The space shuttle astronauts today spent their third day in space "pumping aluminum" and using a $100 million communication satellite for what flight directors said was the clearest space television transmission ever beamed back to Earth.

Although persistent computer troubles at the satellite control center in White Sands, N.M., kept the Tracking and Data Relay Satellite (TDRS) in the dark for half of the day, flight directors here said that voice, data and television transmission through the TDRS the rest of the time was the best in the 20-year history of the manned space program.

"We're looking at television through the Charlie camera," astronaut John Blaha said from the ground this morning at the Mission Control Center, referring to the third of four cockpit cameras.

"It's just a beautiful definition that we have with the TDRS," he said. The satellite, launched from the previous shuttle flight, is a switchboard in the sky to be used by shuttle missions for the next 15 years as their communication relay station.

Later, flight directors used the TDRS to transmit to Challenger printed instructions for the astronauts. The instructions appeared in the cockpit seconds later without a smudge on the reproduction paper, the crew reported.

"These printouts really look like they came off a reproduction machine down the hall," flight director Harold Draughon said. "When the satellite is working, it works beautifully."

But the satellite worked only half of the time. Officials blamed software in White Sands computers that often failed to acknowledge commands and to position correctly three antennas involved in using the TDRS. These are at White Sands, aboard Challenger and on the satellite.

While the TDRS is not essential as a relay station in this mission, it is vital to the planned ninth shuttle mission, scheduled to lift off Oct. 8.

That flight by Columbia, which made the first six shuttle missions, is to carry the $1 billion Spacelab built by the European Space Agency and be equipped with 40 expensive experiments intended for repeated and random testing throughout the planned nine-day flight.

If similar trouble appears during the Spacelab mission, Draughon said, about 25 percent of the astronomical and life-sciences experiments to be performed will be lost.

Draughon said 90 percent of today's problems involving the TDRS were on the ground and in programmed instructions given White Sands computers. The problems have not had an impact on this mission, Draughon said, "and we expect Spacelab to fly on time."

For almost six hours today, astronauts Richard H. Truly and Dale A. Gardner used the shuttle's 50-foot-long robot arm to maneuver an 8,000-pound device to test the limits of the arm's strength. The aluminum device is shaped like a dumbbell.

Tested twice before, the arm was called on for the first time today to move and haul that large a mass in space. Said Truly after just a few minutes of exercising the arm: "It works like a champ."

Pilot Daniel C. Brandenstein flew Challenger, while Guion Stewart Bluford II and Dr. William E. Thornton spent most of their time below decks performing experiments.

Bluford ran a device that uses weightlessness to separate live cells for experiments intended to produce cells that make insulin to treat diabetes.

Thornton attached electrodes near his eyes to study the eyes' role in adaptation to weightlessness.