Abbe Prism
A form of roof prism that can be used to invert an image. Because of the perpendicular end faces, it can be inserted between the objective and eyepiece of a telescope. If the roof is replaced by a plane mirror, it becomes a "k" prism.

Any errors that result in the imperfection of an image. Such errors can result from design or fabrication, or both. See ASTIGMATISM, CHROMATIC ABERRATION, COMA, CURVATURE OF FIELD, DISTORTION, SPHERICAL ABERRATION.

Abrasion Marks
Any scratches, digs or marks on an optical surface caused by friction in polishing, mishandling or weathering.

A material such as silicon carbide, silica, zironium, cerium oxide, emery or rouge that is used to figure, shape or finish optical elements. Abrasives differ from polishing materials mainly in particle size.

Absolute Magnification
The magnification equal to the distance of distinct vision divided by the equivalent focal length of the lens.

Absolute Magnitude
The brightness of a star if it were placed at a distance of 10 Parsecs, or about 33 light-years. The Sun has an absolute magnitude of about +4.9.

Absolute Refractive Index
The refractive index of a medium relative to that of a perfect vacuum.

Absolute Temperature
A modified Celsius temperature scale with the zero point at the point of no thermodynamic activity. Absolute zero is -273.16°C or -459.7'F.

Conversion of radiant energy into other forms through either reflection or passage through matter.

Achromatic Lens
Also known as Achromat. 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.

Air Spaced Doublet
See Fraunhofer doublet.

Airy Disc
The image of a bright point object as focussed through an aberration free lens system. 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.

An instrument so mounted as to permit both azimuth and elevation adjustments.

The process of applying a film of aluminium to a surface, usually by evaporation in a vacuum.

Amici Prism
Also known as roof prism. It consists of a roof edge produced upon the long reflecting face of a right angle prism. Used as an erecting system in right angle telescopes, it erects and reverts the image.

A compound lens system whose astigmatic difference is zero for one or more off-axis zones in the image plane.

Angle Of Convergence
The angle formed by the lines of sight of both eyes when focussing on an object.

Angle Of Incidence
The angle formed between a ray of light and a line normal to the surface it strikes.

Angle Of Reflection
The angle formed between the normal to a surface and the reflected ray.

Angle Of Refraction
The angle formed between a refracted ray and the normal to the surface.

Antireflection Coating
A thin layer of film applied to an optical surface to reduce the reflectance and increase the transmittance of light. The ideal thickness is equal to Is of a wavelength of the light in question.

The opening through which light may pass.

Aperture Ratio
The ratio of the focal length of an optical element to its aperture or width.

Also known as an aplanatic lens. A lens corrected for spherical aberration and coma.

The point in orbital motion when the two bodies are most distant to the Sun.

Also known as an apochromatic lens. A lens in which the secondary spectrum has been reduced, or in which three colours have been brought to common focus. It usually consists of three element of different kinds of glass.

The blocking out of the central part of the aperture, as occurs in a reflecting telescope having a secondary mirror.

Apparent Field
The angular field of view in the image space of an optical system.

Apparent Time
Based on the true day, or time of consecutive transits of the sun. Can differ from Mean Time by 16 minutes.

The near approach of..two celestial bodies.

An optical element having one or more non-spherical surfaces.

Aspheric Lens
A lens element in which at least one surface is not spherical, to attempt to reduce spherical aberration. Also applies to reflective (mirror) elements.

The aberration where the tangential and sagittal image planes are separated axially.

Astronomical Mirror
A mirror, instead of lenses, used in astron­omical applications as it is not limited in size. Generally aspheric to reduce aberrations.

Astronomical Unit
The average distance between the earth and the Sun. Commonly used to measure distances within the Solar System.

Atmospheric Extinction
Absorption of radiation as it passes through the earth's atmosphere. The absorption varies spectrally as well as with the angle of observation.

Atmospheric Refraction
Caused by light bending upon entering the curved atmosphere.

Aurora Borealis (or Northern Lights)
The light emitted when solar wind ions are accelerated in the earth's magnetic field.

Back Focal Length
The distance from the last glass element to the image plane.

A shielding device designed to reduce the effect of stray light in an optical system.

Barlow Lens
A negative focal length (diverging) lens used to increase the effective focal length of a telescope.

