How Hubble Space Telescope Works

By: Craig Freudenrich, Ph.D. & Sarah Goddard

COSTAR Saves the Day

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

Almost immediately after it was deployed in 1990, astronomers discovered a problem with their beloved $1.5 billion, 43.5-ft (13.3-m) telescope. Their new tractor-trailer-sized eye in the sky couldn't focus properly. They realize­d that the telescope's primary mirror had been ground to the wrong dimension. Although the defect in the mirror -- roughly equal to one-fiftieth the thickness of a human hair -- would seem ridiculously minute to most of us, it caused the Hubble Space Telescope to suffer spherical aberration and produce fuzzy images. Surely the astronomers didn't spend years working on the telescope only to be satisfied with unremarkable snapshots of outer space.­

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


NASAastronauts and staff spent 11 months preparing for what would be one of the most challenging space missions ever attempted. Finally, in December 1993, seven men aboard the space shuttle Endeavour rocketed into space for the HST's first servicing mission.

It took the crew one week to make all of the necessary repairs, and when the telescope was tested after the servicing mission, the images were vastly improved. Today, all of the instruments placed in the HST have built-in corrective optics for the mirror's defect, and COSTAR is no longer needed.

There's more to Hubble than COSTAR, though, and we'll talk about some of those critical parts next.