What is an atmospheric dispersion corrector (ADC)?

When light from stars and planets passes through Earth's atmosphere , it is refracted by the layer of air. The intensity of this refraction depends on the wavelength; a phenomenon that is called dispersion , and which causes annoying halos at the edges of planetary images. The atmosphere therefore basically acts like the lens of a telescope with chromatic aberrations. This therefore means that even using high-quality optics, free of chromatic aberrations, you will have these halos. In fact, the dispersion of the Earth's atmosphere cannot be corrected by the optics of your telescope. This is where the small ADC device comes in: it essentially consists of two rotatable prisms made of silicate glass. The prisms generate their own chromatic aberration, so to speak, which compensates for that of terrestrial dispersion. The ADC thus corrects this annoying phenomenon. The position of the prisms is adjustable via a small slide.

Who needs concealer?

The phenomenon of dispersion depends greatly on the height of the observed object on the celestial vault. The lower the object, the longer the path its light has to travel through the atmosphere, and therefore the greater the resulting dispersion. It is precisely the bright planets that are often low on the horizon, especially when you are at high latitudes. Thanks to the ADC you will be able to observe the planets with greater brilliance and without chromatic aberrations not only when they are high in the sky, but also when they are at dawn and dusk . This allows considerable flexibility in choosing your observation times .

Halos are only the most obvious effect of dispersion. Especially in photography this phenomenon is annoyingly visible in contrasts, as can be clearly seen in the examples given. Of course, it is possible to increase the contrast in post-production, but not without loss of resolution. Since the use of the ADC in photography is particularly useful, this device is equipped with a T2 mount on both sides.

To ensure high light passage , the optical surfaces of the ADC are coated with a multilayer treatment. This coating is specially optimized for astrophotography, in the range between 200 and 700 nanometers.