Big Bang Theory
The popular cosmological theory of the conception of the Universe in an explosive event.

Binary Stars
Double stars which are gravitationally connected and not merely being close to the same line of sight.

An instrument in which both eyes are used to view an image. There may be two images or one split image.

An ideal body that completely absorbs all radiation striking it. The radiation emitted by such a body is called black body radiation.

Black Hole
A hypothetical cosmic phenomenon in which the mass and density of a star pass a critical point so that its escape velocity is equal or greater than the speed of light.

A rough optical element shaped either by cutting or molding before final finishing.

A plane that forms one side of the groove of a diffraction FAg.

Blink Comparator
An optical device that rapidly alternates two similar pictures to permit detection of differences.

A sensitive thermometric instrument used for the detection and measurement of IR radiation.

A light-tight box that focusses light on a photosensitive material.

Cassegrain Telescope
A folded reflecting telescope with high effective f/ratios. Light initially reflects from a concave surface to a second, smaller mirror (usually convex) which amplifies the focal length and reflects the signal through a perforation in the primary mirror. See Classical Cassegrain, Gregorian, Ritchey-Cretien, Schmidt.

Catadioptric Imaging System
A system that uses both refraction and reflection to achieve an image.

Celestial Sphere
A convenient term for the celestial bodies and their relative positions.

Cepheid Variables
A very useful variable type; Cepheids have a definite and predictable relation between their period and lumin­osity. This makes them one of the best galactic and inter-galactic distance scales, especially since they are very bright.

Cerenkov (Synchrotron) Radiation
Non-thermal radiation caused by the acceleration of a charged particle in a medium.

Charge-Coupled Device (CCD)
A self scanning semiconductor imaging device that uses MOS technology, surface storage and information transfer.

Charge-Injection Device
A solid state imaging device using an image sensor composed of a two-dimensional array of coupled MOS charged storage capacitors. Converts radiation into electrical signals.

CHROMATIC Aberration
The common lens aberration caused by refractive materials which bend different wavelengths of light at different angles and consequently achieve different focus.

Classical Cassegrain Telescope
A Cassegrain design which features a parabolic Primary, usually around f/4, and a convex parabolic secondary. Typical effective f/ratios are f/12 and higher.

Clear Aperture
The opening in the chassis of an optical system that restricts the rays incident on the optical surface.

Clear Eye Distance
The distance between the rear vertex of the eyelens and the front vertex of the eye cornea in telescopic systems.

Coated Optics
Optical elements having refracting or reflecting surfaces covered with one or more dielectric or metallic layers.

Coherent Radiation
Light in which the phase between any two points in the field has a constant difference.

Cold Camera
A camera cooled usually by dry ice or liquified gases to reduce "bleeding" or contamination of adjacent halide grains from exposed ones.

Alignment of two or more optical axes with respect to each other.

Color Temperature
Temperature of a blackbody that has the same color as the object in question.

An optical aberration, resulting from different magnification TH—ESe various zones of the imaging element, usually occuring some distance from the principal axis. Extra-axis points usually appear as short comet-like images.

Compound Lens
A lens composed of two or more optical elements which may or may not be cemented together.

A hollow curved surface; curved inward.

Loose associations of stars usually of historical interest, on_the Celestial Sphere.

Celestial bodies are in conjunction when their longitudes are the same. (Closest angular approach)

The difference in tone between the lightest and darkest areas.

Spherically shaped, curved outwards.

Cooke Objective
A three element objective used in astronomical cameras.

An instrument to photograph the corona of the Sun.

Correction Window
Optical wedges of very small angle which seal out dirt and prevent turbulence in an optical system.

Corrector Plate
An optical element designed to correct either a lens or mirror for spherical aberration.

The study of the evolution of the Universe.

Critical Angle
The least angle of incidence at which total reflect­ion takes place.

Crown Glass
One of the two principal types of optical glass. Crown is harder than flint glass, and has a lower index of refraction.

Curvature Of Field
A lens aberration that causes a flat object surface to be imaged with curvature.

Dall-Kirkham Telescope
A Cassegrain telescope featuring an ellipsoidal primary mirror and spherical secondary.

Dark Current
The current that flows in a photodiode when there is no incident light.

Dark Nebulae
Dark gaps in the Celestial sphere, probably dust.

The latitude coordinate of the Celestial sphere.

