Can differ from Mean Time by 16 minutes.Īppulse The near approach of.two celestial bodies.Īspheric An optical element having one or more non-spherical surfaces.Īspheric Lens A lens element in which at least one surface is not spherical, to attempt to reduce spherical aberration. It usually consists of three element of different kinds of glass.Īpodization The blocking out of the central part of the aperture, as occurs in a reflecting telescope having a secondary mirror.Īpparent Field The angular field of view in the image space of an optical system.Īpparent Time Based on the true day, or time of consecutive transits of the sun. A lens in which the secondary spectrum has been reduced, or in which three colours have been brought to common focus. A lens corrected for spherical aberration and coma.Īphelion The point in orbital motion when the two bodies are most distant to the Sun.Īpochromat Also known as an apochromatic lens. The ideal thickness is equal to Is of a wavelength of the light in question.Īperture The opening through which light may pass.Īperture Ratio The ratio of the focal length of an optical element to its aperture or width.Īplanat Also known as an aplanatic lens. Used as an erecting system in right angle telescopes, it erects and reverts the image.Īnastigmat A compound lens system whose astigmatic difference is zero for one or more off-axis zones in the image plane.Īngle Of Convergence The angle formed by the lines of sight of both eyes when focussing on an object.Īngle Of Incidence The angle formed between a ray of light and a line normal to the surface it strikes.Īngle Of Reflection The angle formed between the normal to a surface and the reflected ray.Īngle Of Refraction The angle formed between a refracted ray and the normal to the surface.Īntireflection Coating A thin layer of film applied to an optical surface to reduce the reflectance and increase the transmittance of light. It consists of a roof edge produced upon the long reflecting face of a right angle prism. With monochromatic (laser) light, it consists of a bright centre surrounded by alternate dark and light rings caused by reinforcement and interference of the rays.Īltazimuth An instrument so mounted as to permit both azimuth and elevation adjustments.Īluminizing The process of applying a film of aluminium to a surface, usually by evaporation in a vacuum.Īmici Prism Also known as roof prism. A lens consisting of two or more elements, usually of crown and flint glass, that has been corrected for chromatic aberration with respect to two selected colours or light wavelengths.Īir Spaced Doublet See Fraunhofer doublet.Īiry Disc The image of a bright point object as focussed through an aberration free lens system. Absolute zero is -273.16☌ or -459.7'F.Ībsorption Conversion of radiant energy into other forms through either reflection or passage through matter.Īchromatic Lens Also known as Achromat. The Sun has an absolute magnitude of about +4.9.Ībsolute Refractive Index The refractive index of a medium relative to that of a perfect vacuum.Ībsolute Temperature A modified Celsius temperature scale with the zero point at the point of no thermodynamic activity. Abrasives differ from polishing materials mainly in particle size.Ībsolute Magnification The magnification equal to the distance of distinct vision divided by the equivalent focal length of the lens.Ībsolute Magnitude The brightness of a star if it were placed at a distance of 10 Parsecs, or about 33 light-years. See ASTIGMATISM, CHROMATIC ABERRATION, COMA, CURVATURE OF FIELD, DISTORTION, SPHERICAL ABERRATION.Ībrasion Marks Any scratches, digs or marks on an optical surface caused by friction in polishing, mishandling or weathering.Ībrasive A material such as silicon carbide, silica, zironium, cerium oxide, emery or rouge that is used to figure, shape or finish optical elements. Such errors can result from design or fabrication, or both. If the roof is replaced by a plane mirror, it becomes a "k" prism.Īberration Any errors that result in the imperfection of an image. Because of the perpendicular end faces, it can be inserted between the objective and eyepiece of a telescope. Abbe Prism A form of roof prism that can be used to invert an image.
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