A refractor whose objective combines two kinds of glass to bring red and blue to the same focus. Violet stays poorly corrected: a violet halo surrounds bright objects, more so the faster the refractor. At f/12 or slower it becomes discreet.
Related terms
- Refractor
An instrument whose objective is a group of lenses at the front of a closed tube. With no obstruction it gives very contrasty images, cools quickly and almost never needs collimating. For the same aperture it is harder to make and longer than a reflector.
- Apochromat
A refractor whose objective uses special glass (ED, fluorite), often in three lenses, to bring at least three colours to the same focus. Chromatic aberration becomes almost invisible, even at f/6 or f/7. It is the instrument of choice for wide-field imaging.
- Chromatic aberration
A lens bends each colour slightly differently, so they do not all come to focus at the same place. It shows as a violet halo around the Moon, Venus or Jupiter in fast achromatic refractors. Mirrors are free of it; apochromatic refractors correct it almost entirely.
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