Crescent Venus showing red and blue fringing from atmospheric prismatic dispersion. Hold your mouse cursor over the image to see a comparison between the original uncorrected image and the corrected one.

Here's a shot of a crescent Venus. If we look at it closely, we can see that it has a red fringe on the bottom, and a blue fringe on the top. This is from atmospheric prismatic dispersion.

The colors were shifted because it was shot not very high above the horizon and through a lot of atmosphere which acts as a prism. You will see this on a lot of images, some even shot high in the sky.

You can correct for the atmospheric dispersion by aligning the color channels. Every color image is made up of three channels - red, green and blue. All of the colors in the rainbow can be constructed by mixing these three colors.

Before we align the channels, lets resample the image up by 200 percent. This will allow us to register the channels with sub-pixel precision.

• To align the channels, in Photoshop, go to Window > Channels and the Channels panel should become active.

Using Photoshop's Channels Palette to correct for atmospheric dispersion.

• Click on the red channel in the channel's palette and the image display will switch to the grayscale image of the red channel.

• Then click on the eye icon next to the combined RGB channel. This will switch the image back to the color display, but only the red channel will be active.

• Click on the Move Tool at the top of the tool's palette.

• Then move the red channel up one pixel at a time by using the up arrow key on the keyboard.

• Watch the image and keep moving the red channel until you see the red fringe disappear on the bottom of the image.

• Now click on the blue channel and move it down one pixel at a time by using the up arrow key on the keyboard.

• Save the image with a new name when the channels are aligned.

Crescent Venus with the channels aligned to eliminate red and blue fringing from atmospheric prismatic dispersion. Hold your mouse cursor over the image to see a comparison between the original uncorrected image and the corrected one.