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Below are a couple quick and easy tricks for reducing noise and removing gradients in an image.


Noise Reduction

A fast trick for reducing noise in an image is to use the Despeckle filter in Photoshop.  The filter alone will result in lost detail so there are a couple additional steps to keep the image looking sharp but with less noise.

Above:  Unprocessed single exposure of the Horsehead Nebula.  Note the graininess.  Image by James McGaha.


Open the desired image.  Duplicate the image to another layer (Ctrl-J).  Make sure the new layer is the active layer.

Select Filter > Noise > Despeckle.  You might run the filter several times.  Press Ctrl-F to rerun the filter.  For this example, the Despeckle filter was run three times.

Above:  Image after three iterations of the Despeckle filter.  Note the grain is gone, but the sharpness of the image is lost.

Change the layer blend mode to Lighten.  This will let the sharper stars from the lower image show through, but the softness in the nebula and background is retained.

Above:  The new top layer (on which the Despeckle filter was run) set to Lighten mode

Above:  Image after setting blend mode to Lighten.  Note the reduction in noise compared to the original image.


Gradient Removal

The basic method for this trick is to apply a radial gradient to an image to reverse the effect of light falloff due to vignetting.  The technique assumes that the vignetting is circular and uniform, which is not necessarily true.  But for a quick and dirty gradient removal, this trick works pretty well with most images.

Above:  Unprocessed single exposure of the Andromeda Galaxy.  Note the darkening of the corners of the image due to vignetting.  Image by James McGaha.


Begin by sampling the sky background from both the center and the corner of the image.  Select the Eyedropper tool (keyboard shortcut I).  Alt-click on the center of the image to set the background color.  Be sure to avoid sampling the object itself.  In this example, Andromeda takes up most of the center of the frame.  The background was sampled from just outside the left edge of the galaxy.  This will be close enough for this technique.

Select the foreground color by sampling a corner of the image with the Eyedropper.

Create a new layer (Shift-Ctrl-N).  Select the Gradient tool (keyboard shortcut G).  Be sure the radial gradient is selected from the top menu.  Create a radial gradient in the new layer by clicking in the center of the image and dragging outward to the corner.

Above:  The gradient will essentially be the opposite of the sky background, dark in the center and light at the corners

Change the layer blend mode to Screen.

Above:  Layer blend mode set to Screen

The resulting image will have a flatter background, but the background will be bright.

Above:  Image after setting layer blend mode to Screen.  Note the background is even in brightness but is too bright overall.

Flatten the image (Ctrl-E).  Open the Levels window (Ctrl-L).  Move the black point slider (left slider) up to the toe of the histogram curve.

Above:  Setting Levels to darken the background

Above:  Image after Levels adjustment

The image can now be processed normally.  For tips, see the Aesthetic Image Adjustments section.  Below is the final image after full Photoshop processing.

Above:  Final processed image of Andromeda.  Single 10 minute exposure.  Image by James McGaha.

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