Section I. DEFINITION OF COMMON OPTICAL TERMS
Aberration. An aberration is any defect of a lens or optical system which causes the image to be imperfect.
There are six general types of aberrations which are defined and listed below.
Chromatic aberration causes colored fringes around the borders of objects seen through the lens.
Coma is an aberration which causes oblique pencils of light from a point on the object to be imaged as a
comet-shaped blur instead of a point.
Curvature of image is an aberration which causes an image to be focused in a curved plane instead of a
Astigmatism is an aberration which causes the lens to have more than one focal point of the object being
Distortion is an aberration which causes the objects being viewed to appear misshapen or deformed.
Spherical aberration exists when rays of light passing through a lens near its edge are focused to a point
nearer the lens than those rays passing through the lens near the center. The effect of this aberration is
poor definition of the image being viewed.
Auto-Collimation. Establish a line-of-sight perpendicular to a mirror with an infinity setting of the telescope.
This is a process used, in the set up of test fixtures, to insure the proper alignment of the projector collimator and the test
adapter being used. Auto-collimation is accomplished by placing a front surface mirror on the mounting surface of the test
adapter, in front of the objective end of the collimator. Then while sighting through the eyepiece of the collimator, position
the collimator so that its reticle is superimposed on the reflected reticle image.
Collimation. The process of aligning the axis or center of an optical system to the mechanical axis of an
Converge. In a lens this means to deviate light rays toward a common center or focal point as shown in
figure 9-1. When applied to binoculars or any stereoscopic instrument, it means to align the line-of-sight of each side of
the instrument to meet at a common focus to obtain a single image.
Diopter. A unit of optical measurement which indicates the refractive power of a lens or prism. The shorter a
lens focal length the greater power in diopters it will have.
Diverge. In a lens this means to deviate the light rays outward from a common center in different directions,
as shown in figure 9-2.
Change 2 9-1