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