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.
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.
- Achromatic refractor
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.
- 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.
- Field flattener
A lens placed in front of the camera that corrects a refractor’s field curvature, so stars stay sharp to the edge of the frame. It must sit at the right distance from the sensor (often 55 mm). At the eyepiece the eye compensates and it is not needed.
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