Sometimes during processing of planetary images we can pick up artifacts such as ringing and limb artifacts. These are not real details but artifacts created by diffraction and the sharpening process.

These artifacts will often appear as a line inside and / or outside of the limb of a planet.

Figure 1 - After sharpening, Mars exhibits ringing artifacts. Hold your mouse cursor over the image to see a comparison between the original uncorrected image and the corrected one.

The image of Mars seen above (Figure 1) shows both light and dark ringing near the edge of the planet. A lighter ring is seen outside of the edge of the planet, and a darker ring inside the edge (Figure 2).

We will deal with each of these separately.

Figure 2 - The Ringing artifacts are enlarged for better visibility.

Removing the Outer Light Ring

To remove the outer light ring, we will make a circular selection around the planet and the just cut the sky background out to remove it.

• Right Click on the Marquee Tool in the Tool's palette and hold the mouse button down for a couple of seconds. More tools should pop out to the right. Select the Elliptical Marquee Tool to make it active.

• Put your cursor in the center of the planet and hold down the Shift and Alt keys at the same time and left click and drag the cursor towards the edge of the planet. Release the left mouse button when the selection is at the edge of the planet. The Alt key draws the marquee selection from the center, and the Shift key constrains the selection to a circle. If you don't get it perfect, don't worry.

• If the circular marquee is not centered on the planet, simply move it one pixel at a time with the arrow keys on the keyboard.

Figure 3 - The circular marquee is inverted.

• If the circular marque is not the exact right size, go to Select > Modify > Expand or Contract and enter a pixel value to enlarge or reduce the size. You want the circular marquee to be just a tiny bit bigger than the edge of the planet, but not so big as to encompass the light ring.

• Go to Select > Modify > Feather and enter a value of 1 or 2 pixels.

• Go to Select > Inverse to invert the selection (Figure 3).

• Click on the background color at the bottom of the Tool's palette and double click on it and set it to black.

• Go to Edit > Cut. This will cut out the sky background and replace it with black.

Figure 4 - Mars with the light outer ring removed.

Removing the Inner Dark Ring

To remove the inner dark ring, we will use the blur tool.

• 

  Click on the Blur Tool in the Tool's palette to make it active. Use the left and right bracket keys on the keyboard to change its size. Make the tool just a little bit larger than the dark line inside the edge of the planet.

• Left click the mouse in the image, and holding down the left mouse button, drag the blur tool over the dark line. You may have to repeat several times until the dark line disappears.

• If you make a mistake at any point, just go to the History Palette and drag that step into the trash. You can go back multiple steps here to revert to an earlier step in the process.

Figure 5 - The light and dark rings have been removed from Mars. Hold your mouse cursor over the image to see a comparison between the original uncorrected image and the corrected one.

In Figure 5 we can see the results of using Photoshop to remove the light and dark rings around Mars.