NASA identifies what hobbled Hubble.

NASA identifies what hobbled Hubble

Two errors -- one human, one mechanical -- worked in tandem to cripple the Hubble Space Telescope's primary mirror, a NASA-appointed investigative team announced last month. But in contrast to the human goof -- upside-down insertion of a precision measuring tool into an optical system that guided mirror grinding -- the flawed construction of that measuring tool alone would have likely resulted in a defective primary mirror, says Roger Angel, a panel member and a telescope-mirror designer at the University of Arizona in Tucson.

Hubble's mirror makers used laser light and the precision measuring tool -- called a metering rod -- to determine how far apart to space two components of the grinding guide, a device known as the reflective null corrector (SN: 7/21/90, p.39). Measuring distances this way should have provided far greater precision than using a micrometer. However, the technique required passing a laser beam through a tiny hole in a nonreflecting sleeve capping one end of the precision metering rod, so that the beam would bounce off the rod and not its sleeve. And in cutting the millimeter-sized hole in that sleeve, technicians accidentally chipped the cap's nonreflecting coat. This caused the laser beam to erroneously bounce off both the sleeve and the rod. The extra signal returning from the sleeve confused scientists, and inadvertently led them to improperly position the null corrector's components.

Human error appears to have confounded the spacing error, the panel notes. Accidentally inverting the metering rod before capping it -- so that the sleeve loosely covered the opposite, more poorly machined end -- caused scientists to place the test lens and small mirror in the null corrector 1.3 millimeters farther apart than intended, Angel says. Those errors, the panel says, led to the spherical aberration that today renders Hubble's primary mirror virtually useless -- without corrective lenses -- for resolving faint, distant objects in the universe.

As the NASA panel prepares its final report, due next month, it will examine how the measuring error escaped detection. Several clues that were dismissed or ignored hinted something might be awry, Angel told SCIENCE NEWS. Examples he cited include a test of the reflective null corrector that spotted what appeared to be errors in its assembly, and measurements taken with a second type of null corrector indicating that the primary mirror was indeed misshapen.