Fixes may restore Hubble to 90% of its capabilities

December 19, 1990|By Luther Young

NASA has given the green light to a $30 million-to-$40 million "fix" for three instruments on the Hubble Space Telescope that -- in combination with an optically corrected replacement for the main camera -- could restore the observatory to 90 percent of its planned capabilities.

The so-called COSTAR project was recommended to the space agency by a scientific panel assigned to consider ways to compensate for the mirror flaw in the $1.5 billion orbiting telescope that hinders its ability to focus on distant and faint objects in the universe.

"We're going ahead with it," said Dennis McCarthy, deputy program manager for the Hubble Space Telescope at National Aeronautics and Space Administration headquarters in Washington. "The bottom line is, we want to fix what we've got as soon as we can, and this could do it."

Mr. McCarthy said COSTAR -- a device that would fit into one of the instrument bays and deploy five pairs of tiny prescription mirrors to refocus starlight for the three scientific instruments -- could be under way by May and finished in time for installation by astronauts in 1993.

When the flaw in the Hubble mirror, known as spherical aberration, was discovered two months after the April 24 launch, NASA was already building a replacement for the Wide-field/Planetary Camera (WF/PC), the workhorse instrument expected to provide spectacular images and answer basic questions about the universe and its evolution.

That replacement camera, the $70 million WF/PC II, is now being outfitted with prescription optics to compensate for the flaw in the 8-foot primary mirror and is expected to be ready for delivery in December 1992, said Dr. Edward Weiler, Hubble program scientist.

But the other instruments aboard -- the Faint Object Camera, the High Resolution Spectrograph, the Faint Object Spectrograph and the High Speed Photometer -- were initially doomed to diminished vision,although a replacement spectrograph is planned for 1996 or 1997.

"Our charge was to look at all the optical possibilities for recovering the full capability of the telescope, not just for the WF/PC," said Robert Brown, an astronomer at the Space Telescope Science Institute who co-chaired the "strategy" panel.

After considering more than 30 scenarios -- including a spacewalk by astronauts down the telescope tube to place an obscuring ring, or aperture stop, on the mirror -- the panelists decided COSTAR was best and recommended it in a report delivered to NASA last week.

The report also pointed out two additional problems that must be solved on the Hubble: a shaking of the solar panel that is preventing precise pointing of the telescope, and the need for an internal actuator on WF/PC II to align its corrected optics once they are in orbit.

Dr. Weiler said the solar panel problem, caused by uneven heating effects when the spacecraft passes between day and night, should be largely corrected by March through computer software adjustments and solved for good with the replacement of the panels in 1993.

But NASA is being cautious in touting COSTAR, "a very difficult thing to build" because of the extremely precise mirrors required and their small size, less than an inch in diameter, Mr. McCarthy said.

He added that the space agency planned a final design review in the next four months and will require test mirrors to be manufactured by several companies to prove the technical feasibility of the optics.

Dr. Brown called COSTAR "a very elegant solution" and lauded its "great beauty, because it fits flawlessly into NASA's plans to service the instruments."

But it will not totally restore Hubble to the performance originally promised. "It's considerably less difficult to get 90 percent," Mr. McCarthy said. "So we've decided to trade off schedule and risk and cost against 100 percent. We think that's a prudent compromise."

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