PASADENA, Calif. (AP) -- NASA's Deep Impact spacecraft is on course for a July 4 encounter with comet Tempel 1 but mission officials are trying to determine why a telescope that will function as its main science instrument has not reached proper focus, the space agency said Friday.

Officials nonetheless expressed confidence that the mission will not be affected by the problem.

Deep Impact carries an "impactor" that will be released to collide with the comet, possibly creating a stadium-size gouge while the spacecraft's instruments collect data on the material that is hurled off.

The craft was launched on Jan. 12 and then underwent a period of testing in which its subsystems, autonomous navigation system and instruments were checked before it entered the cruise phase of the mission. A trajectory correction maneuver also was performed and went so well that a follow-up maneuver was canceled, NASA said.

The problem with the telescope, known as the High Resolution Instrument, was found after a process called bake-out in which heating is used to remove normal residual moisture from its barrel.

The moisture was absorbed into the instrument as the spacecraft waited on the launch pad and as it flew through the atmosphere into space, NASA said in a statement.

Subsequent test images showed that the telescope hasn't reached perfect focus, and a team has been formed to investigate the issue and ways to bring the instrument all the way to focus, NASA said...