Composites should always be marked as such so that it is clear that parts of the image have been processed separately and then recombined. People can then make a judgement about how “real” the image is. I wouldn’t say that a composite is a “fake” since, if done properly, it shows what would have been seen if the limitations of the telescope and camera had not been present. A good example is stacking a moving object, such as a comet, separately from the fixed stars. I agree though that this is open to abuse from people who are less than honest about how the image has been generated.
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