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OpticsPlanet.com has prepared this quick reference for our customers. It briefly defines more than 200 Microscopy and Astronomy terms. Want more astronomy information? Try Phil Harrington's Starware™ or one of the several astronomy dictionaries. The volumes will more thoroughly cover more than the many subjects about which we included only a few lines. We are just trying to help you get started.

For professional microscopy information, we recommend Barbara Foster's Optimizing Light Microscopy for Biological and Clinical Laboratories, or John Delly's Photography Through the Microscope, a Kodak™ book. There are many excellent introductory books for young microscopists.


abbe condenser A lens used to focus light on a microscope specimen. An abbe condenser usually is rated at a maximum numerical aperture of 1.25 (NA 1.25). This numerical aperture may only be obtained with double oil immersion in which the substage condenser and the objective are oiled to unify them with the specimen. With good optics and a well prepared specimen, this technique can obtain excellent resolution with magnifications of 900x, or more. Without oil, numerical aperture on an abbe condenser cannot exceed 0.90. Named for Ernst Abbe, an optical designer with Zeiss in the 1800s.

aberration, optical Errors in an optical system, lens or mirror, affecting the image. The six types of aberration are astigmatism, chromatic, coma, distortion, field curvature and spherical.

achromat (ic)   A lens with at least two optical components which partially correct for chromatic (color) aberration to increase image sharpness and color accuracy. Light bends (refracts) when it passes through a glass lens. Wavelength determines color, and light normally is composed of multiple wavelengths. Different colors refract at different angles and come into focus at different points. We can be correct this by adding lenses of different shapes and glass formulas to help bring different colors into focus at the same point. While achromatic lenses bring two primary colors (red and blue) of the visible spectrum into focus at the same point, they leave considerable uncorrected chromatic aberration.

achromatic refractor  Usually inferior, overpriced 60mm aperture telescopes marketed by chain stores. Although very good refractors can be made up to about 6" (about 180mm) diameter, Reflecting telescopes are free from chromatic aberration described in achromat. Reflectors use mirrors instead of lenses to collect light rays and bring them into focus.

airy disk Named for George Airy (1801-92), the seventh Astronomer Royal to the British Crown. When observing a star, or a similar point source of light, it is the central disk you see, not a true point.

The radiating diffraction pattern of the not quite point image is surrounded by several fine diffraction rings similar to the rings and bulls eye of a target. You can observe the airy disk by slightly adjusting the focus back and forth until the central disk is visible. The larger a telescopes aperture, the smaller the Airy disk, relative to the field of view. Although some stars are many times larger than our sun, they are nevertheless too distant for us to see their massive spherical shape from Earth, even through a large telescope. Larger telescopes excel at controlling diffraction, thus they form a smaller Airy disk. It is one of the curiosities of astronomy that larger telescopes will make stars appear smaller as they resolve the image to a finer point.

altazimuth mount  A telescope mount which allows the instrument to swing up and down on a pivot for altitude adjustment and side-to-side for azimuth (which see) adjustment. This is usually a simple mount used by manufacturers of inexpensive refractors. Most serious amateur astronomers prefer German Equatorial Mounts, or Fork Mounts. (which see).

altitude The distance of a celestial object above or below the observer's horizon. We measure altitude as an angle with 0° as the horizon and 90° as the zenith.

aluminizing  The process of applying a reflective aluminum coating to the surface of a mirrorHard coatings are added to protect the brilliant surface from abrasion and oxidation. This process replaces silvering, which required silver and toxic mercury compounds. 

anastigmat (ic) An optical system with at least three elements completely corrected for spherical aberration, coma, and astigmatism (which see). ASTELE telescopes are all anastigmatic.

angular resolution The minimum distance between two objects, measured as degrees of angle, that an optical system can resolve or separate.

angular separation The angular distance between two celestial objects stated as degrees, arc-seconds or minutes of arc.

annealing The controlled process of slowly cooling an object created or modified by heat, such as a mirror cast from molten glass. Annealing prevents cracking, stress-related irregularities and assures strength, integrity, and stability of shape. Annealing large telescope mirrors can take months, if not years. LOMO PLC, the manufacturer of Astele telescopes, cast the world's largest one-piece telescope mirror - six meters in diameter and more than 2 tons in weight! Nearly two years were required to anneal this giant mirror.

annulus In microscopy, the etched, laminated, or photographically applied ring on glass lenses to help place light and cause interference in the passage of that light through, or around a specimen, the aim being to increase contrast between the specimen and the surrounding substrate.

anti-reflective coating The special vaporized coating applied in a vacuum to optical surfaces to minimize reflection and improve the passage of light.

aperture Literally, an opening. The diameter of a telescope's objective lens or primary mirror, expressed in inches or millimeters. The larger the aperture, the greater the ability to collect light and detect faint distant objects. In a microscope, the numerical aperture is the measure of the resolving capability of an optical component. The larger the number, the greater the resolving capability and magnification. A convenient rule for estimating magnification an aperture can handle well is 2x per millimeter, or 50x per inch.

aperture diaphragm The substage diaphragm on a microscope.

aperture ratio The ratio of the diameter (aperture) of the objective lens or primary mirror to its focal length, also called -number, or focal ratio.

aplanat, aplanatic apochromat A flat-field, fully color-corrected lens. A microscope condenser which is fully corrected for color and flatness of field.

apochromat (ic) A lens or optical system which is nearly free of chromatic aberration, which for practical purposes means at least three or more wavelengths will focus at precisely the same point. In Astronomy, apochromats are high quality refractors with high prices. Reflectors are apochromatic in performance without the extra expense. There are varying qualities of apochromats, most requiring at least three lenses of different types of glass, but some requiring many more components. The most expensive apochromats use Fluorite crystal and may correct three wavelengths with only two optical elements. Fluorite is expensive to manufacture and because it is brittle, difficult to grind, polish and mount, further raising the product cost. 2. In Microscopy, apochromats are among the most expensive lenses, requiring a large number of elements. Plan apochromats, objectives with extreme corrections for all colors and flatness of field, usually for photomicrography, are very expensive. 

