achromat

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Related to achromats: Achromatic doublet

achromat

[′ak·rə‚mat]
(optics)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
I initially tried out the Quark filter on a 4-inch f/9 achromat with a 20-mm eyepiece, using the supplied AC power adapter and the 2-inch UV/IR blocking filter mounted in the front of my diagonal.
Fast, short-tube achromats are very compact (so are nice travel scopes) and are best for wide-field deep-sky viewing.
Thankfully, you don't need an expensive apochromatic objective lens either, since the chromatic aberration inherent in short-focus achromats isn't noticeable at the low magnifications typical of RFTs.
Some of the scopes listed are plain achromats, others are apochromats, and one is a semi-apochromat.
Aperture Focal length Focal ratio (inches) (inches) 3 27 f/9 4 48 f/12 5 75 f/15 6 108 f/18 8 192 f/24 Many small doublet achromats satisfy this criterion, but in apertures greater than 5 inches or so, such instruments become prohibitively cumbersome and unwieldy.
Good achromats can yield outstanding images, but the false color never completely goes away, and it is generally more pronounced in instruments with low f/ratios.
In actual use, when it came to color correction, the SV85S proved to be as superior in performance to the Nighthawk as the Nighthawk is to many other short-tube achromats.
Typical with achromats, the all-important issue of false color seen in the eyepiece depends somewhat upon the atmospheric conditions at the time of observation.
In practice, however, various good-quality achromats will work, introducing aberrations no more conspicuous than those of fine eyepieces or Barlow lenses.
Most of the telescopes came with good-quality 1 1/4-inch Kellner-class eyepieces, called Modified Achromats (MA).
The color correction in the f/15 folded refractor is vastly superior to the new crop of large f/8 achromats currently in fashion, and yet the tube length is no greater.
To reduce false color to acceptable levels, a long-standing rule for classic achromats of this size has been that their focal length should be at least 15 times the aperture.