Depth Of Focus
The range of image distance that corresponds to the range in object distances.

Secondary wavefronts generated when a light wave passes an edge or opening.

Diffraction Limited
An optical system where the quality of the image is limited only by the diffraction inherent in the system. Sometimes means less than 'Wave manufacturing error.or less. ( Hg. Green light )

The bending of light rays away from each other, either by concave lens or convex mirror.

Dobsonian Telescope
A Newtonian telescope named for its mounting, made primarily of wood; alt-azimuth.

Doppler Effect
The effect produced on wave frequency due to the relative motion of source and observer.

Double Star
Not necessarily physically connected, double stars appear to the eye to be single points of light, but separate under magnification.

A two element lens system, cemented or not.

Dove Prism
A prism which rotates an image at twice the angle moved by the prism. Doesn't work in convergent light.

The measure of the ellipticallity of a celestial orbit.

The blocking of one celestial body by another as seen by the observer.

Eclipsing Variables
A variable brightness binary star system, with the variation caused by eclipse.

The most important Great Circle on the Celestial Sphere. The Ecliptic represents the path of the sun as seen from the Earth over one year.

Effective Focal Length
The product of the f/ratio determined by the distance of the final element to the image plane, and the aperture of the primary mirror or lens.

Electromagnetic Radiation
Radiation emitted from vibrating particles or accelerated charged particles. A self-propagating electric and magnetic wave.

Electromagnetic Spectrum
The total range of EM wavelengths.

Electro-Optical Effect
Where electrons are excited when exp6sed to light.

Elliptical Diagonal
The secondary mirror in a Newtonian telescope. The diagonal mirror reflects light from the primary axis at 90° to incideM for convenient viewing'. Major axis=1.414X Minor axis.

Emulsion Speed
The sensitivity of a photographic emulsion when exposed to light.

Entrance Pupil
The image of the aperture stop as seen from the object side of the lens.

The point at which the inferior planets are highest in the sky.

Equatorial Mount
A telescope mount equipped with an axis alligned with the axis of the earth so that its motion may be corrected with one movement.

The date for which an astronomical chart or catalog has been calculated.

Any table of calculated positions etc. of celestial objects.

Erfle Eyepiece
A popular widefield 5-element design, usually with a 3-element field lens and two-element eye lens.

Exit Pupil
The image of the aperture stop as seen from the image.

The sum total of irradiation.

Eye Lens
The lens of the eyepiece nearest the eye.,

The lens system which magnifies the final real image and projects it onto the retinal surface.

Eye Relief
The distance between the last optical vertex and the exit pupil.

Fibre Optics
Transmission of light through a long flexible fibre of transparent material.

Field Lens
The eyepiece component nearest the objective.

Field Of View
The maximum view angle of an optical instrument.

Field Stop
A mechanical diaphragm used to restrict the useable field. Usually at the image plane.

The geometrical form of an optical surface.

Filar Eyepiece
A measuring eyepiece with a screw micrometer to measure the size of an image.

Film Plane
Position of correct focus for photo-sensitive material.

A device used to attenuate particular light frequencies.

A sighting telescope used to align a larger instrument.

A mirror with zero curvature for reflection without magnification.

Flint Glass
A major type of optical glass. Flint glass is softer than crown glass and has a higher refractive index.

Fluoride Lens
A lens capable of transmission from ultraviolet through to the near infared.

The optical form of calcium fluoride, used for its low refractive index.

Fluorite Objective
A fluorite element used to reduce the secondary spectrum, usually in conjunction with two or more other lenses.

The rate of flow of radiation.

The ratio of focal length to objective diameter.

Focal Length
The distance from the second pricipal plane to the, image of an infinitely distant point of light.

Focal Plane
A plane at the focal point at right angles to the optical axis.

The point in an optical system where maximum signal-to-noise is achieved. (Contrast)

Foucault Tester
A convenient holder for the knife-edge used in testing a concave mirror. The edge is inserted at the point of perfect point image, cutting off light from all points of the mirror simultaneously.

Fraunhofer Doublet
A coma free two-element air spaced objective lens.

Fraunhofer Lines
The dark absorption lines observed in the photo­sphere of the Sun.

The number of oscillations per unit time.

A unit of frequency = 1012 cycles/second.

