NASA tried to get its troublesome $150 million observatory working right Wednesday as more and more star-gazing time evaporated on Columbia's scientific mission, already set back by four years of delays.

Astronaut-astronomers Ron Parise and Robert Parker got in 36 percent of the observation time planned for their 12-hour shift that ended this morning, NASA said. That compares to 17 percent in the crew's previous work period and none before that.Among the targets viewed Wednesday was a galaxy 41 million light years away that is believed to contain a black hole and Crab Nebula, a remnant of a supernova that occurred more than 900 years ago.

"We're trying as hard as we can," Parise told ground controllers.

Ten percent of the Astro observatory's 230 planned celestial targets could have to be scrapped because of the trouble with its automatic aiming systems, NASA scientist Ted Gull said Tuesday.

Several days could pass before the observatory aboard the shuttle is working at peak efficiency, Gull said.

After a day of problems Monday, Columbia's astronauts reported early Tuesday that the observatory's instrument pointing system appeared to be working properly.

The system, which automatically aims Astro's three ultraviolet telescopes, developed glitches later in the day before it was coaxed back into operation early today.

Problems persisted, however, through the morning. Spacelab Mission Operations Control's John David Bartoe at one point informed the astronauts that "this is by far the most stable pointing we've seen so far."

"Except that the target isn't in the (telescope)," replied Parise.

New software later was transmitted to correct the sensitivity of one of the instrument pointing system's three star trackers. Fine-tuning of the system was expected to continue through the day.

The observatory also has an X-ray telescope, which has its own pointing system and is controlled by computer commands from the ground. The telescope unexpectedly fell out of alignment Tuesday night, but ground controllers corrected the problem today and got the instrument operating again.