Hubble's vision is blurred

In focus: Before (left) and after the optical repair

When the Hubble Space Telescope was put into space on 24 April 1990, Nasa called it a new window on the Universe.

It entered orbit and all seemed to go well as the engineers started it up. The thermal system worked, as did the electronics, the stabilisers and the telemetry and solar panels.

Then they opened the door that covered the telescope optics to take the first picture. So-called "first-light" was on 20 May.

To experienced astronomers the first-light image was disturbing. It was the wrong shape. Engineers said not to worry: "The telescope has not been adjusted properly yet."

But behind the scenes, the astronomers and engineers came to realise that something was seriously wrong.

Soft focus

Another image was taken and on that fateful day, in HST's control room in the centre of the Space Telescope Science Institute in Maryland, US, astronomer Eric Chaisson watched the faulty image appear on a computer screen. He later recalled: "I sensed a total deflation in my gut."

Five spacewalks were needed to fix Hubble

It seemed incredible but Hubble's main mirror was the wrong shape and it could not focus properly.

To be fair, Hubble was good, but only as good as ground-based telescopes, and they cost a fraction of Hubble's $2 billion price tag.

It transpired that the central region of the mirror was flatter than it should be - by just one-fiftieth of the width of a human hair. This is equivalent to only four wavelengths of visible light, but it was enough. One insider said that the Hubble mirror was "very accurate, very accurately the wrong shape".

Paint speck

What had happened was that many years before, when the mirror was being made, a speck of paint had affected an optical measuring rod. The subsequent measurements were very slightly wrong.

Hubble history

1977 - Project begins

1985 - Hubble built

1990, 24 April - Hubble launched

1990, 18 May - First light

1993, December - Flaw repaired

2000, 24 April - 10 years of Hubble

2010 - End of Hubble mission

The mirror looked perfect, but in reality it was not. In Nasa's often-ridiculous jargon, it was said to be a "failure to meet a level-one requirement". In normal language, it was a complete catastrophe.

With no explanation, the daily Hubble status reports ceased and a short while later a press conference was called.

At that traumatic presser, Nasa official Leonard Fisk said it was space science's equivalent of the Challenger disaster. It was said he wept.

One journalist quipped: "Hubble is working perfectly but the Universe is all blurry."

Hubble trouble

Astronomers were devastated. Richard Ellis of the University of Cambridge Recalls: "It was desperate and very depressing. We were the butt of jokes.

Foggy view: Hubble before its repair

"We could see just enough to see what we were missing," he says.

But out of defeat came triumph. Hubble had been designed to be serviced and repaired in orbit by astronauts.

The in-orbit repair of Hubble was one of the landmarks of manned spaceflight. A series of corrective mirrors were fixed to Hubble in an unprecedented series of five spacewalks on a single space shuttle flight.

A few weeks later, Hubble once again turned its gaze towards the stars. They were sharp and crisp. As one US Senator put it: "The trouble with Hubble is over."

Hubble's main mirror: Perfectly flawed

Words by Dr David Whitehouse; Images courtesy of the Space Telescope Science Institute