Fresnel Lens
A stepped lens of short focus, designed so as to reduce Franaiums. the mass of the refractive material.

An interference band, such as Newton's Ring.

Front Surface Mirror
An optical mirror on which the reflective surface has been applied to the front optical surface.

Fused Quartz
Crystal quartz melted at white heat and cooled to form amophous glass. Has a much lower refractive index than crystal

Galactic Coordinates
A system of latitude and longitude defined by the plane of our galaxy.

Galilean Telescope
A refracting telescope that forms an erect image with a convex objective and concave eye lens.

Globular Cluster
Very old and large star clusters, found around the central cores of galaxies. Very little known about their evolution.

Geometric Optics
The physical study of light as a ray, ignoring its wave properties.

Ghost Image
A second faint image caused by internal reflection in the internal airspace of an eyepiece. Can be reduced or eliminated with multidi-electric coatings.

A non-crystalline inorganic mixture of metallic oxides fused by heating with glassifiers such as silica.

A ruled dispersion device, usually used to study light spectroscopically.

Gray Body
An imperfect black body.

Gregorian Telescope
A Cassegrain design with a parabolic primary mirror and an elliptical secondary (concave) placed behind the primary's focal point.

The abrasive process used to roughly shape glass.

A term denoting incomplete polishing.

Ground Glass
A plate frosted by loose abrasive to give a high scatter to incident light.

Half Silvered
A process by which an optical element is lightly coated so that roughly the signal reflects and 1/2 passes.

Half-Wave Plate
A plate of electro-optical material that rotates the plane of polarization of a light beam.

Hard Coat
Application of dielectric coatings onto glass such that the hardness of the coating is comparable to the glass itself.

Hartman Test
A test for spherical aberration, coma or astigmatism

A fog from a polished surface caused by a scattering of light.

A device having a plane mirror so mounted as to reflect sunlight onto an instrument.

Hertz (HZ)
One cycle/second.

High Contrast
A high ratio of sinal to noise.

Hour Angle
The difference between an object's right ascension co­ordinate and the hour of R.A. on the meridian at the time of obs­ervation.

Hubble Effect
The basis of the Big Bang theory. The receding velocity of galaxies is proportional to their distance from us.

A process to increase the response speed of a photographic emulsion, done either by light flash or solution.

The science and study of imaging.

Illuminated Reticle Eyepiece
An eyepiece with a lighted reticle at the image plane, used either for guiding the telescope, aligning the polar axis or for making measurements.

The reproduction of an object produced by light rays. A converging beam produces a real image, which can be projected on a screen; a diverging beam produces only a virtual image.

Image Brightness
The apparent luminence as seen through an optical system.

Image Intensifier
An electron multiplier where an incoming photon dislodges an electron which is accelerated by high voltage through multiplying grids. The final cascade strikes a phosphor screen. Made obsolete by modern CID and CCD arrays.

Image Plane
See Focal Plane.

The lack of a fixed relationship between two waves. Image Plane.

Index Of Refraction
The ratio of the speed of light in vacuum to the speed of light in a medium.

The point at which light from a point source has parallel rays.

The invisible portion of the spectrum with wavelengths longer than the deepest visible red light.

Infrared Detector
Most astronomical IR detectors employ chopped signals; channeling devices that show signal then sky in quick succession.

In Phase
When two waves of like frequency travel through their maxima and minima simultaneously.

Flux / solid angle.

The additive process of light waves, where amplitudes of overlapping waves are either reinforced or attenuated.

Inverse Square Law
The non-relativistic law which states that the amount of radiation received is inversely proportional to the square of the distance from the source.

Inverted Image
An image that is similar to the object but rotated 180 degrees about the optical axis.

Julian Day Number
A system to calculate the interval of a long period of time. J.D. 0 is Jan.01 4713 BC at noon.

Kellner Eyepiece
An ocular consisting of a double convex field krgris ina-a-eMehbed doublet eyelens.

Keplerian Telescope
A simple telescope with focussable eyepiece and inverted image.

Konig Eyepiece
A 4-Element design of reasonable field, usually with a doublet field lens followed by two airspaced simple lenses.

The process of wearing down the surface of a softer material by rubbing it under pressure against a preformed harder material.

Metal tools used to grind lenses with loose abrasives.

A monochrome light amplification device which excites or pumps' atoms to an excited state, then utilizes a specific decay.

