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Yesterday, I photographed the comet C/2020 F3 ("NEOWISE"). I took several images and stacked the best ones using Deep Sky Stacker. I post-processed the image using Adobe Lightroom.

Normally, when I sharpen my images, I use the preview function when moving the slider with my mouse, by pressing the Alt (or Option on Mac) key. This shows a processed, grayscale image, that helps to get the "right" amount of the effect I'm working on, without overdoing it (See Adobe's help page on this topic).

I was sharpening my NEOWISE image, and when adjusting the Masking slider of the Sharpening option in Adobe Lightroom CC, pressing Alt/Option showed an amazing detail in the tail of the comet (see picture of my monitor below).

My questions are:

  1. What is the effect/process that Lightroom shows when Alt/Optioning the Masking slider?
  2. Can I recreate this effect in Lightroom or Photoshop, and how?

Processed image of NEOWISE:

enter image description here

Photo of my monitor with the desired effect:

enter image description here

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  • FYI: if you surround part of your text with brackets <>, it will not show up (it likely gets parsed as an unclosed XML-tag or something). There's however dedicated markup that you can use to add decoration to text that makes it look like a key. See this meta question for more info
    – null
    Jul 22 '20 at 12:58
  • Thanks for mentioning that and improving my question!
    – agtoever
    Jul 22 '20 at 13:33
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    That apparent detail isn't real, it's a result of processing artifacts. The sharpening is likely bringing out minute selection differences from the stacking. The worm pattern in front of the head demonstrates it's not actually the comet. The dark center of the comet head is physically impossible to have photographed. The glow is spherical around the head. Seeing the dark center would imply somehow peering through the glow. The dark center is a sharpening artifact. It's a great picture by the way! Jul 23 '20 at 15:36
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    I would experiment with oversharpening and high-pass filter. Jul 24 '20 at 10:58
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Masking is masking, it limits where the sharpening will occur.

Your question is therefore not as simple as it looks as there are at least three steps:

  • the algorithm producing the mask
  • the sharpening algorithm
  • the algorithm using the mask to limit the sharpened area

The artifacts in the image could be the result of the interaction between these three stages.

It is also possible that that these three steps are combined into one, but the resulting algorithm must be complex to say the least.

Personally, I would first try to work with Photoshop in the following way:

  • layer duplication
  • application on the top layer of a sharpening filter
  • on this top layer masking out of the comet with full opacity (black)

Then, as long as the result is not satisfactory or for the fun of it, you would search for a sharpen algorithm for the comet on its layer, as far as one exists because trying to enhance sharpness in blurred areas is rarely a good idea.

This process would limit complexity by avoiding the above-mentioned interaction of different algorithms.

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