522We 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.Revised version, a more complete proof of a statement and some references were added. LaTex, 21 pages, to be published in Int. J. MathDifferential Geometry53C15, 53C25, 53C55text