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How Spirit makes great photos

Photographs o f IMAX quality that were taken by NASA's Spirit Rover suggest photographers should spend their money on lens and not pixels.
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NASA's Spirit Rover is providing a lesson to aspiring digital photographers: Spend your money on the lens, not the pixels.

Anyone who has ever agonized over whether to buy a 3-megapixel or 4-megapixel digital camera might be surprised to learn that Spirit's stunningly detailed images of Mars are made with a 1-megapixel model, a palm-sized 9-ounce marvel that would be coveted in any geek's shirt pocket.

Spirit's images are IMAX quality, mission managers say.

The word pixel is derived from the term "picture element." A pixel is the smallest dot of information that goes into making a digital image. One megapixel is a million pixels set up in an array equal to 1,000 by 1,000.

Intuitively, more pixels means higher resolution. That's generally true on a display screen. But when capturing images, where a pixel is more properly called a sensor, the count is just one of many factors that control quality.

But the Pancam's lenses M there are two, which provides stereo imaging capability — are crafted more finely than anything you'd probably want to plunk down a Visa for. And the light-capturing chunk of silicon, called a charged coupled device, or CCD, was manufactured with no tolerance for the minor flaws that are inherent in mass-produced consumer cameras.

Perhaps most important, the sensors on Spirit's CCDs are bigger, explained Patrick Myles, director of corporate communication at the Dalsa Corporation, which built the CCDs for all of the rover's cameras (Spirit has nine altogether, including hazard avoidance cameras and a microscopic imager).

A Sony DSC-F717, with a street price of around $600, has 5.2 million sensors (or 5 megapixels) on a chip that is 8.8 by 6.6 millimeters (or .35 by .26 inches). The Pancam has just a million sensors spread across a chip that's 12 by 12 millimeters — nearly a half-inch square.

Each tiny Pancam sensor, measured in microns, is nearly four times as big as those on the Sony.

In the consumer market, which Dalsa does not target, 5-megapixel cameras often use the same size CCD as a 3-megapixel camera. More pixels are simply crammed onto the same-size chip.

"The pixels themselves get smaller," Myles said. "This has an impact on image quality."

Why? For one thing, smaller pixels are less light-sensitive.

Also, the lens quality might not support the additional pixels. As the receptors get smaller, a higher quality lens is needed to properly focus light onto each pixel. So where each pixel ought to capture different light information — say perhaps a subtle shading change on the subject's cheek — the same information can get spread across several pixels after passing through a lower quality lens.

"They are the world's highest performing chips in terms of light sensitivity and chip quality," Myles said in a telephone interview earlier this week.

Overall, how does a Pancam stack up to the typical 5-megapixel camera you might purchase at Best Buy?

"There really isn't any comparison," Myles said.

NASA officials say the camera shows what a human with 20-20 vision would see on the surface of Mars. But anyone who has on a distant rock in one of Spirit's color pictures would have to wonder if perhaps Superman's vision might be a better comparison.

Experts argue endlessly about what the human eye can actually see, however. Comparing human vision to what a camera captures "is really up to great speculation," Myles said.

NASA's analogy, Myles explained, is "probably a bit of marketing spin. It helps people visualize the quality." The height and breadth of a Pancam image is roughly equal to what a person would see, taking into account peripheral vision. And the Pancam has a human perspective. It sits atop a mast on the rover, 5 feet (1.4 meters) above the surface.

Myles said the actual image quality probably exceeds human capabilities, especially after the image is processed and a computer is used to provide a .

Tricks with lightThe Pancam does not make a color picture directly. Instead, it records light versus dark in shades of gray. As with other CCD cameras used in high-end astrophotography, such as on the , a series of filters are applied to gather multiple images that are then blended together.

In the most basic application of this process, three images are gathered of a scene, one each recording red, green and blue light. Those are then put together with special software to create a color picture.

A consumer digital camera uses a single coated filter to make the transition from photon reality to electrons and then digital information.

Additionally, the Pancam swivels 360 degrees around and 90 degrees up or down, so that individual scenes can be stitched together to create a view of the rover's . The pictures are expected to reveal important geologic details about rocks, and they're also used for navigation and to pick distant science targets.

Much of the research that ultimately led to today's commercial digital cameras was funded by NASA. A first major step was in developing an 800 by 800 pixel array -- less than a megapixel -- which is what's in the Hubble telescope.

The Pancam results so far have mission managers ecstatic. Cornell astronomer James Bell, who led the development of the camera, called the first Spirit pictures "absolutely spectacular."

Nobody has argued with him.

In fact, Steven Squyres, a Cornell professor who directs the rover science team, called Bell "the Ansel Adams of the Space Age."