Today’s Question: I am into star trails and wondered if there was a way to remove objects (stars/star trails) based on their intensity as I would like to remove the less intense/less bright objects and control to what extent.
Tim’s Quick Answer: Yes, you could define a mask based on brightness values to select the specific areas you wanted to affect, and then either perform cleanup or apply an adjustment to tone down or remove those areas.
More Detail: While this technique can work very well for star trail photography, the same concept could apply to a variety of situations where you can identify specific areas of an image based on their tonal value relative to the rest of the image.
In this example you could define a mask based on a luminance range, such as in Camera Raw (including the Camera Raw filter in Photoshop) or Lightroom Classic. Select the Luminance Range (under the Range option) for the mask. Then define a range that will include only the tonal values between the bright star trails and the dark night sky.
Once you’ve defined that mask, you could use adjustments to help hide the affected areas, such as by using a tonal adjustment (including possibly a Tone Curve adjustment) to darken the areas based on the mask. This would effectively hide (or greatly diminish) the areas of the image that were competing with the star trails a bit, by being darker than the trails but brighter than the night sky.
Note that if you preferred to use an image-cleanup tool to remove these areas, you could similarly create a mask based on tonal range (for Midtones) using the Color Range selection tool. That could then be used as the basis for a mask on an image cleanup layer to protect the rest of the image.
