Rayleigh Criterion

Lord J. W. S. Rayleigh (1842-1919), the Nobel Prize-winning English physicist, empirically determined that telescope optics that yield 1/4th wave accuracy at the final focus (so that all light gathered by the system comes to a focus within 1/4th of a wavelength of the green light to which the eye is most sensitive) will produce results on stars that are visually indistinguishable from an optically perfect system. This is known as the Rayleigh Criterion and is a handy yardstick by which telescope quality can be measured. To achieve a 1/4th wave accuracy overall, each mirror in a reflector must be finished to 1/8th wave smoothness. When observing extended deep space objects (such as nebulas and galaxies), most amateur astronomers find it difficult to see any visible difference between optics made to 1/4th wave accuracy and those made to 1/10th wave accuracy - although experienced observers usually find the higher accuracy to be beneficial on planets. Large optics polished to higher levels of accuracy than 1/10th wave usually gain the observer little additional benefit visually, however, as the performance of the telescope will be limited more by atmospheric conditions than it will be by mirror accuracy. Inexpensive scopes can have mirrors polished to 1/4th wave accuracy and still have a rough surface marred by micro-ripple whose errors might be 1/50th wave or less. Such rough mirrors will have visibly lower contrast and less-sharp images than a well-finished mirror.

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