http://saturn.jpl.nasa.gov/faq/FAQRawImages/ wrote:
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What are those streaks I see in some images?
There are high-energy particles that fly though space called cosmic rays. When one of these particles hit the camera's sensor, it causes a bright spot. When one of the particles hit the camera's sensor edge-on, it can leave a trail across the image. Exposures shorter than a second will not have many of these spots or trails. However, long exposures, like those from a minute to 20 minutes will contain many of these trails.
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What are those dark donut shapes?
Small donut-like dark spots in images are actually out of focus dust specks on the filter wheels, lenses or other parts of the optics of the cameras. Because there is no way to clean the cameras in space, more of these spots may appear as the Cassini mission progresses.
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What is that horizontal waviness in the picture?
There is a low level source of noise in the camera's signal as it comes out of the sensor and gets converted to numbers. This noise adds and subtracts a small amount to the signal in a cycle. When the data is put into an image, one can see it as bright and dark bands in the image. The amount of noise is very small and is not noticeable in most images. Images that are of black sky or very dark can show this noise. The camera records the baseline of the signal for each line so this noise can be removed in later processing. Both cameras are affected by this noise but the Narrow Angle Camera is worse.
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Why does the image look bizarre/psychedelic?
As in the previous question, the other way the camera can send back less data (by sending pixels with values from zero to 255 instead of zero to 4095) is to send back only the lower binary digits of the number. This is like having a list of amounts of money and only recording the amount of cents for each one and assigning the brightness in an image to the amount of leftover cents. Pixels with brightness values just under 255, like amounts just under a dollar, will appear almost white, while pixel values just over 255, like amounts just over a dollar with not many cents, will appear dark. The ideal use of this mode is for image scenes that are dark with almost all of the pixel values less than 255. If the scene is simple with gradual increases in brightness, then even if the original values get over 255 and go dark again, the scientists can figure out what the real value was. If the scene is very complicated or the original values are much brighter than 255, the image can have many bright and dark transitions with strange contours. In this case, the image will look very bizarre but not have much scientific value.
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Why is the image overexposed?
Cassini's cameras have 63 different exposure settings, from 5 milliseconds to 20 minutes. Scientists planning an observation must choose the exposure for each image taken. That can be tough if you're taking a picture of something you've never seen before. Thus, incomplete information on how bright something can be can lead to an underexposed or overexposed image.
Images can be overexposed on purpose too. If the scientist is looking for something dim next to something bright, the bright thing may be overexposed. Finally, Optical Navigation personnel use images to see where Cassini is relative to Saturn and its moons. Often they overexpose images because they need to see where these moons are in relation to the stars in the background sky.
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