ZWO ADC Atmospheric Dispersion Corrector
The ZWO ADC, or Atmospheric Dispersion Corrector, reduces prismatic smearing during planetary imaging, resulting in images with finer details. It also improves the image when doing visual planetary observations, allowing the observer to see more surface detail.
Optical dispersion is an effect caused by the refractive quality of the atmosphere as light passes through it, and is dependent on the angle of the light as well as its wavelength. Optical dispersion spreads the incoming light into a vertical spectrum of colors, causing the object to appear higher in the sky than it truly is. The amount of “lift” that occurs is exaggerated when objects are closer to the horizon, and because optical dispersion is wavelength dependent, it causes the image to separate into different colors. That is why you will see a bluish fringe on the top of an object, and a red fringe at the bottom when atmospheric dispersion effects are particularly bad.
A correctly adjusted ADC, placed between the camera or eyepiece and a Barlow lens, will reduce the effects of optical dispersion and improve image resolution. It does this by applying the opposite amount of dispersion caused by the atmosphere to the image, and then re-converging the light of the different wavelengths at the focal plane.
The ZWO ADC has three components: The ADC body, a 1.25" barrel adapter for the bottom and 1.25" eyepiece adapter for the top (with a brass compression ring). The two ends of the ADC body are female T-threaded. Two slots overlap to allow both levers to move together over a wider range to accommodate a changing orientation of the null location (without having to rotate the body).
A nice feature of the ZWO ADC is the white locking screw on the scale marking the horizontal position and the rotatable scale. This allows the null point to be readily seen to set the levers symmetrically on either side and allows you to finely position the null point without having to rotate the body. The black plastic prism adjusting levers also have a nice adjustable frictional resistance using O-rings at their base.
The two H-K9L (BK-7 equivalent) prisms in the device each have 2° deviation angles and are said to be of 1/10wave accuracy.