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Full well depth is the same really as regular photography where you don't want to overexpose your images.
These days most CCDs have antiblooming features which means once the pixel is fully saturated more exposure simply gets bled off.
So in practice overexposure is hard to do on an astro target as they are usually very dim. Its more prominent on the brighter objects like M42. The core will blow out just like a daylight over exposed image.
There you simply take 2 sets of images, one short exposures so as to keep the core properly exposed and another longer to get the dimmer areas nicely exposed. Then combine them in Photoshop.
65535 is the fully saturated pixel level for a 16 bit CCD (they all are).
Greg.
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