COSTAR

Almost immediately after it was deployed, astronomers found that they could not focus the telescope. They discovered that the primary mirror had been ground to a wrong dimension at the Perkin-Elmer Corporation's factory. Although the defect in the mirror was less than one-fiftieth the size of a human hair, it caused the HST to suffer spherical aberration and produce fuzzy images.

Scientists came up with a replacement "contact" lens called COSTAR (Corrective Optics Space Telescope Axial Replacement) to correct the defect in the HST. COSTAR consisted of several small mirrors that would intercept the beam from the flawed mirror, correct for the defect and relay the corrected beam to the scientific instruments at the focus of the mirror.


Photo courtesy NASA
Small mirrors that made up the COSTAR.

COSTAR replaced one of the scientific instruments when it was installed during a 1993 servicing mission by shuttle astronauts.


Photo courtesy NASA
Space Shuttle astronauts servicing the Hubble Space Telescope.


Photo courtesy NASA
Image of the galaxy M100 before (left) and after (right) the
Hubble Space Telescope's corrective optics were installed.

When the HST was tested after the servicing mission, the images were vastly improved. Now, all of the instruments placed in the HST have built-in corrective optics for the mirror's defect, and COSTAR is no longer needed.

HST is compound telescope design (i.e. Ritchey-Chretien design). Light enters the telescope through the opening and bounces off the primary mirror to a secondary mirror. The secondary mirror reflects the light through a hole in the center of the primary mirror to a focal point behind the primary mirror. At the focal point, smaller, half-reflective, half-transparent mirrors distribute the light to the various scientific instruments. As mentioned above, the corrective optics were initially supplied by COSTAR, but are now built-in to new scientific instruments.


Photo courtesy NASA / STScI
Pre-flight inspection of the Hubble Space Telescope's primary mirror.

HST's mirrors are made of glass and coated with layers of pure aluminum (three-millionths of an inch thick) and magnesium fluoride (one-millionth of an inch thick) to make them reflect visible, infra-red and ultraviolet light. The primary mirror weighs 1,825 pounds (828 kg). The secondary mirror weighs 27.4 pounds (12.3 kg).