Atmospheric Dispersion Corrector (ADC) significantly improve the quality of images or you live image in eyepiece. Enemy number 1 in high resolution plantery imaging is our atmosphere. However, the seeing effects are not the only causes of smearing and fading of fine details.
At large distances from the Zenith (near to the horizon) there is another cause: Atmospheric Dispersion. This effect is also known as wavelength dependent refraction (blue light is dispersed more than red light). This wavelength dependent refraction causes blue and orange-red sides around objects while imaging them at relatively low declination.
RAW unprocessed video bellow show planet Saturn captured through 30-cm Newton with ASI120MC planetary camera. Left video show Saturn captured without ADC and right captured trought our ADC! Need more explanation?
Normally this effect is not quite visible, but during imaging with modern planetary cameras. and a telescope, the image is greatly enlarged and the negative effects of dispersion are then certainly visible.The further you are away from Zenith (closer to the horizon), the worser this effect becomes because of the larger dispersion. As a result of this the quality of the image becomes worse. The position of the planets will be quite low in the coming years. This will cause problems while imaging and observing them, not only because the seeing effects are more visible at lower declination, but also because of the higher dispersion. During planetary imaging, the seeing effects can be corrected by making movies with a high number of frames per second, in this small movie there will always be images where the seeing is frozen. The best images (with frozen seeing) can then be added together with software (e.g. Autostakkert, Registax). The result of this is a sharper planetary image, but the atmospheric dispersion still causes red and blue edges around the image. Stacking software cannot compensate for this (yet). The angle of refraction in the atmosphere is dependent on the wavelenght of the incoming light. Because of this we need to apply different corrections to enable us to put the colors correctly "on top of each other". Only then we can see the image without refraction effects. There is no software yet known to us that can compensate for this (except professional software used by professional astronomers). Not even the RGB shift functionality in Registax can correct this sufficiently.