Hubble Space Telescope

After the Hubble's launch in 1990, astronomers eagerly awaited its first observations. When they saw the test images, however, it quickly became clear that something was seriously wrong: the Hubble had defective vision. Scientists soon realized that the primary mirror of the space telescope suffered from a spherical aberration, an error in its shape that caused it to focus light in a thin slab of space rather than at a sharply defined focal plane. In the focal plane, therefore, a star's image appeared as a blurred disk instead of a sharp point.

The fabrication of a large astronomical mirror such as the Hubble's primary is a painstaking task. The mirror is first cast in the rough and must be ground and polished down to its precise final shape. The computer-controlled tools used for this process remove glass from the rough cast one micron at a time. After each grinding or polishing step, the mirror is re-measured to determine how closely it approximates the desired shape. With these measurements in hand, engineers can tell the computer how much glass to remove in the next grinding or polishing pass and where the glass must be removed. This cycle of grind, polish, measure, and re-grind, a single round of which can take weeks, must be repeated dozens of times before the mirror's final shape is achieved.

During the metrology (measuring) step, a repeated or systematic error caused the manufacturers to produce a mirror with a shape that was slightly more flat around the edges than specified. The error was small—the thickness of extra glass removed was a fraction of the width of a human hair—but it was enough to produce a significant spherical aberration. Although useful science could still be performed with the telescope's spectroscopic instruments, the Hubble was unable to perform its imaging mission.