SIM: Science Instrument Module
The ACIS and HRC focal plane instruments are both attached to a single sliding stage, the Science Instrument Module (SIM). The SIM slides in the spacecraft Z-axis to move one of the flight instruments into the mirror (HRMA) focus, and has a focus adjustment in the spacecraft X-axis. The SIM also carries the fiducial lights use in the aspect system.
See the Proposers' Observatory Guide for technical details.