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Hubble Space Telescope - Endeavor To The Rescue

shuttle designed optics mirror

The design and manufacture of a space telescope like the Hubble is a large project that takes many years; of necessity, the design must be finalized early on. As a result, by the time the observatory reaches orbit its scientific instruments rarely represent the state of the art. Having this constraint in mind, the telescope engineers designed the Hubble's instruments as modular units that could be easily swapped out for improved designs. The Hubble was thus, engineered for periodic servicing missions by space shuttle crews over the course of its planned 15-year lifetime (since extended to 20 years). Its housing or outer shell is studded with a host of handholds and places for astronauts to secure themselves, bolt heads are large-sized for easy manipulation by astronauts wearing clumsy gloves, and more than 90 components are designed to be replaced in orbit. The Hubble's housing also includes a fixture that enables the shuttle's robot arm to seize it and draw the Hubble and shuttle together. The shuttle's cargo bay includes a servicing platform to hold the telescope while the bay doors are open, and astronauts can affect repairs while standing on small platforms nearby.

One benefit of the primary mirror's precision fabrication was that despite the error imparted by the systematic metrology error, the mirror's shape—error and all—was precisely known. Its surface is so smooth that if the mirror were the width of the United States, its largest variation in surface height would be less than 3 ft (1 m). Once scientists understood what was wrong, therefore, they knew the exact correction required. Replacing the primary mirror would have required bringing the Hubble back to Earth, re-building it, and re-launching it, much too expensive to be feasible; instead, designers developed an add-on optics module to compensate for the focusing error. This module would correct the "vision" of the telescope to the level originally designed for, much as a pair of glasses corrects for defective eyesight.

This module—the Corrective Optics Space Telescope Axial Replacement (COSTAR)—contained five mirrors that would refocus light gathered by the primary and secondary mirrors and relay it to the instruments. The challenge was to build the module to fit into the compact interior of a telescope that was, and would remain, in orbit, and which had never been designed for such a fix. Engineers also produced an improved version of the Wide Field/Planetary Camera, the WF/PC2, that included its own corrective optics to allow it to capture images of the clarity that astronomers had originally hoped for.

In addition to the flaw in its optics, the observatory was experiencing difficulties with its pointing stability and with its solar arrays, which turned out to be prone to wobbling due to thermal stress created during the transition from sun to shadow. This wobbling further degraded observation quality. NASA planned an ambitious repair mission that would attempt to correct all the Hubble's problems at once.

In December, 1993, the space shuttle Endeavor took off to rendezvous with the Hubble Space Telescope. During the course of the mission, astronauts performed a total of five space walks. They captured the Hubble with the shuttle arm, repaired some of the pointing gyroscopes, replaced the wobbling solar arrays, and installed the WF/PC2 and COSTAR.

The mission was a success; the contrast between the images taken before and after the repairs was stunning. Suddenly the Hubble was dazzling the world and astronomers were lining up for observing time. Since the 1993 repair, the Hubble's available observing time has invariably been booked for years in advance; in fact, it is so over-subscribed that only one out of every ten proposals for observing time can be accepted.

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