Reducing Noise

Often when shooting long exposure astrophotography, noise places limits on what can be done with the image.  Noise is that random snow in our images and for the most part all noise reduction routines blur the image to some extent. Fortunately noise reduction is required more in the dimmer portions of the image where detail is at a minimum. Noise reduction can be tailored in such a way that it has minimum effect on the bright sections of the image and applies maximum blurring in the dark areas to limit background noise. The example below uses Paint Shop Pro, but the method works equally well in Photoshop or Gimp.

Now let's zoom in around the bottom portion of the nebula used in the fixing background colour cast example to see the noise.






Now let's apply the following algorithm to reduce the noise in the background without blurring the brighter portions of the nebula.

  1. Duplicate the image on another layer
  2. Make the bottom layer visible
  3. Apply your favorite noise reduction function to the bottom layer and overdo it slightly
  4. Make a mask out of the luminance of the bottom layer on top of the topmost layer
  5. Make all layers visible
  6. Adjust the brightness and contrast of the mask to show the desired detail
  7. Apply a small gaussian blur to the mask (4 to 6 pixels)
  8. You can play with thresholding on the mask as well or simply paint it white where you want additional detail to show through

Here I used Pain Shop Pro's digital camera noise reduction command to produce the noise reduced background on the bottom layer and the layer stack is shown to the right. The results, below, show a smooth sky background while maintaining detail in the brighter areas of the image.