By skyforbes 
The mirror was supposed to bring all incoming light to one sharp focal point, but due to a small 1.3-millimeter spacing error in a testing device called a null corrector, the mirror’s outer edge was too flat. This caused spherical aberration, meaning light focused at different points and produced blurry images.
A NASA investigation traced the issue to mistakes made during mirror testing and confirmed that both of Hubble’s original cameras showed the same distortion. The problem was similar to how irregularities in a human eye can cause blurry vision. To fix Hubble, engineers redesigned the Wide Field and Planetary Camera 2 (WFPC2) with built-in corrective optics and created an additional instrument called COSTAR, which worked like a pair of “eyeglasses” for the telescope’s other scientific instruments.
Astronauts installed both systems during a 1993 servicing mission, restoring Hubble’s ability to capture the clear, detailed images that have since transformed our understanding of the universe.
Source: NASA