arc minute  See minute of arc arc second  See second of arc

arm  The component of a modern microscope rising from the base which contains the focusing mechanism and supports the stage, as well as the body, or head, which contains the oculars.

aspect The earth observers perspective of an object in the Solar System relative to the Sun. The four main aspects are conjunction, greatest elongation, opposition and quadrature.  

aspheric The shape of a lens or mirror which is not part of sphere.

asteroid Any of about 1 million small, naturally occurring, objects in the Solar System, most of which orbit in the main asteroid belt, a zone between the orbits of Mars and Jupiter. Those detected to date range from 10 meters in diameter to nearly 1000 kilometers in diameter for Ceres, the first asteroid discovered by astronomers. Total weight of all asteroids is less than 5% that of our moon and their orbits are far more eccentric than that of the planets. To date, orbits of more than 7500 asteroids have been charted, and the composition of many has been analyzed by spectometry. See minor planets

asteroid belt Also known as the main belt, it is a zone between the orbits of Mars and Jupiter containing irregular concentrations of asteroids.

astigmatism An optical aberration in which the focal of a lens or optical system varies at different diameters. A typical example will make a star change from a short horizontal line, to a circle at the best focus, to a short vertical line past the point of best focus. By slowly focusing through an object and watching for this shift, you can determine if there is astigmatism.

astro-sital A high quality, well-annealed glass that, with Pyrex, is a preferred choice for optical blanks from which mirrors are made. Telescope mirrors can collect considerable heat and require superior heat-tolerating traits. ASTELE mirrors are made of Pyrex, astro-sital, or other precisely annealed glass.

astronomy   The science of space beyond the earths surface and lower atmosphere. In addition to observational astronomy, it now includes such branches as astrometry, the measuring of the positions and movements of celestial bodies, astrophysics, includes spectrometry, the analysis of the compositions of celestial bodies through measurement of wavelengths of light emitted and reflected, celestial mechanics, which specializes in orbit calculation, including the influence of gravity, mass, acceleration and inertia, and cosmology, which investigates the origins and evolution of the universe. 

astrophotography  The marriage of photography with astronomy. Long exposures on hyper-sensitized films have revealed much which cannot be seen during real-time observation. Images of faint nebulae and galaxies can continue to improve during exposures ranging from several minutes to many hours, revealing detail that cannot be seen without tremendous light amplification. Such photography requires excellent motorized mounts to correct for the earths rotation, perfect alignment, and periodic correction during the exposures.

autoguider  An electronic device that monitors the position of a guide star and automatically corrects for drift on both axes to achieve precise postioning for photographs, recording, etc.

averted vision Using peripheral vision to view faint objects instead of central vision. This takes advantage of the high concentration of the very light-sensitive rod structures surrounding the human retina. These rods are totally absent in the eyes tiny optical center the foveola where there are only cones -- the color and form discriminating structures. An observer can multiply light sensitivity many times by slightly averting the head so the projected image from the eyepiece falls outside of the foveola and onto the area of high light sensitivity.

azimuth The compass direction of a celestial object measured in degrees clockwise from north around the observers horizon. Due north is 0, due east is 90, due south 180, due west 270.

Barlow lens  A magnifying lens placed in the drawtube of a telescope before the eyepiece to add to the apparent magnification.

binary star Paired companion stars held together by mutual gravitational attraction and orbiting their common center of mass. These differ from optical binaries which do not orbit one another but appear to be companions due to location on a common radius from the observer. Optical binaries may be huge distances from one another while true binaries are in relative proximity. In addition to binary star systems, other multiple star systems with as many as seven suns orbit a common center of mass. Specific true binaries are useful indicators of telescope performance, particularly if the stars are of different colors. Some amateur astronomers specialize in studying the numerous types of multiple star systems.

binocular In Astronomy -- a binocular is really a pair of low-power prismatic spotting scopes mounted together for comfortable three-dimensional viewing. Prisms and/or porro mirrors shorten the instrument size and deliver a corrected image to the viewer. The common incorrect usage, a pair of binoculars, really means 2x2 oculars, or four oculars. A good binocular with large objectives is a very useful astronomical tool. Non-prismatic joined telescopes are field glasses.  

binocular In Microscopy a viewing system in which prisms permit both eyes to observe the specimen through separate eyepieces using one objective.

Cassegrain telescope A reflecting telescope with a folded optical path achieved bytwo mirrors, one -- a large concave primary with a center hole the other, a small convex mirror mounted on the large front corrector lens. Light passes through the corrector plate, or lens, strikes the primary mirror, which reflects the image back to the smaller convex secondary mirror, which then reflects the magnified image through the center hole and onward to the eyepiece. Maksutov-Cassegrainsuse a spherical miniscus lens as a corrector and traditionally are superior to Schmidt-Cassegrains at apertures up to 150mm. With very precise, proprietary manufacturing techniques, ASTELE is building Maksutov-Cassegrain telescopes of outstanding performance with apertures up to 203mm (8).

Cassinis Division The dark gray band indicating a gap between two of Saturns outer rings. It is named after French-Italian astronomer Giovanni Domenico Cassini (1625-1712) who discovered the division in 1675. Use of a blue filter will greatly assist in seeing this feature, while yellow filters help define more features of the gaseous planet..

catadioptric system An optical system utilizing lenses and mirrors to form a magnified image. The Maksutov-Cassegrain and Maksutov-Newtonian telescopes from ASTELE are excellent examples of catadioptric instruments. Schmidt-Cassegrain telescopes (SCTs) are easier to manufacture than Maksutov instruments, but are subject to more optical errors than Maks.

