The Third Servicing Mission continues this tradition. Its two challenging flights promise to upgrade the Hubble Space Telescope with the latest tech-nology hardware for spacecraft systems and to incorporate advanced instruments that will significantly expand Hubble's scientific capabilities. Figure 1-5 lists specifications for the Telescope. The OTA consists of two mirrors, support trusses, and the focal plane structure. The optical system is a Ritchey-Chretien design, in which two special aspheric mirrors form focused images over the largest possible field of view.
Incoming light travels down a tubular baffle that absorbs stray light.
Shortly after launch in 1990, a spherical aber-ration of the primary mirror was detected. An investigation revealed the specific errors that caused the fault. This knowledge allowed optical experts to design and fabricate small pairs of corrective mirrors that successfully refocused the light reflected from the primary mirror before it entered the axial science instruments. In addition, WFPC2, a radial instrument with corrective optics incorporated, was installed on the same mission. The subsequent increase in optical performance helped to restore the Telescope's capability to original expectations.