An investigative team last week concluded that the flawed mirror aboard the crippled $1.5 billion Hubble Space Telescope was caused by a measuring rod that was inserted backwards in an optical device used to guide grinding of the mirror.

The rod was used a decade ago to achieve the proper spacing between mirrors and a lens in the device, called a reflective null corrector. By shining light through the device onto the Hubble mirror, and letting the light reflected from the mirror shine back through the device, technicians could tell whether the Hubble mirror had the right curvature.

Unfortunately, whoever handled the measuring rod made a profound bungle. It was inserted backwards. Because its ends are slightly different, they did not fit properly into other parts of the device intended to receive them. As a result the spacing between a mirror in the device and a lens was off by 1.3 millimeters. To make the Hubble mirror look right when viewed through the device, technicians ground it to the wrong curvature. The curvature is off by less than the thickness of a sheet of paper.

The chairman of the investigating team, Lew Allen of the Jet Propulsion Laboratory, said the team will next try to determine how the mistake could have escaped detection.