Artificial Flats by Corius Astronomy
http://www.corius.net/?page_id=395

Summary: Apply large median filter, remove target(s), blur, subtract from original

  1. Open the image to which an artificial flat needs to be applied. Create a copy of it using File > Save As... Open this and the original image. The copy will be processed to become the flat.
  2. Apply a Median filter to the copy using Filter > Noise > Median Filter. For regular (full) size images use a Median filter with a radius of about 50 pixels. The optimum setting may vary for your image.
  3. Remove any remaining nebulousity and stars by using either the Clone or healing tools. Steps 2 and 3 can be reversed if you find it easier to remove the target object before it becomes a smear.
  4. Apply a Gaussian Blur. Use Filter > Blur > Gaussian Blur with a radius of about 30 pixels (could be less, try experimenting with different sizes). Note that a gaussian blur can produce black edges on the image, so you may need to do some minor cropping.
  5. Subtract the artificial flat field from the light. Select the original image, then use Image > Apply Image... with Source = flat file, Blending = Subtract, Scale = 1, Offset = 20 to 40. [You can experiment with the offset value.]
  6. Reset the black and possibly the white points using Image > Adjustments > Levels.
  7. Save your new flatted image.
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