These anastigmatic lenses, which are manufactured up to X 40, are chromatically and spherically corrected, and for a middle diaphragm the errors of lateral pencils, distortion, astigmatism and coma are eliminated.
If measures are made by placing the image of a star in the centre of the disk of a planet, the observer may have a tendency to do so systematically in error from some acquired habit or from natural astigmatism of the eye.
These are: (I) chromatic aberration, (2) spherical aberration and (3) astigmatism (see Aberration).
To eliminate distortion and astigmatism when the centre of rotation of the eye coincided with the point where the principal rays crossed the axis.
Airy, the astronomer, about 1827, corrected his own astigmatism by means of a cylindrical lens.
Photog., 1891, 5, p. 225; 18 93, 7, p. 221), cemented objectives of thin lenses permit the elimination of spherical aberration on the axis, if, as above, the collective lens has a smaller refractive index; on the other hand, they permit the elimination of astigmatism and curvature of the field, if the collective lens has a greater refractive index (this follows from the Petzval equation; see L.
The correction of astigmatism is in many cases a matter of considerable difficulty, but the results to vision almost always reward the trouble.
It must be so designed as to give as flat an image as is possible consistently with freedom from astigmatism of oblique pencils.