Hi Nigel,
Along with the excellent example that Vicent has just put, I think this is better described with some practical examples. This is a sample of things that are wrong or inadmissible in our opinion:
- Paint a mask by hand with a brush and apply a sharpening filter through that mask.
- Enhance or diminish some particular features arbitrarily. For example, tame down a bright star by direct intervention with a brush or a similar tool.
- Define an arbitrary selection, with a lasso tool for example, with the purpose of isolating one or more selected objects from the background.
- Replace the existing sky background with a synthetically generated background.
- Repair a prominent blooming artifact with a clone stamp tool, when some significant structures (other than the background or a smooth and constant region) must be "invented" on the regions occupied by the blooming.
- Apply image processing tools and techniques incorrectly. Incorrect application of image processing tools leads to generation of artifacts (I mean questionable artifacts such as ringing, excessive noise, or patterns), to generation of false structures (or the impression of them), or to the removal of existing structures.
Things that are OK in our opinion:
- Define a mask based on the brightness (luminance or lightness) of the whole image, activate it, and apply a sharpening filter in a reasonable way —without generating artifacts such as ringing or false structures— through that mask.
- Define a star mask based on morphological properties of the stars on the whole image, or using appropriate multiscale analysis techniques, and apply an erosion filter with caution —no star must be removed or diminished to the point it becomes practically invisible— through that mask.
- Separate all the objects on the image from the background with a mask based on the brightness of the whole image, then apply appropriate processes to the background or to the objects, separately. The kind of processes applied must be well adapted to the characteristics and physical properties of the selected regions. For example, it makes no sense applying deconvolution to the background, due to its inherent lack of signal.
- Use the clone stamp tool to repair relatively small artifacts or cosmetic defects. Bloomings can also be repaired manually, as long as one doesn't feel guilty of guessing too much. When in doubt, it is always preferable leaving the blooming as it is than falsifying what the blooming is supposed to be blocking.
An example of things that can be considered borderline (note: this is my personal opinion; other DSA signers may think differently):
- Increasing color saturation for a particular color. This can be admissible, in my opinion, as long as it is clear that a particular color is selecting a specific type of objects, and as long as the saturation increase is being applied with good doses of common sense. For example, I see no problem in raising saturation of the reds to enhance Ha structures. Another example would be raising saturation of the blues to enhance reflection nebulosity. In both cases, the applied transformation should only improve the visibility of some objects without creating false relations or false proportions between them. The point where these techniques begin falsifying an image is always a matter of opinion; this is definitely a gray area.
I hope this clarifies our opinions. I want to stress the fact that this is not intended to be, nor should it be understood as, a threat of any kind. This is only our particular vision of astrophotography, and we have formalized it publicly as a contribution to the development of this discipline.