Latent Image
The chemical change on an emulsion before development.

Law Of Reflection
The angle of reflection equals the angle of incidence, when measured from a line normal to the surface.

Law Of Refraction
The product of the sine of the angle of incidence and refractive index of the first medium equals the product of the sine of the angle of refraction and the refracting medium.

Law Of Reversibility
If the direction of a light beam is reversed, it will follow the same path.

A transparent optical element consisting of one or more pieces 6?-6gtical glass with curved surfaces designed to converge or diverge light.

Lens Mount
The assembly that holds lenses in their proper relationship.

Lens Speed
See F/Number

Electromagnetic radiation visible to the eye; 400-750nm.

Light Chopper

Light Pollution Blocking (LPB) Filter
Very similar to the nebular filter. Allows passage of emmission lines from ionized H2O, and N but blocks out mercury and sodium vapour lamp lines.

Light Ray
The path of a given point on a wavefront.

Light Time
The time taken for light to travel from a celestial body to the observer.

Limiting Magnitude
The faintest magnitude observable by an instrument under ideal conditions.

Limit Of Resolution
Resolution of a telescope 1imited by its aperture WHEN air conditions allow. The theoretical limit is roughly equal to 4.50/Aperture for visible light in arcseconds.

Linear Motion
The component of a star's motion perpendicular to a line of sight to the Earth.

Line Spectra
Discrete wavelengths emitted when an electron drops a discrete energy level in an atom.

Long Period Variables
Usually M or N class Red Giants which range several magnitudes over periods of hundreds of days.

A low power magnifier with a single lens assembly.

Magellanic Clouds
A large irregular galaxy very near our own. Visible only from the southern hemisphere.

Magnesium Fluoride
A colourless compound evaporated on lenses in vacuum to aid light transmittance.

The apparent magnitudes of stars as seen from earth are a measure of brightness; an increase of 5 magnitudes= 0.01X luminosity. The star Vega is the standard magnitude 0.0

Main Sequence
Main Sequence stars are quite young and are fusing H to He. When the luminosity of these stars is plotted against their mass the resulting curve is called the main sequence, or Mass-Luminosity relation.

Magnifying Power
The ability of an optical system to appear to make objects larger.

Maksutov Telescope
A cassegrain design with spherical mirrors and thick corrector lens.

Mangin Mirror
A second surface spherical mirror whose spherical first surface acts to correct the spherical aberration of the reflecting surface.

Massive Optics
Optical components exceeding 24 inches aperture.

Any substance through which light can travel.

Mean Time
Time as shown by ordinary clocks.

The Great Circle on the Celestial Sphere which passes through both celestial poles and the zenith of the observer.

Mercury Green
The standard spectral line for optical testing.

Messier Catalog
103 objects compiled by Messier to be avoided when hunting comets. Almost all are of considerable interest.

Any piece of material burning upon entering the Earth's

A smooth, highly polished surface for reflecting light.

Mirror Blank
The material on which the reflective coating is applied. It may be glass, metal or plastic and must be thick enough to resist distortion by gravity.

Mohs Hardness
A surface hardness standard, talc=1; diamond=10.

Molded Blank
A blank whose basic curves are shaped under heat.

Molded Lens
A lens fire polished to its final surface.

One colour.

Application of two or more dielectric coatings to an optical surface to inhibit reflection.

Multiple Ocular Holder
A device that rotates about the optical axis to discretely engage different eyepieces.

The symbol used to denote refractive index.

Faint, luminous patches of light, usually gas illuminated by starlight.

Nebular Filter
See LPB filter.

Newtonian Telescope
A telescope with a concave primary a small plane (diagonal) mirror that reflects the image primary's axis for convenient viewing.

Newton's Rings
The series of rings or bands formed when reflected from adjacent surfaces and separated by a thin interfere.

New General Catalog of astronomical objects; one of the largest.

Nodal Points
Of all the rays passing through a lens from an off-axis object point to its corresponding image point, there is always one ray which maintains the same direction. The nodal points are the two points at which the external rays appear to intersect the axis.


Northern Lights
See Aurora Borealis.

New Star. A Stellar explosion which causes a rapid increase in a star's luminosity.

The precessional path of the Celestial Pole. The Earth's axis makes one complete rotation in about 26,000 Years.

