Correct Oblong Stars (for selected area of image)

The description below will give you a sense of the power of layer masking.

You may find that the elongation of stars varies by position on the image. For example, if you're using a short focal length refractor without a field flattener field curvature will cause the stars to show radial elongation that increases with distance from the center. This might not be bothersome until you get into the image's corners. This kind of elongation can't be corrected with a single application of the offset filter.

Another case for making corrections to star shape only in selected areas is if your sensor plane isn't perpendicular to the imaging telescope's optical axis.

An even worse case can be caused by a very poor polar alignment combined with autoguiding and long exposures. The field of your image will rotate around the guide star, leading to tangential elongation around the guide point.

Let's look at how one would correct one portion of an image:

  1. Open your image
  2. Copy it to the clipboard and then to a new layer (Ctrl-A, Ctrl-C, Ctrl-V). We'll call the new layer SOS (selected oblong stars)
  3. Select the SOS layer create a Hide All mask: Layer > Layer Mask > Hide All.
  4. Set the brush color to white and an appropriate size with Opacity and Flow at 100%.
  5. Click on the mask to make it the active drawing surface and paint the area you want to correct. You'll be moving th paintbrush over the image, but the white will show up on the mask.
  6. Apply a Gaussian blur to soften the edges of the mask. (Filter > Blur > Gaussian Blur...)
  7. Change the SOS layer's mode to Darken
  8. Click on the SOS layer's thumbnail and use Filter > Other > Offset... to reshape the stars in the masked area.
  9. Merge down and save*

*To handle multiple areas of the image using different offsets, just duplicate the background, slide it to the top of the layer stack, and give it a Hide All mask (Let's call this layer SOS2). Repeat steps 4-8 for SOS2 using the appropriate offset. When you're done you can merge all the layers and save.

An extension of this is to make changes that vary in amount from one place to another. For example, if stars are oblong on the left side of your image but not the right, give your mask a white to black gradient from left to right and apply the offset. The offset will be applied with varying opacity, greatest on the left and least on the right. Note that this is not the same as gradually decreasing an offset from full to zero across the image, but it will look a little like that's what you did.

 

This method can be extended to cIf you want to go really crazy you can apply this method to individual stars, but please don't! Life's too short for that kind of processing.

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