Fixing Background Blobs

Sometimes, after noise reduction, you are left with a blobby background that spoils an otherwise excellent image. This is a common defect in many DSLR images as they tend to be a little noiser than dedicated astro cameras. The usual problem is that many noise reduction algorithms do a great job on high spatial frequency noise, but have mor difficulty with low frequency noise. It is a much more difficult task to remove the lower frequency noise because it is very difficult to distinguish it from the image data. Take the example of the image to the right. There are darker blobs throughout the background that detract from an otherwise nice galaxy image.

 

In order to correct the problem use the eyedropper tool to sample the brighter area around one of the dark blobs. Create a new raster layer on top of the image and fill it with the sampled colour. Set the combine mode to lighten; this will cause the darker areas to fill with the sampled colour. Any areas of the image that are brighter than the sampled colour will be left alone. The problem now is that the background looks unnatural because of the flat appearence as shown to the right. 

To solve the flat sky add a slight amount of gaussian monochrome noise to the new layer, just enough to produce a natural looking sky background and ensure proper blending into the DSO.

Layers

Final Image