The first optical element in a system which receives light from the object and forms the primary image.

A structure housing a permanently set-up instrument. Observatories usually have either a rotatable dome or removable roof.

Occultation Period
The time between lunar eclipses of stars near the ecliptic. On average, each potential star will be occulted twice in a 18.59 year period.

Off Axis Guider
A light sampling. device that allows telescope guidance while simultaneously exposing a negative.

Closest approach of a superior (further from Sun) planet.

Optical Axis
The line passing through the centers of curvatures of the optical elements.

Optical Chopper
A mechanical or electro-mechanical device for inter­upting the light beam.

Optical Flat
A piece of glass polished flat on both surfaces.

Optical Glass
Glass manufactured with controlled composition so that its physical characteristics are consistent and homogeneous.

Optical Window
A flat glass plate sealing the front of a telescope tube to restrict dust and air currents, and to eliminate diffraction spikes caused by secondary mirror vanes.

The science of light.

Orange Peel
The uneven or dimpled appearance of an optical element which has been insufficiently polished.

Corrected for distortion.

Orthoscopic Eyepiece
One of the most popular eyepiece designs. The Ortho usually has a field of 40-45° and features a 3-element cemented field lens and simple eyelens.

A layer of material applied to a reflective coating to protect it from physical or chemical damage.

Parabolic Mirror
An astronomical objective which reflects a signal to a perfect focus on axis.

The optical illusion that causes a viewed object to change position due to movement of the observer.

Having the same positioning on the focal plane.

Parfocal Eyepieces
A series of eyepieces which may be inserted without having to re-focus.

The nearest point of the orbit of a planet or comet to the Sun.

Periplan Eyepiece
A well-corrected flat-field eyepiece; similar to a Huygenian.

Chemical substance that exhibit fluorescence when excited by light or particles.

A 2-electrode light sensitive semi-conductor.

Photographic Exposure
The product of time and flux of radiation.

A colour sensitive light measuring device.

The science of the measurement of colour intensity.

Photomultiplier Tube

A quantum of light.

Photon Counter
A low light level light-generating events.

The apparent surface to radiate.

Physical Optics
The study of light as a wave phenomenon rather than as rays as in geometrical optics.

A surface with no curvature

Plane Grating
A grating requiring either a lens or mirror to focus the spectrum.

Large relatively circular orbit about the sun.

Planetary Nebula
A circular nebula caused by stellar explosion. All have a central originating star, usually a White Dwarf.

Plate Scale
The angular amount of sky/inch at the image plane. Degrees/inch = 57.3/Focal length. (in inches)

Plossl Eyepiece
Commonly a 4-Element eyepiece featuring two cemented doublets cemented by an airspace. Recent designs feature 5 and 6 elements.

Plastic Optics
Optical elements made from molded plastic.' Not suitable for astronomical application.

Polar Axis
The axis of an equatorial telescope that aligns with the Earth's axis, correcting for the Earth's rotation with an equal and opposite movement.

Polar Axis Finder
A finder or allignment telescope, sometimes built into the polar axis, which helps align the polar axis by sighting on Polaris. (North Star)

The restriction of light vibrations to a single plane.

Polarizing Filter
Attenuates light by allowing only one polarity to pass.

The optical process following grin ing that smooths the optical element to its final figure.

Porro Prism
A 45-90-45° prism used to correct the inverted and reverted images of most astronomical telescopes.

Usually taken to mean magnification.

Precession Of The Equinoxes
The advancement of the vernal equinox by about 20 minutes per year, due to the precession of the Earth's axis

Prime Focus
The focal point of the objective mirror or lens.

Prime Focus Cage
In very large reflector telescopes, the observer's position at the prime focus within the telescope chassis.

A transparent body having at least two polished plane surfaces through which light is reflected or refracted.

Protective Coating
See overcoat.

Or neutron star. A star of more than 1.3 but less than 3 so ar masses which has exhausted its nuclear fuel and has collapsed into a mass of degenerate neutrons.

Quasar (QSO)
Quasi-stellar object. Highly red-shifted star-like objects of considerable controversy.

Quantum Optics
Optics defined in terms of discrete quantum amounts of energy. (photons)

Quarter-Wave Plate
A plate of double refracting crystal such that a difference of one quarter wavelength is formed between transmitted light.

When fused, an excellent substrate for astronomical figuration due to its low expansion.

