An atmospheric dispersion corrector, or ADC, is a clever little device that corrects for the dispersion effects of the Earth's atmosphere on light rays. Put simply, when viewing a bright object, such as a planet, through a telescope, you often see colour fringes. A large object such as Jupiter may therefore have a red and blue fringe in addition to distortion. This is caused by the effect that the Earth's atmosphere has on the light from the object as it passes through. The atmosphere basically acts like a prism. This effect changes depending on how high the object is above the horizon. The lower it is, the greater the effect. This is because in Northern Europe, bright objects such as planets are never very high, so we suffer more from this effect.

An ADC corrects this dispersion effect and allows you to see and image objects free of these distortions and aberrations. No more waiting for an object to be high enough in the sky to provide an acceptable view. Now you can view it at any time in crystal-clear quality.

How does the ZWO ADC work?

As we mentioned earlier, light from planets or stars is ‘refracted’ (deflected at an angle that depends on its wavelength) as it passes through the Earth's atmosphere. Similar to a prism, this effect causes a significant colour shift known as chromatic aberration. This is very detrimental to both the viewing and imaging of these objects.

The ADC, or atmospheric dispersion corrector, is an optomechanical accessory that corrects this by using the chromatic superposition of two counter-rotating prisms. Its simplified operation is described by the small diagram above.

The levers on the ADC move the counter-rotating prisms and adjust the compensation from nothing to maximum effect when the levers are 180 degrees apart.

Different wavelengths are affected more by the dispersion effect than others. The ADC makes it easy to compensate for the effect by simply adjusting the levers.

Imaging

When you display a planet in colour without ADC, colour fringing and distortion occur. Even if the appearance of this fringing and distortion is eliminated during processing, this has led to a loss of resolution in the image that cannot be restored. In mono imaging, capturing individual red, green and blue images and superimposing them helps to compensate for the image shift to some extent. However, to maximise the resolution of the image, a luminance image should be created. This causes the same problems as with a normal full-colour image. The improvement can be seen in the images above.

The use of an ADC eliminates distortion and image shift problems, resulting in a much more detailed image. Further processing results in a final image that is significantly improved.

Important note for operation:

For the correct effect, the corrector must be correctly aligned with the horizon. This is usually the case when both levers of the corrector are stacked parallel to the horizon, as can be seen in the focuser.

If you turn the levers by the same amount but in opposite directions, you will rotate the prime numbers so that their effects do not cancel each other out. If the image deteriorates, both levers must be turned in the opposite direction.

Please note that the direction of the horizon changes during the hours when using an equatorial mount telescope. This must of course be taken into account.

Scope of Delivery:

• ZWO ADC corrector
• 1.25" adapter on the telescope side
• 1.25" adapter for eyepieces