CCD Charge-coupled device -- the silicon wafer that captures the image in a video camera. It is analogous to film in a 35mm camera.

celestial axis The imaginary north-south line joining the celestial poles around which the celestial sphere rotates. (See celestial sphere).    

celestial coordinate systems One reference system which helps astronomers map the heavens. (See celestial sphere).

celestial equator An imaginary circle projected outward from Earths equator onto the celestial sphere.

celestial latitude The symbol b (lower case Greek beta) indicates celestial latitude in the ecliptic coordinates system and denotes in degrees the angle above or below the ecliptic the imaginary line that shows the Suns apparent orbit through the sky. Because we know the Earth orbits around the Sun, the line actually traces the plane of Earths orbit. Also known as ecliptic latitude.

celestial longitude The symbol l (lower case Greek lambda) indicates celestial longitude and is measured eastward from the vernal (spring) equinox, when the amount of day and night are approximately equal. Also known as ecliptic longitude. This occurs about March 21.

celestial meridian The large circle on the celestial sphere that joins the north and south points on the observers horizon, circling the zenith, north and south celestial poles. When a celestial object crosses the observers meridian, it is in transit.

celestial poles The north and south points connected by the celestial axis around which the celestial sphere appears to rotate daily.

celestial sphere The imaginary rotating sphere that seems to surround the planet earth and to which celestial bodies appear to be affixed. The canopy of heaven. The celestial sphere is a holdover from the geocentric days when astronomers believed the universe rotates around our planet. It remains a very convenient system for mapping locations of celestial objects, forecasting their positions, the seasons and for navigation. Except for zenith, nadir, and ecliptic, practically all the reference points and lines on the celestial sphere are outward projections of similar coordinates on the earth.

chromatic aberration Color errors in an optical system such as fringing and false color are caused by the refracting (bending) characteristics of different wave lengths of light. Each wavelength focuses at different points after passing through different materials. Red light bends less than blue when it passes through a lens, hence it focuses sharply at a point beyond the focus point of blue light. The two colored images cause fringing. Long focal length lenses show less chromatic error than short lenses, but true correction with refracting telescopes normally requires use of at least two different types of glass in combinations of lenses to cancel the error. Achromats are lenses corrected for red and blue light. Apochromats are corrected for at least red, blue and green. Reflecting telescopes are free from this aberration.

circle of confusion The cross-section of the smallest circle to which a lens can focus a point. Telescope aperture, quality of optical correction, accurate collimation, precise alignment and fineness of the focus mechanism all contribute to making that circle as small as the theoretical design limit will permit.

c-mount  A television industry-standard thread for mounting lenses and adapters on commercial video cameras. C-mount cameras easily fit microscopes and telescopes that accept c-mount adapters.

clock drive (drive correctors) An electric motor, or motors, set to keep a properly aligned telescope slowly turning to match Earths rotation (or celestial sphere rotation). The corrector drive makes objects appear stationary in the telescope as the instrument meets the right ascension. Declination drives slowly drive the declination axis, which is at right angles to the polar axis. An accurate drive is essential for very long exposures in astrophotography, but is not a requirement for simple observation. With manual slow motion controls, a reflex viewer and good reticle in the finder scope, excellent long exposures can be made by the attentive astrophotographer. Basic clock drives rotate to counteract sidereal motion, but other drive rates may also be available.

coaxial  Concentric shafts sharing a common axis so two types of motion can be accessed without a need for the operator to reach for another control. For example, microscopes frequently have coaxial fine and coarse focus, and coaxial control of bi-directional mechanical stages.

collimation The process of truing an optical system so all components convey parallel beams of light. In an equatorial mount, when the optical axis and declination are in precise perpendicular alignment for accurate star tracking, the mount is said to be in collimation. 

coma (optical) One of the six types of optical aberrations or errors. If a telescope exhibits coma, star images appear as tiny fans spreading radially away from the optical axis.

concave lens  A lens with an inward curve.

constellation The 88 relatively stationary star patterns which represent boundaries of sky areas designated by the International Astronomers Union. Constellations are excellent references to locate celestial bodies in, or near, their sector. To the ancient Greeks, there were 48, mostly representing figures and scenes from mythology, but every culture has connected the dots to make pictures out of the stars in the heavens. Many of the figures, such as The Great Bear (Ursa Major), hav virtually the same name in completely isolated cultures. Three-quarters of the constellations are visible from latitude 40 north. 

condenser A lens which collects light and limits the diameter of the cone of light. 1. The substage collector lens that receives the light rays from the illuminator and relays them to the specimen (see substage condenser). Located between the light source and the specimen, the condenser may be fixed in place, or moveable to enable precise control of the angle of light passing through the specimen. It may include an adjustable iris or disk diaphragm, a filter holder, different phase rings (annuli) for special contrast effects. It may have specific optical qualities such as very flat field (aplanat) to provide a quality of light superior for photography. 2. The light concentrating lens in an illuminator.

convex lens  A lens with an outward curve.

convex-concave  A lens with one side curving outward and the other side curving inward.

Copernicus, Nicolaus (1473-1543) Polish astronomer whose realization that the Earth orbited the Sun, and not the other way around, caused him great difficulty as the Church regarded such thought as heresy. His Heliocentric Theory (Earth orbits the Sun) replaced the Geocentric Theory (Sun orbits Earth) after Kepler and Galileo confirmed it 200 years later.

Coverglass, or coverslip The extremely thin, flat, glass or plastic cover for a specimen on a glass slide. These glasses, available in different thicknesses, may particularly affect optical performance of objectives of 40x and higher magnification. Certain objectives optimally perform with coverglasses of specific thickness, or with no coverglass at all. A number engraved on the objective, such as 0.17(mm), indicates the objective has a design criterion for a No. 1 coverglass (0.16 0.18mm). Some objectives have an adjustment ring with a numeric scale which will accommodate a range of coverglasses. As they are are sold by the ounce, there are more No. 1 coverglasses of 0.13 - 0.15mm thickness in the box than there would be in a box of No. 2 coverglasses of 0.18 0.20mm thickness. When there is no such number engraved on the objective, opt for a No. 1.

dark adaptation Heightened sensitivity to light when the eye is subjected to darkness for about 20 minutes. The chemical changes which occur in the retina during this period actually continue for up to two hours, greatly improving the observers ability to see faint objects, but they can be cancelled quickly by a sudden exposure to light. That is why astronomers carry red-filtered flashlights into the field to enable use of star charts, setting circles and telescope controls. Hint: The red plastic screw-top from a plastic Coca-Cola bottle makes a perfect filter for the pocket Mini-Maglite.

darkfieldIn Microscopy,an illumination technique which shuts out background light and allows only scattered light to reach the specimen in order to heighten textural detail. In simple darkfield, a filter with a center spot (darkfield stop), eliminates the intense direct light from the illuminator. A dedicated darkfield condenser produces advanced darkfield illumination for maximum effect. This achieves an even higher level of detail with a condenser lens formula that adds a more discrete, sharper, more intense, scattered light to the specimen.

