Surveying the Scene – Martian Style
Surveying the Scene – Martian Style
|The new Pancam design has a camera bar that contains Pancam and Navcam (navigation camera) heads. A "visor" changes the elevation of the cameras so the rover can look up or down.
Credit: Cornell University
Like all good exploration vessels, each of the Mars Exploration Rovers, Spirit and Opportunity, includes a mast. Instead of a crow’s nest and a lookout, however, the rover mast-a little more than a meter tall-is topped by a pair of cameras-collectively called the Pancam (panoramic camera)-that can image the scene around the rover as sharply as a person with 20-20 vision.
The Pancam’s two lenses aim slightly inward from parallel, allowing twin images to be used as a stereo pair, showing the scene in three dimensions. And the whole camera head can rotate 360 degrees, as well as tilting up and down 90 degrees.
The Pancam design began, literally, from the ground up.
"The governing requirement was that we be able to take images of potential obstacles about the size of the wheels of the rover about 100 meters away," says Jim Bell, Pancam lead scientist and an astronomer at Cornell University.
Beginning with that requirement-and some cost constraints-the scientists designed the Pancam’s one-megapixel digital "film," a charge-coupled device (CCD) similar to those used in consumer digital cameras. The same space-certified CCD is used on other rover cameras as well.
The requirement of being able to make a sharp image at a distance also determined the Pancam lenses’ focal lengths, about the same as a moderate telephoto lens on a consumer digital camera or a 110-mm lens on a 35-mm-film camera.
|The Pancam CCD housing assembly. Here the Pancam CCD itself is covered by a protective piece of orange Kapton tape.
"Each image is just going to cover a very small part of the landing site," Bell says, "so it will take literally hundreds of images to make these beautiful mosaics."
Onboard software will perform some image correction similar to what astronomers normally do with astronomical images. This software will also compress the image data using a method similar to those used for serving images on websites. Finally, ground-based software will help researchers stitch hundreds of images into mosaics, for example an uninterrupted 360-degree panorama.
The rover’s navigation cameras will do most of the range finding, telling the computer how far away objects are. But the Pancam may also prove useful as a rangefinder in certain circumstances.
"The niche for Pancam is range finding at far distances, because we have much better resolution than the other cameras," Bell says. With its sharper resolution, the Pancam can make sharper images of distant objects for the rover to investigate-or to avoid.
|Each "eye" of the Pancam carries a filter wheel that gives Pancam its multispectral imaging capabilities.
The CCD used in the rovers cannot record color. To produce a true-color image, the Pancam must take three photos, each with a different-color filter. Pancam includes eight filters per lens (all of the optics are protected by sapphire lens covers), so in addition to true-color images it can make images at various wavelengths, including those slightly beyond the range of human vision in both the ultraviolet and the infrared parts of the spectrum.
"We’re looking to use the color information to help determine the composition, especially of iron-bearing minerals that have very small changes in their colors in the infrared wavelengths beyond where the human eye is sensitive," Bell says. Iron is important to Mars geologists because it provides clues to the history of the rocks and soils, perhaps implicating watery environments that could have harbored life.
Each lens also includes a special filter allowing the Pancam to image the Sun directly. To color calibrate the cameras, rover designers included a calibration target with three gray regions of varying darkness, color patches that reflect blue, green, red and near infrared, and a central post that will cast a shadow, allowing the scientists to determine the color cast created by sky light, as opposed to sunlight. The calibration target can also act as a sundial for K-12 education purposes.
In addition to showing individual rocks and distant dunes, the Pancam’s photo mosaics will reveal how wind or surface water may have shaped the Martian landscape.
The Pancam images will be the best-ever pictures of the Mars landscape, Bell says. The Pancam "is about three times higher resolution than the Mars Pathfinder and the Viking landers’ highest-resolution mode. These are very close to 20-20 human vision resolution."
"We land on January 4 [in 2004] UT or late night on the 3rd Pacific time," Bell says. "So we’re hoping to be able to get some images down that first day. If not on the first day, then definitely on the second day. Immediately on landing we’ll start doing the various instrument checkouts and making sure things are working well and then we can really start diving into the investigation."