Hubble's pointing assistants, always face the same direction, like the
needle of a compass. They sense the telescope's angular motion and provide
a short-term reference point to help Hubble zero in on its target.
are Hubble's "steering" system. The reaction wheels spin one way, and
Hubble spins the other. Flight software commands the reaction wheels to
spin, accelerating or decelerating as needed to rotate the telescope toward
a new target.
are Hubble's targeting devices. They aim the telescope by locking onto
"guide stars" and measuring the position of the telescope relative to
the target. The sensors provide the precise reference point from which
the telescope can begin repositioning.