Dawes, Will Rutter (1799-1868) English physician/churchman/amateur astronomer devoted to studying double stars. His application of splitting double stars to measure a telescopes resolving power became widely accepted as a method to demonstrate telescope resolving power. The table he published in 1867 showed how large a telescope aperture would be required to resolve specific double stars. His standard, called the dawes limit, required only that the observer must be able to discern that there two stars, but did not require a gap between them. See resolution

declination (dec., symbol δ) The celestial spheres equivalent of Earth latitude is measured in degrees north or south of the celestial equator.

declination axis The pivot point on an equatorial mounting that permits the telescope to move in declination (up and down) at right angles to the polar axis.

declination circle The setting circle on an equatorial mount that is divided into degrees of declination to indicate the angular declination of an object to be observed from 0-90 north(positive) to 0-90 south (negative).

deep sky object Any object outside of the solar system. The term is usually used to describe celestial bodies of in the Messier catalog and the New General Catalog (NGC).

degree of arc 1/360 of a circle.

depth of field  The farthest and nearest points in the field of view which are in simultaneous sharp focus. In microscopy, this is an inverse function of the magnifying power of any objective. Stronger objectives have less depth of field than low-magnification objectives.

depth of focus   The farthest and nearest points in the film plane (photomicrography) or CCD plane (videomicrography) which are simultaneously in focus. Paradoxically, higher magnification objectives have greater depth of focus than low magnification objectives.

dew cap Similar to a protective lens shade on a camera, but usually much longer ideally 2-3x longer than the telescope aperture. A 70mm scope should have a 140-210mm dew cap. The extra length is necessary to shield the objective from cooling night air currents that cause moisture to condense on the lens. Excessively long dew caps may generate convection currents which distort views.

diagonal The secondary mirror on a Newtonian reflector, or a prism which reflects a light beam at right angle. See also star diagonal and sun diagonal.

differential interference contrast  A microscope illumination technique which improves visibility of low-contrast specimens by revealing their gradients. Also known as Nomarski, DIC clearly displays very slight differences in thickness and texture using a three-stage system, i.e. 1) splitting a beam of polarized light with a slight shear, and 2) the differential displacement of the slightly sheared wave fronts by a compensator, with 3) a second polarizer to perform analysis with a heightened effect. The result is an almost three-dimensional modeling of the specimen surface and structure. DIC can accomplish much more by differentially changing the colors and intensities of the sheared wave fronts.

diffraction The tendency of light to bend around an edge, or any obstacle.

diffraction-limited optics  There are a variety of definitions, but the term indicates high quality optics which minimize diffraction. When an optical system meets the Rayleigh criterion, (which see under resolution) it is said to be diffraction-limited. To meet this standard of optical quality, there must be no more than wavelength of light between wavefronts at the focal point. Modern optical systems can exceed this theoretical design limit, but it remains a standard. No lens truly brings light to focus at a true point there is always an airy disk. If the optics can keep the focus point to the theoretical minimum of the airy disk, they are diffraction limited.  

diffraction rings   The concentric rings that surround an airy disk (which see).

DIN  Deutche Industrie Normen (German Industrial Standards). In microscopy this refers to the 45mm parfocal distance and RMS thread for standard objectives. This measurement extends from the mounting hole in the nosepiece to the point of focus on the specimen. Precision microscope objectives conforming to this specification should remain in approximate focus as the nosepiece rotates to change magnifications.

dispersion  The splitting of light into different colors. Associated with diffraction, refraction and scattering.

distortion  An optical aberration in which there is a variation in magnification across the visual field resulting in a pincushion or barrel shape compared to what should be perpendicular lines. If the magnification becomes greater towards the edges, the pincushion shape results, if less, the barrel shape.

Dobsonian  A Newtonian reflector with a low, altazimuth mounting, typically using Teflon pads as bearings on a flat board, in which the telescope is supported by v-shaped cuts in the side of a wooden box. Named for John Lowry Dobson, who advocated simplicity in equipment, the Dobsonian has been enhanced with clock drives, computers and many other niceties.

Dobson, John Lowry  The father of sidewalk astronomy, (1915 -), who devised the Dobsoniandesign so amateur astronomers could easily create their own large-aperture reflecting telescope mounts in home workshops. Dobson is credited with popularizing astronomy and inspiring thousands of observers to consider philosophical implications of observational cosmology, i.e. We are occupants of a small world in a giant universe where so many worlds may abound.

double star  Two stars close together which may appear as one until observed with magnification.

drawtube  The tube in a telescope or microscope that holds the eyepiece.

drive  The motor, shaft and gear system which causes the telescope to move opposite to the movement of celestial objects and the Earths rotation, effectively causing the image to stand still. Drives may rotate only on the polar axis, or also move the declination axis.

eclipse  When one astronomical body, or the shadow of an astronomical body, obscures or passes over another, there is said to be an eclipse.

ecliptic  The path of the Suns annual orbit as viewed from Earth.

effective focal length  In a compound lens system, the apparent focal length which results from the combination of lenses projecting an image.

elongation  Specific to the Solar System, elongation is the angular distance of 0- 180 between any Solar System body and the Sun, measured from the Sun as to the east or west up to 180.

epi illumination In Microscopy, a form of incident illumination in which the light comes from above the specimen, commonly used with flourescence microscopy (antibody detection) and reflective microscopy, such as material surface inspection, metallographic analysis, surface mount printed circuit board inspection.

equator The circle around the circumference of a spherical body which is perpendicular and equidistant to the poles.

equatorial mount(ing) A telescope mount in which one axis may be made parallel to Earths polar axis while the telescope is attached to the declination axis at 90 to it. By rotating the telescope around the polar axis in the opposite direction to Earths rotation, celestial objects appear to stand still. Also called German Equatorial Mount.

equinox  One of the two points on the ecliptic in which the Sun, intersects the celestial equator. The dates on which these events occur.

erecting prism  An optical device inserted into the eyepiece tube of a telescope before the eyepiece is installed in the extended eyepiece tube of the erecting prism. The prism flips the inverted image rightside up for easier viewing, although the image is still reversed from left to right.

