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The refractor follows the classic telescope design - a long, thinner tube pointing into the sky and an eyepiece at the bottom. The incident light is collected by the objective, consisting of one or more lenses. Apochromats are refractors (APO´s) whose lens design is designed to combine at least three colors (red, green, blue) into one focal point. This corrects the chromatic aberration (a shift in colors, fig. A) typical of refracting telescopes.
The FFC allows the focal length to be significantly extended without affecting the sharpness of the image.
A Barlow lens is an optical component that is inserted between the eyepiece and the focuser. Inside there is a negative lens that extends the focal length of the telescope.
Technical details: