Hubble Checks Out Well, Say Relieved Scientists

FOR scientists and engineers working with the Hubble Space Telescope, the new year has been encouraging.

``Activities continue to go extremely well'' in checking out the repaired orbiting observatory, says public information officer Mike Finneran at the National Aeronautics and Space Administration Goddard Space Flight Center in Green Belt, Md. In fact, NASA canceled last week's scheduled teleconference progress report. The checkout was going so smoothly there was ``nothing new to report,'' the agency said. Instead, NASA said it expects to release the first new Hubble pictures later this week.

This is good news for astronomers who now look forward to having an observatory that finally can deliver its design performance. That includes having a well-focused view of objects at the edge of the universe, some 15 billion light years away. A light year is the distance light travels through empty space in one year at the rate of nearly 300,000 kilometers a second. Since the universe itself is only about 15 billion years old, light from objects at that distance would have begun the journey when the universe was very young. Studying the universe's infancy was one of the Hubble telescope's primary mission objectives.

To do this, the telescope must concentrate 70 percent of an object's light within a core image only 0.1 arc-seconds in diameter, as originally planned. Because of the spherical-aberration defect distorting the telescope's 2.4-meter (94.5-inch) main mirror, only 15 percent of the light made it into that sharp-focus zone. Hubble's new ``eyeglasses'' seem to be taking care of this problem.

Endeavor astronauts installed two sets of corrective optics during their 11-day mission last month. One set is in the Wide Field Planetary Camera. This is a major Hubble instrument that was due for replacement anyway.

The other set is a general purpose device called the Corrective Optics Space Telescope Axial Replacement, or Costar in NASA-speak. It's a 2-meter-long assembly of five pairs of coin-size mirrors that can be positioned to correct the vision of the remaining Hubble instruments. The Hubble testing team has exercised both sets of corrective optics to the point at which it is ready to go ahead with test pictures.

Hubble's other observing problem was jittering caused by flexing of its two solar-cell arrays. This was due to rapid heating and cooling as the telescope crossed the day-night boundary. Stresses from this or another cause were so severe that astronauts found 1 of the 12-meter-long arrays so badly misshapen that they couldn't stow it in Endeavor's cargo bay and had to jettison it. The European Space Agency, which supplied the arrays, provided a new set that is working well. Hubble's chief scientist, Edward Wieler, has said there still are ``a few twangs.'' But the jitter problem seems to be solved. The Hubble team had controlled the jitters - which lasted for five to 10 minutes - by varying the speeds of the telescope's attitude-control flywheels.

Other repairs made during the astronauts' five work sessions Dec. 4-9 included installation of new gyroscopes, magnetometers, new computer electronics, and miscellaneous electronic and electrical repairs. These have all checked out well.

It will take several more weeks of testing before engineers can certify that Hubble is ready to resume regular service. David Leckrone, senior Hubble project scientist at the Goddard center, has warned against declaring victory ``until we have victory in hand.'' Nevertheless, he told a press teleconference Dec. 29 that so far ``people are very pleased with what they have seen.''