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.
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.
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.