ACS is mounted in one of the axial instrument bays behind the HST
primary mirror. The relative locations of the science instruments in the focal plane and their fields of view are shown schematically in Figure 3.1.
When referring to the HST
and its focal plane, we use a coordinate system that is fixed to the telescope and consists of three orthogonal axes: U1, U2 and U3. U1 lies along the optical axis, U2 is parallel to the solar-array rotation axis, and U3 is perpendicular to the solar-array axis. (Note: Some HST
documentation uses the alternative V1, V2, V3 coordinate system for which V1=U1, V2= –U2 and V3= –U3.)