Hubble's Mirror Flaw
Shortly after the Hubble Space Telescope's launch in 1990, scientists and engineers discovered that the observatory's primary mirror had an aberration that affected the clarity of the telescope's early images.
Launched on April 24, 1990, NASA’s flagship Great Observatory has far surpassed its original mission goals. Hubble has more than doubled its 15-year life expectancy and significantly expanded our understanding of the universe while simultaneously unveiling new mysteries that prompt further investigation. Hubble’s more than one and a half million observations have generated some of our most iconic views of the universe, enchanting their way into the global zeitgeist of the late 20th and early 21st centuries. But this Great Observatory hasn’t always had such a positive reputation.
Two months after its launch, on June 27, 1990, NASA announced that the telescope was flawed. Both of Hubble’s imaging cameras (the Wide Field and Planetary Camera and the Faint Object Camera) showed the same characteristic distortion, called spherical aberration, which must have originated in the primary mirror, the secondary mirror, or both. To investigate the cause of the flaw, NASA formed the Hubble Space Telescope Optical Systems Board of Investigation on July 2, 1990. The board, which included optical scientists and senior NASA personnel, reviewed documentation related to Hubble’s fabrication and testing. They analyzed and tested the equipment used in the creation of the telescope’s mirrors, and interviewed the people involved in the telescope’s construction and testing. Their investigation uncovered that Hubble’s primary mirror had the wrong curve. It was too flat near its outer edge. The error was 10 times larger than the specified tolerance needed for its design to work properly.
What Happened?
Hubble is a type of telescope that uses mirrors instead of lenses to bring distant objects into focus. Its primary mirror is one large 7.9-foot (2.4-meter) diameter disk of Ultra-Low Expansion Glass® weighing 1,825-pounds (828-kilograms). Its specific design, a Ritchey-Chrétien Cassegrain, is compact and relatively lightweight compared to other telescope designs of this size, making it a good choice for launching into space. The design is also very good at seeing distant objects like galaxies.
The contractors that fabricated Hubble’s large mirror improperly set up a device used to test its curve or figure during the final shaping and polishing process. Investigators found the device used to test Hubble’s mirror, called a null corrector, undisturbed and in the same configuration as it was when fabricators tested the mirror nine years earlier. By studying the null corrector, they were able to determine that Hubble’s main mirror had the wrong shape because of a lens spacing error. The spacing was off by 1.3 millimeters, which resulted in a mirror that was too flat away from its center.
Hubble’s primary mirror should have brought all of the incoming light to a specific focal point on Hubble’s secondary mirror. Because the outer edge of the mirror was flatter than specified, light landing on that outer edge had different focal points than the other parts of the mirror. This flaw, called spherical aberration, gave Hubble’s primary mirror more than one focal point, which made its images blurry.
The same thing can happen in our eyes. If there are irregularities in the curve of the eye’s lens or its cornea, light entering the pupil can have multiple focal points, making our vision blurry. We correct for those irregularities with eyeglasses, contacts, or laser surgery. And that’s what NASA did with Hubble.
Solving the Problem
When the Wide Field and Planetary Camera 2 (WFPC2) team learned of the problem with Hubble’s primary mirror, they started researching ways to correct the flaw within the optics of the camera itself. They redesigned WFPC2 to include internal optics that brought Hubble's blurry data into focus. Their work led to Hubble’s Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument package. About the size of a large refrigerator, COSTAR acted like a pair of eyeglasses for Hubble’s remaining scientific instruments, which included the Faint Object Camera (FOC), Goddard High Resolution Spectrograph (GHRS), and the Faint Object Spectrograph (FOS). Astronauts installed both WFPC2 and COSTAR during Hubble’s first servicing mission in December 1993.
Like WFPC2, all instruments installed after the flaw’s discovery had built-in corrective optics. This eventually made COSTAR obsolete. Astronauts removed COSTAR in May 2009 during Servicing Mission 4 to make room to install the Cosmic Origins Spectrograph. They returned COSTAR to Earth where visitors to the Smithsonian National Air and Space Museum in Washington, D.C. can see it on display.
Thirty Years of Unprecedented Science
Hubble may have had a rocky start, but through the hard work and dedication of NASA’s scientists, engineers, and astronauts, Hubble’s view came into focus. Hubble’s discoveries have rewritten textbooks and won a Nobel prize, while simultaneously presenting us with new questions to answer. Today, Hubble continues to add to its unparalleled archive of observations, making it one of the most productive scientific instruments in history.