Erfle eyepiece  A widefield eyepiece with a field of up to 68. See Eyepieces.

exit pupil  The back focal plane of an eyepiece or any optical system.

extra low dispersion glass, or ED glass  Optical glass formulated to bring red and blue wavelengths in perfect simultaneous focus to eliminate color fringing.

eyepiece (Ocular) The magnifying optical unit which fits into the drawtube of an instrument and through which the observer views the image. Modern telescope systems can use many types, from very simple achromats to complex eyepieces with many optical elements. Microscope eyepieces are available in several designs but typically are variants of Huygenian, Ramsden, Kellner, or Orthoscopic. Standard inside diameters of telescope drawtubes into which oculars fit vary from .96, 1 and 2, although adapters may be purchased to step up or down in size. Most eyepieces are roughly tubular in construction and contain a field lens through which the image first passes, a circular aperture called a stop and the eyelens through which the image is viewed. Usually, the more complex an eyepiece, the more optical corrections, the better eye relief and wider field of view. But adding many glass surfaces may also increase internal reflections, or ghosting.

Eyepiece Characteristics

Huygenian eyepieces give good eye relief, good color rendition, but fall short with excessive spherical aberration and perform acceptably only with long focal length telescopes and f-ratios greater the f10.

Ramsden eyepieces offer an improvement over the Huygenians weak performance insofar as spherical aberration is concerned, but have considerable chromatic aberration and poor eye relief.

Kellner eyepieces are a fairly aberration-free improvement on the Ramsden with the addition of an achromat for the eyelens. Sometimes called an achromatic Ramsden, the Kellner gives a moderately wide field of view up to 50 -- and is an eyepiece of choice for many binocular manufacturers, but requires excellent internal coatings to minimize ghosting..

Orthoscopic eypeieces are more expensive than Kellners, but the long eye relief makes them a solid choice for eyeglass wearers. Orthoscopic eyepieces perform well in all categories but have a limited field of view, from 35-50.

Plossl eyepieces offer 5 wider fields 40 -- than Kellners and are an excellent compromise, combining good color correction, flat field, adequate eye relief and modest cost.

Wide Field Eyepieces

In Astronomy, wide field eyepieces can dazzle with spectacular views, but frequently will not work in certain telescope draw tubes and with certain telescope designs. Telescopes lacking good optical performance around the edge of field, will have their worst traits emphasized through the use of these eyepieces. At high magnifications with some telescopes, some ultra-wide eyepieces are unusable. Before committing to an expensive wide field, or ultra wide field eyepiece, obtain permission for a brief inspection period

Erfle eyepieces are very wide field, between 65 and 70 -- but extreme edges of the field are affected by curvature, astigmatism and chromatic aberration.

Konig eyepieces are short focal length versions of the Erfle, giving high magnification.

Nagler eyepieces are ultra wide-field up to 80 -- and solve aberration and distortion problems while also giving excellent eye relief. Although expensive, they are an excellent choice for observers with very fast focal ratio telescopes (up to f4).

eyepoint The position in which the eye is placed to see the full circumference of the image. See eye relief.

eye relief  The distance between the observers cornea and nearest surface of the eyepiece.

field curvature  A form of optical distortion in which the focus changes from the center to the edge of the field of view.

field diaphragm In microscopy, an iris diaphragm placed in the illuminator to control ray bundles for the light quality required to set up Koehler Illumination.

field lens  In an eyepiece, the lens which receives the image from the objective and relays it on the eyelens.

field number  In microscope eyepieces, the diameter of the aperture in a baffle between the field lens and the eyelens which limits the field of view to the useful coverage of the eyepiece and removes from view the inferior edge of the image.

field of view   The viewable span visible through an optical system at a specific working distance, or the angular diameter viewable through that system (also called angle of view).

figuring  The grinding and polishing process by which a lens is shaped.

filar micrometer  A measurement device and positioning device built into an eyepiece. It consists of one or more scaled reticles and separately moveable crosshairs capable of precise movement to accurately measure, or describe the position of, an object.

filter  In Optics, a dyed, or colored medium (glass, gelatin, plastic, or liquid) which permits transmission and/or rejection of particular frequencies of the ultraviolet spectrum. Valuable to improve detail and contrast, reduce light intensity, remove glare and to delineate detail that would be less visible without the filter.

finder  A wide field, low-magnification telescope, gun-sight/aperture or laser spot device mounted on a telescope to help locate celestial bodies.

flat field  A lens or optical system that corrects spherical aberration and produces uniform focus from the center to the edge of the field. In Microscopy, objectives may be achromats with 50-70% flat field, semi-plan and part-plan with 75% flat field, or plan, with 90-100% flat field. A caveat is that astigmatism may increase as field curvature decreases. This requires more correction to minimize astigmatism with plan lenses. A simple achromat or semi-plan lens can be sharper in the center than a poorly corrected plan lens.

fluorescence  In Microscopy, a phenomenon in which specific wavelengths of light are directed at the specimen which, if excitable by this wavelength, will emit energy detectable as visible light. The emission may denote a specific identification for certain antibodies, disease conditions and materials, some of which may be previously conditioned by their affinity for substances which will fluoresce as they are bombarded with certain wavelengths.

f number (f stop) Also called focal ratio, it is the ratio between the focal length of a lens and its diameter.

focal length  The distance between the optical center of a lens system and the focal point, or focal plane.

focal plane  The flat plane at right angles to the optical axis on which a lens will focus the image.

focal ratio  see f-number

focal reducer  An auxiliary lens used to shorten a telescopes focal length and widen the field of view.

focus, optical  The convergence point, or sharpest point of focus, for an image.

fork mount  An equatorial mount frequently combined with large aperture catadioptric reflectors.

fringing  A chromatic aberration caused by the failure of the lens system to focus all colors at the same point, so a red and violet image, for example are slightly offset from one another.

