From 10 feet away, Pancam has a resolution of 1 millimeter per pixel. “It’s Mars like you’ve never seen it before,” says Steven Squyres, Cornell professor of astronomy and principal investigator for the suite of scientific instruments carried by the rovers. The word pixel is derived from the term “picture element.” Digital images are composed of touching pixels, each having a specific color or tone and the eye merges different colored pixels into continuous tone images. One megapixel is a million pixels set up in an array equal to 1,000 by 1,000.
Pamcam seeks to perfect digital technology
The technology used to make Spirit’s Panoramic Camera, or Pancam, is essentially the same as what goes into any digital camera. 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. As each twin-lens CCD (charge-coupled device) camera takes pictures, the electronic images will be sent to the rover’s onboard computer for a number of image processing steps, including compression, before the data are sent to Earth. Perhaps most important factor is that the sensors on Spirit’s CCDs are bigger. 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. The Pancam has a million sensors spread across a chip that’s 12 by 12 millimeters — nearly a half-inch square.
In the consumer market, a 5-megapixel camera often uses the same size CCD as on a 3-megapixel camera, it’s just that more pixels are crammed onto the same-size chip. So as the pixels themselves get smaller, this has an impact on image quality. 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 such as shading, the same information can get spread across several pixels after passing through a lower quality lens.
Dalsa, based in Waterloo, Ontario, Canada, makes cinema-quality video components and other high-end imaging devices and was called on to make the CCDs for the Pancam and the other cameras on Spirit and its twin rover, Opportunity. 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.
Tricks with light
The 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 Hubble Space Telescope, 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 on the other hand, uses a single coated filter to make the transition from photon reality to electrons and then digital information.
Pancam’s mast can swing the camera 360 degrees across the horizon and 90 degrees up or down. Scientists will know a rover’s orientation each day on the Martian surface by using data gained as the camera searches for and finds the sun in the sky at a known time of day. The pictures are expected to reveal important geologic details about rocks, and they’re also used for navigation and to pick distant science targets.
The Viking missions, as well as the Voyager missions to the outer planets, used technology similar to antiquated television vacuum tubes. CCD technology was first developed in 1969, but it took decades before arrays were big enough to be useful. NASA funded much of the research that ultimately led to today’s commercial digital cameras.
Processing pictures from 100 million miles away will be no easy feat. It took three years for Cornell faculty, staff and students to precisely calibrate the Pancam lenses, filters and detectors, and to write the software that tells the special camera what to do. 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.”
