Processing: Synthetic Flat

If you didn't shoot flat frames there's still hope. It's not too difficult to create flat frames that help reduce all flat frame issues except dust shadows. Here's how to create a flat frame:

  1. Open your image.
  2. Create a duplicate file of your image using File > Save As... The copy will become the flat so you should name it to avoid confusion.
  3. Reopen your original image and select the flat image.
  4. Apply a Median filter to the flat 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. This should get rid of most of the stars in the image.
  5. Remove any remaining nebulousity and star remnants by using the Clone Stamp tool. To use the Clone Stamp tool, set its Mode to Normal Opacity and Flow to 100%, and Alt-click on a background area with no stars and an intensity that matches the star remnant's surroundings. Then click on the remnant.
  6. Apply a Gaussian Blur. Use Filter > Blur > Gaussian Blur with a radius of about 30 pixels (Ttry 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 later.
  7. 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.]
  8. Reset the black and possibly the white points using Image > Adjustments > Levels.
  9. Save your new flatted image.

Needless to say this won't work well for cases in which the target is spread over much of the image. For that you really should shoot some internal flats!

 

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