However, something was wrong. The same problem that had haunted telescopes for hundreds of years reared its head again: spherical aberration.
It wasn’t supposed to happen. But Hubble’s primary mirror had been ground incorrectly. Its curve was wrong – it was just a tiny bit too flat, off by a depth about 50 times smaller than the thickness of a human hair. The pictures being beamed back to Earth, while still better than the images from most ground-based observatories, were not the quality scientists had expected. They were blurry.
Fortunately, Hubble had been specially designed with interchangeable parts so astronauts could visit it to perform repairs and update the telescope’s instruments during servicing missions. It was the first NASA mission designed to be visited and improved by astronauts.