Space camera flaw traced to earthly mirror

The optical flaw that blurred the vision of NASA’s comet-smashing Deep Impact probe has been tentatively diagnosed as the result of overlooking a simple law of physics, sources familiar with the investigation have told MSNBC.com.

But despite widespread rumors that the same flaw had also infected the cameras for other space probes, including a Mars orbiter now on the launch pad at Cape Canaveral and a mission to Pluto due for launch next January, these sources insist that the focus problem was unique to Deep Impact — and has already been solved.

As Deep Impact fixes its sights to send out its Impactor subsatellite for a planned collision with Comet Tempel 1 on July 4, space engineers back on Earth have been literally "fixing the sight" on the main spacecraft's High Resolution Imager, which is intended to relay the best pictures of the resulting debris cloud and crater.

Shortly after Deep Impact's launch in January, engineers found that the camera was sending slightly blurred test images back to Earth, but now they have succeeded in developing mathematical methods to sharpen the focus.

In the rush to repair the damage, discovering the original cause of the focusing flaw was a less critical task than being “focused on the success of the mission,” explained Lindley Johnson, the mission's program executive at NASA Headquarters in Washington. “I’m the person who keeps track of everything and works all the issues,” he explained by telephone.

“There hasn’t been a complete investigation,” he said, “and nothing’s been signed off.” However, he said he believes the problem, which arose at the Ball Aerospace facility in Boulder, Colo., is now understood.

Johnson explained that the imaging system undergoes final calibration by projecting a simulated comet image into the optics, and a standard glass mirror 8 inches (20 centimeters) in diameter was used to reflect light from the image projector and the flight imager. “The flat mirror is the suspected cause,” Johnson said.

Low temperatures warped test mirror
The mirror became warped during the test because of the need to simulate the low temperatures that would be experienced in space, almost twice as far from the sun as Earth is. “The test chamber shroud was flooded with liquid nitrogen to bring the temperature of the detectors down to the flight operating temperature” of 225 degrees below zero Fahrenheit (-143 degrees Celsius), Ball Aerospace spokesman David Beachley explained.

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As a result, the mirror bowed slightly concave, bringing the test image into perfect focus about a quarter of an inch (7.4 mm) in front of the design position. The imager’s 12-inch-diameter (30-centimeter-diameter) mirror was in perfect shape, and the unit was calibrated by moving the light detector array farther down the telescope tube. Once the image appeared to be in perfect focus, the hardware was firmly fixed in position for flight.

“This is not a problem in the figure of the mirror,” Johnson explained, referring to the actual parabolic shape of the reflector and implicitly contrasting his spacecraft’s problem with the focus flaw that cursed the Hubble Space Telescope before astronauts installed corrective optics. “This is a problem of structural placement,” he said.

Investigators still need to determine what it was about the material properties of the flat mirror that caused the front face to contract in the cold more than the back face, creating a very small curvature. They also will be asking whether the required correction in the imager’s array position was larger than the expected uncertainty and should have been recognized as an indicator of trouble in the test rig.

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