Poised on the threshold of Victoria Crater
Posted by Emily Lakdawalla
29-08-2007 13:26 CDT
Although Mars' atmosphere continues to be rather dusty, the storm has abated enough that both Mars Exploration Rovers have resumed a relatively normal level of activity. For Opportunity, that means a drive to the very edge of Victoria crater. Opportunity originally planned to enter the crater nearly two months ago. But the storm intervened. Opportunity finally made its first move toward the rim on sol 1271 (August 21), and now has forward wheels perched right on the edge:
There are two reasons Opportunity made this move. One was to get ready to enter the crater; the other was in case of a resurgence of the dust storm. The slope that Opportunity is pointed at faces south. If the skies get dustier again, being parked on a south-facing slope will point the solar panels more directly at the Sun, improving the power situation.
Eduardo Tesheiner, who made both images, called the Navcam mosaic (the top image) "quick and dirty." That's a true description in more than one sense. You may notice, if you examine the panorama, that there appear to be fuzzy blobs superimposed here and there on the picture. Those blobs are caused by dust, freshly deposited on the exterior sapphire windows through which the Navcams see Mars. The Navcams aren't equipped with windshield wipers, so that dust will remain in place until and unless some strong puff of wind blows it away.
Although annoying, a relatively small amount of dust deposited on the windows isn't too big a deal for the rover camera team. They can actually correct their images for this problem by something called "flatfielding." What they will do is to take photos of a featureless target -- like the sky in a direction well away from the Sun. These photos will not be featureless; all the features will be due to dust on the windows (or other imperfections elsewhere in the camera system, like "hot pixels" on the detectors). Then, as part of their usual image calibration process, the camera team will take each image and divide it by the flat-field image. This division will cancel out the features that are caused by dust, leaving behind actual features in the image. Flat-fielding is already incorporated into the imaging team's calibration process; it's just gotten more important to the quality of the images now that there are some really big features to cancel out. The images that show up on the raw images pages have not been through any calibration process -- that's one of the compromises required to get those pictures out to the public quickly -- so all the images on the raw images page will continue to show this dust blemishing until (or unless) the dust gets blown away again. No big deal -- just part of the challenge of working with a long-lived mission.
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