NASA finished testing the $1.5 billion Hubble Space Telescope aboard Discovery and started final pre-countdown work Thursday, raising the space agency's hopes for a timely launch next week. It is the most comfortable NASA officials have felt about Tuesday's launch date since a power outage interrupted the testing Monday, said George Diller, a spokesman for the space agency. ``Basically, the work's done. We're just down to the things you can't do before now anyway,'' Diller said. ``We're essentially on or near schedule on all of that work. I'm really not aware of anything that's unusually shaky,'' he said. The countdown is scheduled to begin Saturday afternoon. Early forecasts show an 80 percent chance of weather favorable for launch Tuesday, with the weather worsening slightly during the next two days. Functional testing of the telescope and shuttle was completed early Thursday, and the launch pad was cleared of all non-essential personnel for final preparation of the payload and orbiter. Technicians were in the middle of the 52-hour test when a power outage at Kennedy Space Center knocked out air conditioning and forced the shutdown of heat-sensitive computers. Testing resumed Tuesday evening and had to be completed by Thursday morning for the mission to stay on schedule. As soon as the testing ended, technicians began the routine procedure of placing explosive devices on the shuttle's solid rocket boosters, external tank and the orbiter itself to be used in the event of a serious malfunction, said NASA spokesman Bruce Buckingham. Power was turned off to the telescope and shuttle so that the hazardous work could be performed. Charging of the telescope's nickel-hydrogen batteries also was halted and was to resume. The telescope's batteries will be charged until the shuttle's payload bay doors are closed two days before launch. The 24,250-pound, 43-foot-long telescope, named for the late American astronomer Edwin P. Hubble, is the most expensive unmanned spacecraft ever built by the National Aeronautics and Space Administration. It originally was scheduled for launch in 1983, but was delayed due to technical problems and the 1986 Challenger explosion. Discovery's five astronauts plan to deploy the telescope on the second day of the five-day mission. The Hubble will orbit 380 miles high for 15 years, enabling astronomers to study stars and galaxies so distant that their light has been traveling to Earth for 14 billion years. It will be capable of detecting objects 50 times fainter and with 10 times greater clarity than the best ground-based observatory.