How is the Atmospheric Dispersion Corrector made?

The secret is in two very quality coated wedge-prisms.

Astro Systems Holland Atmospheric Dispersion Corrector  - ZWO ADC

The corrector consists of a main body. In this main body there are two coated wedge-prisms that can be adjusted individually against each other to compensate for the atmospheric dispersion.The wedge prisms are made from 1/10th lambda Schott N-BK7 or fused silica and are of the highest quality. Astro Systems Holland The prisms have a broadband coating on both sides (see graph in PDF) and as you can see in the graph for N-BK7, this coating blocks the shorter wavelengths (near UV) below 380nm. This causes problems while imaging Venus (for example). The cheapest solution is to use N-BK7 prisms without coating, then the light passes up to 320nm. The best solution however (but unfortunately also the most expensive), is to use the fused silica (quartz) version with a broadband coating, then the light will pass up to 250nm.

Both prisms can rotated against each other in a continously adjustable way. By rotating the two prisms the "broken" lightrays are corrected for the different colors and are refracted in such a way that they come together in the same spot. At 0 degrees adjustment of the prisms between each other there is no effect, at 180 degrees there is the maximum effect. With this the amount of dispersion can be corrected depending on the height of the object. The result will be a good image, without chromatic abberation caused by the atmosphere.
Atmospheric Dispersion Corrector  - ZWO ADC We assume that the used optics are in good order and of good quality and that they don't cause possible other chromatic abberations! A real APO refractor or Newtonian reflector have of course less (or no) chromatic abberation than an ordinary achromat, or similar system.

When the seeing is bad, or the relative humidity is high, the atmospheric dispersion can be higher than normal. In this case there is a chance that even the atmospheric dispersion corrector cannot fully neutralise this.