Asked by James Beed
An apochromatic telescope has a multi-lens setup to eliminate chromatic aberration when stargazing. Chromatic aberration is caused by the difference in wavelength of various colours of light. In a single-lens setup, blue light is focussed closer to the lens and it has a shorter focal length. Red light is focussed further away from the lens. This variance in focal length results in small degradations in image quality. This effect is more prominent when viewing certain objects such as planets. To eliminate this aberration a series of lenses are used to manipulate the light so that the focal lengths converge. By converging the focal lengths, quality can be improved. However, the extra material and expertise required to build these compound lenses increases the price of telescopes using this setup.
Answered by Josh Barker from the National Space Centre
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