Five Spacewalks Later, Hubble Repairs Deemed a Success

THANKS to astronaut skill and the fact that its mirror flaw is a ``perfect'' error, the Hubble Space Telescope now should have the capabilities for which it was originally designed.

``Perfect'' means that the flaw that has blurred the telescope's vision was ground into its main mirror very evenly. That enabled scientists to diagnose the flaw with such fine precision that they were able to design small lenses that correct the mirror's focusing.

Astronauts on board the shuttle Endeavour installed those lenses this week - along with other new equipment - in a series of five spacewalks. It will take several weeks of testing to check out these repairs. But, right now, the Hubble team on the ground is delighted with the repair mission's success.

Hubble telescope senior project scientist David Leckrone acknowledged, ``We don't know what we have in orbit today.'' He explained: ``We know what we have electrically.... But optically, it's going to take this [testing] period of two months [to know what we have].'' However, he also admitted: ``It is extremely difficult to keep from getting excited.''

The $1.6 billion telescope went on orbit April 24, 1990, with an optical fault called spherical aberration. The outer part of its main mirror is too flat. It should be 2.34 millionths of a meter higher in relation to the center. Light from the outer edge focuses at a different place than does light from the inner area.

Astronauts have replaced a key instrument - the Wide Field Planetary Camera - with a new model equipped to compensate for this aberration. This camera was scheduled for replacement anyway.

In addition, astronauts have installed a $50 million set of lenses that will correctly focus light from the mirror for other instruments. This 670-pound device, the size of a standard refrigerator, is called the Corrective Optics Space Telescope Axial Replacement. It replaces the telescope's least-used instrument - a high-speed photometer.

The astronauts also have given Hubble three new gyroscopes to replace failed units in its pointing system and installed a new pair of solar-cell arrays. They have repaired some wiring and installed a new computer with enhanced memory. As Hubble goes back into space, it is a fully revitalized spacecraft.

The repair has been a demanding exercise. But officials point out that the fact that the telescope is designed for astronaut servicing has made the work relatively easy. With a planned working life of 15 years and 11 of those years yet to go, more service calls will be needed. The next such call may be made in another three or four years.

Even with faulty vision, Hubble has delighted astronomers with the discoveries it has enabled them to make. This includes such objects as young stars in the Orion constellation that are surrounded by shells of material from which planets might form. It has revealed new details in distant galaxies. Now astronomers expect to find even more wonders with Hubble's new sharper vision.