The ability to produce good quality high-resolution planetary images is hampered by atmospheric dispersion, an effect in which white light is separated vertically into a spectrum of colors, with blue at the top and red at the bottom. The atmosphere effectively acts like a weak prism, causing light entering at an angle to be bent by refraction to a slightly steeper angle. Because refraction is wavelength dependent, an effect known as dispersion, the apparent 'lift' that the object gets from atmospheric refraction varies according to the color of the light (see figure 1 below). Atmospheric dispersion gets more pronounced when the altitude of the object is lower. That's bad news for those in the Northern hemisphere because the brightest planets Jupiter, Saturn, and Mars will be at low elevations during their oppositions for the next several years through 2020 and beyond.

You can align the separate red, green and blue images manually or automatically during processing to remove the worst effects of atmospheric dispersion. But you can't use this method to correct any dispersion occurring within the color band itself. Dispersion within the color band is worse for shorter wavelengths like blue, worsens with decreasing altitude, and is much more problematic for one-shot color (OSC) cameras. That's because these cameras have much wider color bands than monochrome cameras which are usually used with dedicated high-quality R,G, and B filters.

To help planetary imagers and visual observers combat atmospheric dispersion, adjustable devices called Atmospheric Dispersion Correctors (ADCs) have become commercially available in recent years. These devices contain a pair of thin circular prisms which act to nullify the dispersion caused by the passage of light through the atmosphere by introducing dispersion of the opposite direction.

ADCs can give considerable benefits in image quality when planets are less than 30° high, but they also help improve detail when the planet is as high as 60°. An ADC can help with RGB mono imaging, one-shot color imaging, true luminance imaging, and even during visual use (You can learn more about the operation of atmospheric dispersion correctors at this link)

ADCs operate best at higher focal ratios (>f/20), and this generally means they should be placed between the Barlow lens and the camera. Adjustment of an ADC involves setting the mid-point of the prism's horizontal and then moving the two prism control levers apart by equal amounts until the colour fringing from the atmosphere is cancelled out. Again, the link above gives more information on how to actually do this.