Product Description

ZWO ADC is voted as  “HOT SKY PRODUCT – 2017” by SKY&TELESCOPE magazine!

ZWO Atmospheric Dispersion Corrector is used to reduce the effect of atmospheric dispersion on lunar and planetary images

Why use an ADC?

(Section credit: https://skyinspector.co.uk/atm-dispersion-corrector-adc/)

Light from any star or planet that enters our atmosphere at an angle will suffer refraction effects which bend to light to a slightly steeper angle. This refraction effect makes the object appear higher in the sky than if the atmosphere wasn’t there. The degree of refraction depends to a small degree on various things like temperature, humidity, observers and height above sea level, but does increases strongly with decreasing altitude of the object – for example, the image of the sun at sunset can actually be elevated by over 0.5°, which is slightly more than one solar diameter.

Like most optical media the refractive (bending) power of the atmosphere is actually dependent on the wavelength of light. This is called optical dispersion. Atmospheric dispersion means that actually the degree of ‘lift’ that the refraction causes depends not only on the angle of the light, but also the colour of that light. Atmospheric dispersion spreads the light from any point source into a vertical spectrum of colours whose length increases the lower the object is in the sky.

The schematic below illustrates this dispersion effect as light enters the atmosphere and also the lifted elevation of the image. The lift is wavelength dependent and hence the image is separated into different colours. A planet exhibiting bad atmospheric dispersion effects will have a blue fringe on the top edge and a red fringe on the bottom edge.

You can see an example of this dispersion effect on the image of Polaris below for an altitude of 52° and taken with a colour digital video camera with no atmospheric dispersion corrector in place. The airy disc is spread into a small vertical spectrum whose height is considerably longer than the diameter of the airy disc.

Here we see Polaris at an altitude of 52° showing the effects of dispersion which has spread the Airy disc of the star into a vertical spectrum (222mm scope).
For a great comparison between real and modelled dispersion of stars at different altitudes and with different filters please have a look at the webpage of Andre Paquette

A correctly adjusted ADC placed between the camera or eyepiece and a barlow lens will reduce this spectral spread, improving image resolution as a result. It does this by applying the opposite amount of dispersion to that caused by the atmosphere, reconverging the light of the different wavelengths at the focal plane.

The prisms are the heart of the ADC

Material: H-K9L(Schott BK7)

Surface Accuracy: λ/10@632.8nm

Deviation Angle: 2°

UV friendly AR Coating

Martin Lewis tells you what an ADC is and why do we need it?How to use it?

Some test images show the difference between using ADC and without ADC

Jupiter from Emil with ASI174MM and ZWO ADC