Galaxy, galaxies  Huge star systems, bound together by gravity, which comprise most of the matter in the Universe. The Galaxy, with a capital G, always denotes our spiral Galaxy, also called The Milky Way. The bulging center of The Galaxy lies about 30 million light years from the Solar System and is one of the largest in the Local Group -- 31 known galaxies which are associated in a cluster.

Galilean Telescope  The type of telescope developed by Galileo Galilei in 1609. It consisted of a long focal length, double-convex objective lens, a long tube and a double-concave eyepiece, mounted in a long tube. Capable of 30x magnification, it enabled Galileo to make discoveries, confirm Copernicuss Heliocentric Theory, and propel observational astronomy into the forefront of scientific and religious controversy. Galileos pioneering telescope could not approach the performance of the telescopes on this Web site!

graticule  see reticle

guide telescope  A reticle-equipped telescope piggy-backed on the same mount as a camera or other instrument and aimed at a guide star to help the observer maintain position.

incident light  Light falling on a subject.

infinity corrected optics  In microscopy, an optical design in which the rays passing through the objective do not converge to form an image in the eyepiece without the assistance of additional optics in the drawtube. As the rays remain parallel, they are said to continue to infinity.

inner planets  Also known as the terrestrial planets, they are the four planets which lie between the Asteroid Belt and the Sun Mercury, Venus, Earth and Mars.

interpupillary distance (IPD)  The distance, measured in millimeters between the microscopists pupils. The IPD is noted on a scale between adjustable binocular eyepieces to enable speedy setting for comfort when different persons are using the same microscope.

Koehler illumination A microscopic illumination technique to optimize light quality and sharpness by aligning and adjusting each component of the optical system, starting with a focusing illuminator equipped with a field diaphragm, an iris which valves the ray bundles to precisely cover the substage diaphragm which in turn reduces the aperture to optimum levelsas the substage condenser is focused directly on the specimen.

lens  Transparent material shaped or ground to bend light.

light  The visible part of the electromagnetic spectrum.

light pollution  Ambient background and direct light, usually caused by human activity, which interferes with dark adaptation and which reduce the contrast of the celestial bodies with the sky and limits our ability to see fainter bodies. A natural cause of light pollution would be a bright Moon.

light year  The distance electromagnetic radiation can travel in one year through space.

limiting magnitude  The faintest detectable magnitude that can be observed or detected by an instrument or telescope. Larger numbers indicate dimmer objects, and larger apertures are required to view extremely dim objects. A combination of ideal conditions must exist to view the dimmest celestial object listed for a particular telescope.

luminosity In Astronomy, the total amount of electromagnetic radiation emitted by a celestial body per second, expressed logarithmically as compared to the Sun.

magnification  The enlargement of the image of an object. In Microscopy, for the sake of standardization, we say the human eye has a near focus point of about 10, or 254 millimeters. Each time we halve that distance, we double magnification. Microscope optical systems can effectively halve that distance many times. Working distance of a typical 100x oil immersion objective is 0.18mm. This is similar to halving the distance 10 times to obtain about 1,024x magnification with the 10x ocular. Although not a perfect explanation of how a microscope achieves magnification, the concept describes the microscope as a tool to overcome the inability of the eye to focus very close to the object. If resolution fails to show sharp detail, we call the magnification empty magnification. (See resolution.)

In Astronomy, to calculate magnification of a telescope, divide the focal length of the objective or optical system by the focal length of the eyepiece.  The quotient is magnification.

magnitude  A brightness rating system for celestial objects invented by the ancient Greeks in which the most brilliant objects have the lowest number, as the first visible celestial objects seen before sundown had to be the brightest. This scale originally extended from Magnitude 1 to Magnitude 6, but the scale has since extended to include millions of celestial bodies now visible through large aperture telescopes. Modern light measuring instruments added considerable accuracy to this system. Now, there are two principle measurements of brightness apparent magnitude -- as viewed from Earth -- and absolute magnitude, or intrinsic brightness of the heavenly body itself, without consideration of the effects of distance and Earths atmosphere.

major planets  All nine planets in the Solar System, i.e. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, as distinguished from minor planets that are large asteroids.

Maksutov telescope  A catadioptric telescope (an instrument combining mirrors and lenses) comprised of a short focal length, fast spheroid mirror with a spheroid miniscus corrector plate and a silvered-on-miniscus spheroid secondary mirror. Named after the Russian optical designer D. D. Maksutov, who published his 1941 design in 1944, the Mak design, when well made, outperforms Schmidt-Cassegrain telescopes (SCTs) of equal quality. But the heavy miniscus corrector plate is difficult to manufacture in large apertures, and until recently, Maks over 5 had been fairly unusual in the amateur market. Much larger Maks, up to 8, or even 10, should soon arrive for the amateur market. Maks are the among the best reflecting telescopes for planetary viewing. They compare very well to expensive, high-end, apochromatic refractors. The triple-folded optical path makes a very compact telescope of long focal length.

mechanical stage  A precision mechanical device permitting discrete movement of a microscope slide in x and y axes (forward and back, and side-to-side) under the objective. Controls may be separate for each axis, or the two controls may be mounted on concentric shafts called a coaxial control.

meridian, celestial  The imaginary great circle on the celestial sphere which passes through the north and south celestial poles and the observers zenith while intersecting the north and south points on the observers horizon.

Messier Catalog  A list of 110 celestial objects, the first 103 of which were catalogued by 18th century French astronomer Charles Messier. Messier numbered the objects in the sequence he identified them. On celestial maps and charts, Messier objects include the letter M before each number. Messier was a comet hunter who was trying to identify objects that may be confused for comets. Now, observing and noting the Messier list is a passion among many amateur astronomers.

mounting  The stable structure which holds the telescope and permits it to be pointed at any part of the sky. The two primary types are equatorial and altazimuth. The base consists usually of a tripod, pillar, pier or platform, and the top of the mount is usually referred top as the head. Electric drives may adapt to many mounts allowing them to rotate on the polar axis to oppose Earths rotation. Mounts should be capable of holding at least twice the weight of the telescope assembly, including counterweights.

