This frame from Spirit's panoramic camera is the 50,000th image from NASA's pair of Mars Exploration Rovers. It shows the camera's calibration target, the most photographed subject on Mars, with a glimpse past it to rocks and soil at the location in the "Columbia Hills" where Spirit was examining soil during its 260th martian day, or sol (Sept. 25, 2004). The outer circle of the calibration target is 8 centimeters (3.15 inches) in diameter. The target includes reference colors and a shadow-casting post. Most days when the panoramic camera is used, it takes a series of images of the calibration target through different filters to aid in accurate interpretation of the other images it takes. This frame was taken by the left eye of the camera's stereo pair, through a red-orange filter admitting light with wavelengths centered at 600 nanometers.