JTSAN [How] do you use them?
They are kind of a shadow/highlight control, but the actual equations appear to keep changing on each release of ASIAIR app, so who knows what it actually is.
Roughly speaking, they are similar to highlight and shadow controls in photo editing programs, for example like the controls in the Affinity Photo screenshot here:
The controls should only affect the displayed image, not the file. The file should always be the unadulterated raw image.
Roughly speaking, you want the shadow control to be to the left of the peak of the histogram, and the highlight control to be on the right of the peak. You then fine tune them to bring out details of parts of a nebula that you want to inspect.
If you are imaging from a less than optimal dark sky, you will often see a histogram with two humps, with large overlaps between them. With visible nebulas, the hump to the right comes mostly from the actual histogram of the nebula, while the hump on the left is mostly from the histogram of the background sky illumination. In such a case, placing the shadow control somewhere near the dip between the two peaks will give the best view of the nebula. If you place the shadow control to the left of both humps, you will get a washed out appearance, since you are also seeing full contribution from the sky background. If you place the shadow control to the right of both humps, the nebula also eventually disappear.
Sometimes, when the nebula is barely brighter than the sky background, the two histograms will merge into a single-peaked histogram. In such a case, you can often make the nebular appear in the preview by placing the shadow control at the peak of the histogram itself.
Pulling the highlight in from the right towards the peak will the nebula brighter.
In short, just treat those the same way you treat shadow/highlight controls in a photo-editing program and you will get a roughly similar effect.
Chen