Nagler eyepiece  see eyepiece.

NA  Numerical aperture (Which see).

nebula  A luminous -- or dark, obscuring cloud -- of interstellar gas and dust made visible by its contrast, whether dark or light. Many nebulae are invisible without enhancement due to the lack of emissions in the visible spectrum and a non-contrasting background.

negative phase contrast  A form of phase contrast microscope design in which the diffracted light from the specimen is advanced by half a wave to produce a bright specimen against a dark background.

Newtonian telescope The first, and still the most popular, reflecting telescope design, was created in 1670 by Isaac Newton as a solution to the chromatic aberration shortcomings of refracting telescopes. He mounted a paraboloid primary mirror in the bottom of the telescope to reflect the image to a flat diagonal mirror that in turn would reflect the image to a side-mounted eyepiece near the top of the telescope. The first Newtonian primary mirror was made of polished metal and was only one inch in diameter with a magnification of about 30x. Newtonian telescopes require an open tube which is susceptible to invasion by dust and moisture. They frequently may require cleaning and re-calibration. Properly maintained, and with occasional mirror re-silvering, Newtonians make excellent observation instruments.

NGC, or New General Catalog of Nebulae and Clusters of Stars, originally published in Ireland in 1888 under the authorship of Danish astronomer, J. L. E. Dreyer. He added to data gathered by the F. W. and J. F. W. Herschel to document some 7,840 celestial objects. The much larger current volume includes the entire sky and has more than 13,100 objects cataloged with Right Ascension, position and description.

Nomarski  See differential interference contrast

North Star, see Polaris

nosepieceThe rotating turret of a microscope which holds objectives of different magnifications.

numerical aperture Also shown as NA on a microscope objective, it is the measure of the resolving capability of an optical component. The larger the number, the greater the resolving capability and the higher magnification it can achieve with the appropriate objective and eyepiece.  To obtain the NA, multiply the sine of half the collecting angle times the refractive index of the specified immersion liquid. See resolution

nutation  A gravitationally imposed irregularity in the precessional circle traced out by the celestial poles introduced by the varying distances of the Sun and Moon from Earth which alters their gravitational attraction.

objective lens In Microscopy, the magnifying lens closest to the specimen. The shortest focal length objectives give the highest magnification.

In astronomical refracting telescopes, the lens or lens system aimed at the object -- the large diameter glass lens at the end of the sealed optical tube. Long focal length astronomical objectives give highest magnifications.

  • Telescope objectives May be achromats or apochromats. See definitions.
  • Microscope objectives As above, but great emphasis is given to curvature of field due to the short focal length, nearness of the subject, shallow depth of field. Plan objectives should have an edge-to-edge flat field, semi-plans about 85% and achromats 60-75%
  • Microscope objectives may vary in power from 1x to 160x, but the common objectives are seen from 4x to 100x. A quadruple nosepiece microscope will usually have 4x (scanning), 10x (low dry), 40x (high dry) and 100x (oil immersion).
  • Hematology laboratories prefer to add a 50x oil immersion lens for analyzing morphology: (shape) of white cells.

occultation The obscuring of one astronomical object by another, such as an eclipse

ocular See eyepiece.

oil immersionA microscopy technique to close the air gap between the tip of an objective and the top of the specimen with a drop of oil compounded with a refractive index to match that of the glass in the objective. Once the objective contacts the oil droplet of matching refractivity, that portion of the optical system becomes a unit. Immersion oil is the only suitable oil for that purpose in order to achieve very high magnifications, typically from 500x to 1,500x, and avoid damage to the objective. Because very high magnifications require a very oblique angle of entry for ray bundles, they will tend to reflect, refract, scatter and disperse, without forming a sharp image, unless a unifying medium is used. Immersion oils are available in indices of refraction to match optics.

opposition  When the celestial longitude of an object in the Solar System is 180 from that of the Sun, or opposite in the sky from the Sun, it is said to be in opposition.

optical axis  The imaginary line passing perpendicularly through the center of an optical system, from front to back.

orbit  The path of a body in space, usually around another body.

outer planets  The five planets beyond the Asteroid Belt from the Sun Jupiter, Saturn, Neptune, Uranus and Pluto.

paraboloid  An ellipsoid curve with sides that become more nearly parallel as they extend toward infinity. A parabolic lens can be made by deepening the curve on a spherical lens.

parallax  The difference in position of an object viewed from two separate points.

parfocal distance  The distance from the nosepiece objective mounting hole to the specimen when in focus. If they meet this standard, objectives of different magnifications will be nearly in focus when the nosepiece rotates. The DIN standard parfocal distance is 45mm. (See DIN). Some manufacturers have abandoned the 45mm DIN standard and the older RMS standard of 33mm, but LOMO objectives conform to both of these two standards. LOMO Multiscope objectives use the RMS standard of 33mm. Laboroscope objectives meet the 45mm criterion. Laboroscope microscope design also includes a patented adjustment permitting usage of either system. Microscopists who own classically fine objectives such as Fluor-apochromats requiring a 33mm system may use them on the Laboroscope. Such objectives could cost thousands of dollars to replace with newer DIN objectives.

phase contrast A microscopy illumination method to enhance contrast of certain phase objects such as a cell, and delineate the texture and structure of membranes which would be lost in brightfield or very difficult to see.

A phase annulus (ring) mounted in the condensers front focal plane partially modulates the ray bundles that pass through and around the specimen, where they are slowed wave, then are retarded another wave when they pass through the phase plate in the rear focal plane of the objective. The system also diminishes background light by about 85%, providing a darkened background to contrast with the illuminated structure of the phase object.

