ZWO ADC Atmospheric Dispersion Corrector - T2 and 1.25" connection

So what is it?

An Atmosperic Dispersion Corrector or ADC, is a clever little device which corrects the disperse effects of the earth´s atmosphere on light rays. Put simply, when you view a bright object such as a planet with a telescope, you will often see colour fringing. So a large object like Jupiter may have a red and blue fringe in addition to being distorted. This is caused by the effect the earth´s atmosphere has on the light from the object as it passes through it. The atmosphere is basically acting like a prism. This effect changes depending on how high the object is above the horizon. The lower it is the greater the effect. Because in northern Europe bright objects like planets are never that high, so we suffer from the effect more.

An ADC corrects this dispersion effect, allowing you to see and image objects free of these distortions and aberrations. No more waiting for an object to get high enough in the sky for it to offer an acceptable view. Now you can view it anytime with crystal clear quality.


How does the ZWO ADC work?

As we mentiones, when light from the planets or stars passes through the Earth´s atmosphere, it is "refracted", (deflected at an angle which is dependent on its wavelength). This effect, similar to a prism, causes a significant colour shift known as chromatic aberration. This is very detrimental to both viewing and imaging these objects.

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

The levers on the ADC move the counter rotating prisms and adjust the compensation from nothing to a 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 simply by adjusting the levers.


Imaging

When colour imaging a planet without an ADC, you will end up with colour fringing and distortion. Even if the appearance of this fringing and distortion is removed in processing, it has led to a loss of resolution in the image which cannot be recovered. When mono imaging, the taking of individual red, green and blue images and overlaying them helps to compensate somewhat for the image shift. However to maximise resolution of the image a luminance image should be taken. This will suffer from the same issues as a normal full colour image. The improvement to this can be seen in the pictures above.

By using an ADC the distortions and image shift problems are eliminated resulting in a massively more detailed image. When processed further, this results in a final image which is dramatically improved.


Important hint for operation:

For the correct effect, the corrector has to be properly aligned to the horizon. Usually, this is the case if both levers from the corrector are stacked parallel to the horizon as seen in the focuser.

Turning the levers by the same value, but in opposite directions, will rotate the prims so that their effects do not cancel each other out. If the image deteriorates, both levers have to be turned to the opposite direction.

Please take into account that the direction of the horizon changes during the hours when using an equatorial mounted telescope. Of course, this has to be taken into account.