Space camera flaw traced to earthly mirror

Problem comes into focus
The focus flaw was at first masked by an expected problem that was supposed to heal itself. The graphite-epoxy material comprising the 40-inch-long (meter-long) telescope tube had absorbed moisture from the air while on the launch pad at Cape Canaveral, and had swelled somewhat. Engineers expected the focal point to shift by about a quarter of an inch, but once in flight that deviation would be reversed by exposure to space vacuum and by cycling heaters to encourage evaporation.

NASA's Johnson said the first test observations of bright stars indicated that the focal point was as much as half an inch off, twice the expected distortion. Three bake-out heater sequences were performed, and “there was some collapse, but not as much as was needed.”

“After the second bake-out, it was not coming in as rapidly as needed,” he said. More images were taken, and a third bake-out made no difference. There was still a quarter-inch error in the focal point.

The loss of focus would blur the images significantly, threatening one of the top science goals of the mission — to measure the actual shape of the hole gouged into the comet’s surface. The camera would be able to make out details only down to 8 meters wide (roughly 24 feet), rather than the 2-meter (6-foot) resolution that scientists hoped to achieve.

Because the actual defocus cause was so well understood, mathematicians could develop complex computer programs to remove most of the blur and restore the imager’s precision. Experts say this works only because the problem is so well known and there is so little static, or “noise,” in the blurred images. They say the technique cannot be used to improve the output of imagers that are already in focus.

How common is this error?
As to the possible contagion with imagers on other probes, the high anxiety expressed in Internet traffic among space imaging scientists has somewhat diminished, now that the unique thermal-based nature of the mistake has become clear. There had even been concern that a camera aboard the Mars Reconnaissance Orbiter, already delivered to Cape Canaveral for launch in August, may have had a similar calibration error.

“Our look at it has not indicated that it has affected any other instruments,” Johnson said. The same test rig was used to calibrate the flight imager for the New Horizons mission to Pluto, due for launch in six months. “This source of error is not an issue” for this imager, he explained. Because the imagery will be in the infrared, with a much shorter focal length, the same problem would have practically no effect on the focus of the instrument.

Ball Aerospace's Beachley concurred. “We are taking every measure to ensure that the problem will not recur,” he said.

Similar optical systems for the Kepler planet-finding spacecraft are being built and tested by the same facility, and the Deep Impact flaw has led to heightened care at all stages of production.

A third source involved in the New Horizons mission, commenting on condition of anonymity, said in an e-mail mission that even though the camera was delivered late, threatening the planned launch date, an additional test was scheduled just to make sure about the focus.

“To be extra safe, we are in the process of planning a focus check with another set of equipment,” the source wrote on April 13, “in order to independently show that we are fine and thereby retire this issue from the worry list.”

The source confirmed to MSNBC.com that the instrument passed the extra test.

MORE FROM SPACE

Space Section Front

The coldest places in the solar system   Decrepit stars locate the fountain of youth Trio brings holiday cheer to space station Rover makes discovery while spinning wheels Cosmic Log: The year in space   2009's best images from space How much real science is in ‘Avatar’? Mystery swirls around ‘dark stars’ Cassini image confirms liquid on Saturn moon Long-defunct Mars rover wheel spins Space Section Front   Add Space headlines to your news reader: