Low-Pass Noise Removal

Stacking images is the best way to reduce noise in an image.  However, images of faint objects, which must be significantly enhanced, can often still be noisy in the dimmer areas of the image.  A perfect example of this is the image below of the very faint nebula Sharpless 2-170 in Cassiopeia.  Imaged from a suburban location through a hydrogen-alpha filter, this picture shows more detail than the Palomar Sky Survey image taken through the 48" Schmidt telescope!  But there is some noise visible in the faint outer areas of the nebula because the image was contrast stretched to enhance the faint details.

Above:  Nebula Sh2-170 imaged through a 4" refractor.  Notice the noise in the outer regions of the nebula.

A low-pass filter reduces noise.  However, it also blurs the image.  Running a standard low-pass filter such as a Gaussian blur on this image eliminates the noise but reduces the sharpness of the image and reduces the amount of visible detail.

Above:  A Gaussian blur smooths out the noise but reduces the detail and sharpness in the image.

A better method is to blur only the noisy part of the image.  Since noise tends to appear in the faint areas of the image, it is possible to select only these parts of the picture and then apply the low-pass filter to that selection.  Photoshop has the best features for this application, so it will be used to demonstrate the technique.  Other program may have similar capabilities.

Begin by choosing Select > Color Range from the main menu.  The Color Range selector allows you to select portions of the image based on the brightness of that area.  Use the eyedropper tool to select a medium-brightness portion of the image.  Then drag the Fuzziness slider up until the preview window shows most (but not all) of the nebula selected.  You will notice that the stars have also been selected.  Click the Invert checkbox.  This will select everything in the image except the bright nebulosity and stars -- in other words, only the noisy parts should be selected.

Above:  The Color Range window after selecting the non-noisy parts of the image and then inverting the selection to choose the noisy areas.

Above:  The image now shows the selected regions.

Note:  Remember that by running the Fuzziness slider up so high, parts of the image are selected but Photoshop cannot display that selection.  The actual selection will always be larger than what is shown if the Fuzziness slider is raised.

Running the Gaussian blur filter now smoothes only the selected portions of the image, which should be the noisy parts.  The other areas, such as bright nebulosity and stars, are unaffected.

Above:  Final result after blurring selected regions of the image.

Next, Filters for Removing Artifacts....

Next Page