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Tips & Tutorials

This section contains tips and tutorials on astrophotography including imaging techniques and image processing tips that I have collecting over the years.

Star Trail Removal in Adobe Photoshop

Star Trail Removal in Adobe Photoshop

Easy Approach:

  1. In Photoshop, make a copy of your main layer.   Call it, say, "Darken".
  2. (Optional) Use a layer mask for the copied layer to mask out the main object, like a galaxy or nebula. This will leave just the stars that are trailing visible on that layer.  (You can try it without this step, but often the effect on the main object is too severe).  To do this, I will frequently select "white" for the foreground color, then 'select/color range' to pick up just the stars in the photo.  I will expand the selection by 4 pixels, the feather by 2.  This ensures that only the stars in the photo will be adjusted.
  3. Change the blend mode of the "Darken" layer to "darken".  The theory is that wherever a pixel in this layer overlaps the main layer, the program will choose the darker of the two pixels to display.
  4. So... now click on the "Move Tool" in the toolbox.
  5. Make sure the image section of the copied layer ("Darken") is selected - click on it.  You don't want another layer, or the layer mask of the copied layer to be selected.
  6. Use your cursor keys to 'nudge' the copied layer around - you will be overlaying the darker pixels of the copied layer so that they eliminate the trailed sections of the stars!
  7. Use your cursor keys to 'nudge' the copied layer around - you will be overlaying the darker pixels of the copied layer so that they eliminate the trailed sections of the stars!  You want to get within one pixel accuracy in both directions.

If You Need More Adjustment Accuracy:

  1. Instead of just using the cursor keys to nudge things around, use the "Filter/Other/Offset" command to move that "Darken" layer for that final portion-of-pixel differential.  Now, use the "Edit/Fade Offset" command to finely adjust the amount by which the filter is applied.  (I generally do one axis at a time.)  This gives you the appearance of extremely precise pixel control.  (In actual fact, the before and after states are blended to produce your final results.)  Therefore, don't use this method except for the portion of the final pixel.
  2. Alternatively, you can upsize your document 400-500% before nudging the layer around.  In this example, it gives you 1/4 or 1/5th of a pixel accuracy once you resize back to the original size.
Courtesy Fleming Astrophotography

Star Bloat Reduction in Adobe Photoshop

Star Bloat Reduction in Adobe Photoshop
  1. Make sure you've selected the image layer, rather than one of your adjustment layers
  2. Use the eyedropper tool to select the color one of the stars that you wish to work on
  3. Use "Select/Color Range" to select all similar stars in the picture. You can set the degree over which the color range is selected
  4. Use "Select/Modify/Expand" to increase the selection by 2 pixels
  5. I then use "Select/Feather" to "blur" the area of the selection by 1 pixel
  6. Go under "Filter/Other/Minimum", set the range as low as it can go, i.e., 1 pixel, and apply this filter to the selected stars
  7. Generally, the effect is too extreme, so IMMEDIATELY go under "Edit/Fade Minimum Filter" to adjust the amount of this last edit to a more appropriate level. The example above was probably done at 50% of the original "minimum" filter amount.
  8. Hit CTL+D to deselect the stars, and you can use the History window to view the various states of your work.

    And there you go!

Courtesy Fleming Astrophotography
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