Radial Astigmatism
Astigmatism in a lens system caused when light enters the system at an oblique angle.

Radial Distortion
Any alteration in magnification off the optical axis.

The unit angle formed when the radial distance is imposed on the circle. One radian=57.3°

The emission of energy through space or a medium.

Radio Astronomy
The study of extra-terrestrial radiation in the radio area of the spectrum.

Radio Telescope
Usually a large concave wire collector with detectors for the study of radio waves.

Raman Effect
The effect of light vibrating the molecules of the medium it is passing through.

Ramsden Eyepiece
An eyepiece consisting of two piano-convex lenses of the same focal length, with their convex surfaces facing each other.

A geometric representation of a light path.

Rayleigh Criterion
When a lens or mirror is free of aberrations, the images of point objects appear as diffraction patterns. The size of the point image is limited by the first destructive interference. With respect to circular objects the angular resolving power=1.22W/a where W is the wavelength of the light and a is the diameter of the objective.

Real Image
An image that can be physically projected on a screen.

The amount of exposure is equal to any product of radiation and time.

Red Giant
The first move of a star off the main sequence, once He fusion has become established.

Red Shift
The displacement of spectral lines towards the red end of the spectrum, due either to receding motion or gravity.

Reflecting Telescope
A telescope with a concave reflective primary mirror.

Return of radiation from a surface, without change in wavelength.

Reflective Coating
A substance applied to an optical element to aid reflection.

Refracting Telescope
A telescope having a lens as primary objective.

The bending of oblique incident rays due to light's lower velocity in the refractive medium.

Relay Lens
A lens used to transfer a real image from one point to another within an optical system.

Replication Optics
Optical elements cast from polished molds.

The quality of reproduction of an optical system.

Resolving Power
The ability of a telescope to separate closely positioned stars.

A pattern mounted in the focal plane of an eyepiece to be superimposed on the image of the object in view.

Retrograde Motion
The apparent reversing of a planet's true motion due to parallax from the faster moving Earth.

Richfield Telescope
An astronomical telescope designed to show a wide field of view; usually f/4 or less.

Right Angle Prism
Allows convenient viewing of the zenith in refracting and Cassegrain telescopes.

Right Ascension
The longitude of the Celestial Sphere; the Great Circle perpendicular to the Meridian when at the equator.

Advanced photographic Cassegrain design with hyper­bolid primary and secondary mirrors. Large coma-free field.

Ronchi Test
A modified, more accurate, version of the Foucault test, with the knife edge replaced by a transmission grating.

A fine powdered ferric oxide used in polishing lenses.

Ross Lens
A corrective lens system placed near the focal plane of a Newtonian telescope to increase the effective field of view by correcting for coma.

An orbiting body.

Schmidt Camera
A camera consisting of a concave spherical mirror with an aspheric filmholder and correcting lens at the centre of curvature of the mirror. Coma and astigmatism are automatically corrected, leaving only chromatic aberration.

Schmidt Plate
An aspheric plate placed at the centre of curvature of a spherical mirror to correct for spherical aberration.

Secondary Colour
An aberration that remains after primary colour is corrected.

Setting Circles
Mechanical scales denoting declination and right ascension to aid in the targeting of the telescope.

Sidereal Day
The true sidereal day is the time between successive transists of a star. The sidereal day is shorter than the regular day (23h56m) due to the Earth's movement on it's orbit.

Sidereal Time
Used for measuring Right Ascension and based on the sidereal day.

A mechanical or electronic device used to control exposure time.

Signal-To-Noise Ratio
The ratio of power in a desired signal to the background incoherence.

Silicon Monoxide
SiO. A material used as a protective layer on aluminized or silvered mirrors.

Silver Halide
Used as emulsion on photosensitive material.

A mechanical aperture, used to increase resolution in a spectrograph.

Solar Prominence
A jet or cloud of very hot gas forming the chromosphere of the Sun.

Speckle Interferometry
Resolution of stars with ultra-high speed exposures.

An optical instrument for forming the spectrum of a star. The dispersing medium may be a prism or grating.

Spectroscopic Binaries
Double stars so close together that they cannot be visually separated, but are identified as doubles by their spectra.

The entire frequency range of electromagnetic radiation.

Spherical Aberration
The failure of a lens or mirror system of spheric design to form a perfect image.

