Hubble uses three Fine Guidance Sensors for steady pointing and measurement. Astronauts will exchange one of these optical sensors with a refurbished unit that has enhanced, on-orbit alignment capability during Hubble Space Telescope Servicing Mission 4.
Wide-Field Planetary Camera 2 (WFPC2) was installed in Dec 1993 and used to obtain high resolution images of astronomical objects. This camera was removed in the last servicing mission so it is no longer in service.