There are three primary steps involved in post-processing planetary images (or any astronomical images, for that matter). They are
1. Stacking and averaging
3. Adjusting levels
The second and third options will be familiar to anyone who has ever used an image editing program like Photoshop. The Unsharp Mask filter is a miracle of digital photography. However, the stacking process may need a bit more of an explanation.
How does the stacking process work?
|1. A single raw image from a run of 100. I selected one of the clearer ones for this example. The first step in processing is to eliminate all unacceptable images from the run. This means images blurred by excessive atmospheric shimmering or by vibration of the telescope. In this case, I eliminated all but 27 images, which are combined, or stacked for step 2.|
2. The 27 best images from the original 100, now stacked together. The basic idea behind stacking is to make a sandwich with multiple pictures of the same object to boost the faintest details and their color saturation while the graininess of the picture and other defects are greatly reduced.
Note that the stack in step 2 appears blurrier than the individual frame above. This is only temporary as the 2nd image contains much more information than the single frame, and is ready now for additional post processing.
For more info on how image stacking works, see this site.
3. The stack after an initial round of processing. The primary mode of processing is sharpening, which can be done by most image manipulation programs such as Photoshop. The sharpening process brings out the wealth of detail provided by the stacking step, and produces an image composite far better than any of the individual frames.
Other processing steps could include adjusting brightness and contrast, as well as RGB levels to enhance the colors and bring out detail in the rings and clouds.
It's all about signal-to-noise ratio. Stacking multiple images multiplies the amount of signal present in the composite, but it also multiplies the noise. However, signal (actual physical features visible on the planet) increases at a higher rate, or co-adds. Noise is random (caused by atmospheric refraction, vibrations of the telescope, etc.) and thus it grows at a much slower rate. Thus a stack will always contain more good information (because signal co-adds) and less spurious info (signal-to-noise ratio will always increase), than any individual photo.
For a much better explanation that I can provide, check this link.
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