An instrument for measuring the radius of curvature of a spherical lens or mirror.

A type of coating process in which the coating material is generated by application of gases in a vacuum and then applied to the substrate through excitation by RF energy.

Standard Time
The division of the Earth into standard time zones, with Greenwich taken as the universal Zero.

A celestial body which emits radiation.

Star Cluster
A group of stars bound by gravity.

Stark Effect
The shifting of spectral lines caused by application of a strong transverse electric field.

Steady State Theory
The theory of the Universe which advocates flat space-time and unchanging density.

Stefan Boltzmann Law
The thermal energy emitted by a body is proportional to the fourth power (T) of absolute temperature.

Stellar Parallax
The angle of apparrent stellar motion due to the diameter of the Earth's orbit about the Sun.

Stepper Motor
State of the art motors for telescope drives. These motors have a discrete number of steps/revolution which advance under the control of a logic sequencer.

Superchromatic Correction
Correction of an optical system for separate wavelengths, usually accomplished with three elements.

Nature's most violent event. When a massive star has exhausted it's nuclear fuel it begins an unstoppable collapse and implodes. The gravitational energy release causes the brightness of the star to increase tens of thousands of times in a few hours.

The exterior faces of an optical element.

Symmetrical Lens
An eyepiece made up of two sets of similar lenses, each of which compensates for many of the aberrations produced by the other.

Synchrotron Radiation
Radiation of a continuously accelerated electron.

Telephoto Lens
A compound lens constructed so that its overall length is equal or less than its effective focal length.

An afocal optical device made up of lenses or mirrors with a magnification greater than 1.

Telescope Mount
The pointing mechanism for an astronomical telescope. It may be either altazimuth, with horizontal and vertical axes of rotation, or equatorial with one axis parallel to the Earth's and the other perpendicular. The polar axis is usually mechanically driven for ease of use.

Terrestrial Telescope
A telescope that produces an erect image.

Test Flats
Certified flat glass used to test new surfaces.

Thermal Radiation
Energy released through molecular vibration.

The minimum incident light detectable by the eye.

The crossing of a Meridian.

Conduction of light through a medium.

Transmission Grating
Transparent diffraction grating.

Triple Aplanat
A compound lens with two negative flint glasses surrounding a double convex crown glass.

A three lens component, usually cemented.

True Field
The size of the field of view in object space. How much sky that can be seen.

The invisable region of the spectrum with wavelength just shorter than violet.

Universal Time
Mean time for the meridian of Greenwich.

All known and observable matter.

Vacuum Apparatus
Equipment used for coating optical elements. For most evaporated coating applications, a near perfect vacuum is required; this is usually accomplished with a combination of mechanical and oil diffusion pumps.

Variable Stars
Stars that vary in intensity with time. There are many different types of variables; all stars fluctuate somewhat.

Vernal Equinox
The zero for celestial measurement for longitude, is the point of intersection of the Celestial Equator and the Ecliptic. Also called First Point of Aries.

Loss of light in an optical system due to mechanical restriction. Sometimes a problem in Newtonian telescopes.

That portion of the spectrum detectable by the eye. It extends from approximately 400-750nm in wavelength.

In light, the wavefront is all the energy equidistant from the source.

The distance of one cycle of light.

Wave Number
The number of waves per unit length.

White Dwarf
A spent star, reduced to a very dense, small, non-luminous state. The eventual fate of the Sun.

Widefield Eyepiece
An ocular of more than 50u ApArrent field. Usually composed of 5 or more elements.

A piece of transparent material used to keep dirt, moist­ure and turbulence out of a telescope tube.

Working Aperture
The maximum aperture of a lens or mirror which is usable, even though the physical aperture may be larger.

X-Ray Astronomy
The study of celestial bodies relative to X-ray emission. Advanced study requires orbiting telescopes.

X-Y Recorder
A device used to measure the distances on photographs, particularly sky surveys.

Zeeman Effect
Splitting of spectral lines due to a magnetic field.

Zenith Telescope
A telescope with limited movement in vertical.

Directly overhead the observer.

The constellations found around the ecliptic.

Zonal Aberration
Spherical or chromatic aberration in a lens or mirror having a wide aperture.

Zoom Lens
An optical system of variable focal length. Not suit­able for most astronomical applications