1. The Problem of Atmospheric Dispersion
Like a big glass prism or lens, the Earth’s atmosphere refracts or bends the light from stars or planets on its way to a telescope, and it bends light of different colors by slightly different degrees. This effect is called atmospheric dispersion and it results a greater refraction of blue and green light than red light, for example. It’s the same effect that causes the well-known ‘Green Flash’ on the upper edge of the Sun as it sets below the horizon.
Of course, when you’re imaging the planets or the Moon, especially when their light travels through the thick atmosphere above the horizon, dispersion is a big problem. If you’re doing color imaging of planets low near the horizon, dispersion results in color fringing of the image. It can be removed with processing but at the cost of detail and resolution. If you image in monochrome with separate LRGB frames, you still suffer from atmospheric dispersion in the luminance frame. Even visual observers can notice the effects of dispersion when observing planets and the Moon low on the horizon where the effects of atmospheric dispersion are greatest.
2. Dispersion Compensation for Astroimagers and Visual Observers
The ZWO Atmospheric Dispersion Corrector (ADC) is an ingenious device that compensates for the effects of atmospheric dispersion. The ZWO ADC uses a pair of prisms that can be precisely adjusted with manual levers to apply the opposite dispersion induced by the atmosphere. The device then directs all colors of light to re-converge at the focal plane. The adjustment can range from null (no correction) to maximum refraction suitable for correcting for the well-understood effect of atmospheric dispersion at a range of altitudes above the horizon.
The effect of using the ZWO ADC? Greatly improved image resolution and detail for both imagers and visual observers, even when the planet is just above the horizon. You no longer need to wait hours for the planet to get higher in the sky during the night, or years for the planet to reach a more favorable opposition.
3. Designed for Ease of Use and Optimum Image Quality
The ZWO Atmospheric Dispersion Corrector consists of three pieces. The main body holds the corrective prisms and adjustment levers. The two lever slots overlap to allow both levers to move together over a wider range. A 1.25″; barrel adapter for the bottom of the main body and a 1.25″ adapter for the top of the body each mount into the female threads on the body to accept the camera/eyepiece and Barlow. The ZWO ADC works best at large focal ratios, so you get optimum results when you place the device between a Barlow lens and the camera or eyepiece.
The body of the ADC also has a white locking screw on the scale to mark the null point about which the levers are rotated to get the optimum correction of a given planet at a given altitude. This allows you to finely position the null point without having to rotate the body of the ADC. The levers also have an adjustable frictional resistance for ease of use.
The prisms in the ZWO ADC have a 2° deviation angle and are made from H-K9L (Schott BK7) glass with surfaces polished to 1/10 wave accuracy. All optical surfaces are AR-coated to ensure >98% transmission over the visible spectrum.
Priced at a fraction of competing ADC solutions, the ZWO Atmospheric Dispersion Corrector is one of the few devices available to counteract the distorting effects of our atmosphere. Designed by amateur astronomers for amateur astronomers, the ZWO ADC helps you extract maximum detail when observing or imaging the Moon and planets.