Taking the Picture
Taking the picture is the job of the spacecraft’s imaging system: the digital camera, computer, and radio. When the Cassini camera looks at its “target” — a planet or moon, for instance — light from the target object passes through the lens and then through a color filter before falling on an electronic chip called a chargecoupled device, or CCD.
The surface of the Cassini camera’s CCD is divided into 1,024 parallel lines, each of which is further divided into 1,024 light-sensitive pieces — a total of 1,048,576 picture elements, or pixels. Each pixel records the scene brightness on a scale of values from 0 (black) to 4,096 (white). Pictures of the same scene are taken through various filters — red, green, blue. After all the pictures are transmitted to Earth, a color image of the scene is constructed from the red, green, and blue images.
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Numerical values from 0–255 (black–white) assigned to the separate elements, or pixels, making up this Martian crater image are shown in the photo below. How the shades of gray compile into a picture can be seen in the bottom image.
Cassini has two digital cameras — a narrow-angle camera, which has two filter wheels with 12 filters each, and a wide-angle camera, which has two filter wheels with nine filters each. The Cassini spacecraft’s onboard computer takes all 1,048,576 values recorded by the pixels — for the narrow-angle camera, that’s 12,582,912 (1,048,576 x 12) pieces of information! — and converts the values into digital code, made up of 0’s and 1’s called “bits.” The radio transmitter then relays the “bitstream” of data to Earth.
Above: Cassini’s Imaging Science Subsystem consists of two framing cameras: a narrow-angle camera (top), which is a reflecting telescope, and a wide-angle camera, which is a refractor.
Images from Space | Taking the Picture | Getting the Picture
Making the Picture | Deep Space Network