That tube is equipped with light baffles to keep stray light from reaching the main mirror. The tube terminates in an aperture door that closes off the tube when not in use. The support systems module that contains the various electronic and mechanical systems required to operate the observatory surrounds the lower end of the tube. The Hubble Space Telescope was designed to be serviced and upgraded in space using the Space Shuttle. The exterior of the satellite is thus equipped with handrails to be used by astronauts.
Hatches provide access to the interior to permit replacement of the modular instrument packages.
Pointing of the telescope is accomplished by the use of reaction wheels. Since only two at one time are used for pointing, the third provides data for mapping celestial objects. In its original launch configuration the observatory included five additional instruments. The remaining instruments were located axially behind the focal plane. The spherical aberration evident in the main mirror was soon found as the instrument was being calibrated.