Most of the talk among the press covering the first secret launch of a space shuttle for the Defense Department last week was about the measures taken to keep them in the dark about the flight. Cars arriving at the Kennedy Space Center were stopped and some searched. Reporters were asked to show drivers' licenses. Time magazine was denied a permit to fly a plane over the Atlantic to photograph Soviet trawlers offshore to monitor the launch.

One of the most bizarre aspects was an order to replace the mission clock that shows how long the shuttle has been in orbit with a clock showing Greenwich Mean Time.

When asked why the clock had been changed, an unidentified Air Force spokesman said: "We have our reasons." Whatever the reasons, the clock was changed back Friday to reflect what NASA calls "Mission Elapsed Time." WINDOW OF SECURITY . . .

When Discovery commander Henry W. Hartsfield was inspecting the shuttle before his crew rode it into orbit last August, he discovered that a window in the mid-deck of the shuttle cabin had been replaced with a so-called "optically flat" window that is longer than it is wide.

Hartsfield asked who had ordered the new window and what it was for. He was told that the Air Force had required it.

The new window, it turns out, allows the astronauts to take better photographs of the Earth. It magnifies objects and allows them to be photographed with less distortion.

The Air Force is not saying whether the window was used on Discovery's secret mission last week. HALLEY'S UPDATE . . .

Not only is Halley's Comet on target for its once-in-a-lifetime visit to Earth this year, but it is also much brighter than scientists expected.

"We were very surprised to see an extensive coma of gas surrounding the comet's nucleus," said Raymond Newburn of the Jet Propulsion Laboratory, one of the codirectors of the International Halley Watch, which is preparing for the 31st recorded visit of the world's most famous comet. "The coma of gas extending toward the sun is now 60,000 kilometers wide, which makes it much larger and brighter than we would have thought."

Halley's Comet is now about 403 million miles from Earth and moving at a speed of 10.6 miles per second.

The comet is expected to reach the orbit of Mars on Nov. 29, 1985, and cross the Earth's orbit on New Year's Day, 1986.

The comet will come closest to the Earth after it swings around the sun and starts moving back toward Neptune. That moment will come on April 11, 1986, when the comet will be 38 million miles from Earth.