While the effect diminishes the resolution of the image, it makes detail visible that one could not see without it. The phase plate annulus must precisely superpose the condenser phase annulus or the system will suffer diminished phase effect. Compared to darkfield, phase contrast has the advantage of showing interior structure as well as surface texture of a specimen.

phase telescope (focusing telescope) In Microscopy, a special eyepiece with adjustable focus and long focal length use to focus on and align phase plates and phase annuli for Zernike phase and other components in various types of phase microscopy.

plan  The description of a microscope objective which has been well corrected for field curvature so it yields a flat field requiring minimal focus adjustment from the center to the edge of an image.

plan achromats  Microscope objectives well correct for flatness of field and the primary colors of red and green.

plan apochromats  Microscope objectives with extreme corrections for all colors and flatness of field, usually for photomicrography. They are very expensive

planet  A body orbiting a sun that does not emit light, although it can reflect light that falls on it from another source. In the Solar System, it is one of the nine bodies too large to be asteroids. If a body that has similar non-emitting characteristic to planet is 10 times the size of Jupiter, it is classified as a brown dwarf.

planisphere  A circular map of the heavens with a time-marked calendar on a rotary overlay to approximate the seasonal and hourly changes in heavenly views.

Plossl eyepiece  See eyepiece.

Pointers,or Pointer Stars  Dubhe and Merak (Alphae and Beta Ursae Majoris), the two stars in the Big Dipper that point the way to Polaris, the North Star.

polar axis  The axis on an equatorial mount that is parallel to Earths axis. Earths axis extends from pole to pole through the center of the planet.

Polaris  The North Star, or star which is closest to the north celestial pole, so it appears fixed in place. Due to the slow wobble in the Earths rotation (Precession), the Pole Star has changed among Vega, Thuban and Polaris over thousands of years.

polarizer  A filter or material that transmits light in only one plane of vibration. This material can have various degrees of polarization.

positive phase contrast  In Microscopy, an illumination technique in which the phase plate in the objective retards light from the subject by one-half a wave, producing a light background and dark subject.

precession  A term applied to the slow wobble of a spinning bodys rotation which causes the poles to circle a conical shape rather than a point. This phenomenon leads to a change in pole stars over thousands of years.

primary mirror  The curved, figured, mirror in a reflecting telescope which collects the light reflected from a heavenly body and reflects it to a secondary mirror and thence to an eyepiece.

prime focus  The point or plane on which an optical mirror or lens forms an image without intervening optics.

prime lens  A lens designed with a single focal length (non-zoom).

prime meridian  The imaginary and arbitrary circle designated as zero degrees longitude on a

celestial body. On Earth, we call the prime meridian, the Greenwich Meridian.

Ramsden disk  The small disk of light visible in the back focal plane of an eyepiece.

Ramsden eyepiece  See eyepiece.

Rayleigh criterion  See resolution.

reflecting telescope  A telescope which uses a front-surface mirror to collect light rays and bring them to focus at a single point. Because light bends as it passes through a transparent medium such as glass, and different colors of light rays bend at different angles, chromatic aberrations result from using glass lenses. Reflecting telescopes eliminate these aberrations. They also fold the light path as they reflect the image from the primary mirror to a secondary mirror, and therefore shorten the tube length of the telescope.

reflection  A change of direction in light caused by inability to pass through a surface. On a polished surface, the angle of reflection equals the angle of incidence. Incident light is the light that falls on the subject.

refracting telescope A telescope which uses a lens to collect and bend light to magnify the image. Most are achromatic refractors corrected for two colors. Light bends, or refracts, as it passes from one transparent medium to another, such as from air to glass, or from crown glass to flint glass. As different colors of light refract at different angles, they focus at different points unless the optical designer corrects this tendency with the optical design and choice of materials. Chromatic aberrations such as fringing and false color degrade the sharpness and accuracy of the image. An apochromatic refracting telescope avoids these errors. All primary colors focus on the same point.

refraction The bending of light as it passes through materials of different thickness or refractive index.

refractive index (RI)  The ratio of the speed of light in a vacuum versus the speed of light through another medium. It is always greater than one, unless in a perfect vacuum, as the presence of matter retards the speed of light. When passing through a medium, long wave lengths refract less than short wave lengths, so the refractive index of a medium depends on the wavelength of light passed through it.

resolution  The ability to detect fine detail. The ability to discriminate between two closely set points. The closer the points that can be discerned, the greater the resolution. Resolving capability is not only a factor of precision design and workmanship, but depends strongly on aperture the sheer size of an optic!

Two standards for testing resolution are the Rayleigh criterion and the Dawes Limit. Both of these criteria were developed before modern advances in lens coating, glass formulation, and improved optical precision figuring and design. They state resolution in terms of degrees of arc. Todays skilled manufacturers regularly exceed these limits.

Rayleigh studied diffraction patterns of closely spaced point sources. He theorized that if the central maximum of the one pattern coincided with the central minimum of the other, the point sources could barely be identified as two separate sources -- any closer, and they would appear as one. He measured this distance and found that the distance between the centers would equal the radius of the central disk. This theoretical limit is the Rayleigh Limit of Resolution. Rayleighs criterion stated that the limit was reached when the center of the Airy disk of one point fell on the first dark diffraction ring of the other.

The Dawes Limit also compares two closely spaced bright objects, but Dawes recognized that even point-source lights have larger centers called Airy Disks. When the two points are so close together that if they move any closer, the observer cannot discern they are two separate entities, then they have reached The Dawes Limit. The Dawes Limit claims about 12% more resolution than the Rayleigh Limit.

Formulas for these limits appear in many astronomy and optics texts.

reticle  Any measuring, pointing, angular, or grid line(s) in the focal plane of an eyepiece, same as graticule. (From the Latin for network or mesh.)

rich field telescope   (RFT)  A telescope with a relatively short focal length relative to the fairly large aperture with an optical system designed to take in the largest possible area of the sky. RFTs are especially useful when searching for unusual events such as comets, novae and meteor showers.

right ascension  (RA) Right ascension alludes to the apparent rising of celestial bodies in the East due to Earths rotation. Right Ascension is an equatorial coordinate on the celestial sphere measured clockwise around the equator. The increments of 24 hours are divided into 60 minutes and each minute consists of 60 seconds. In sidereal time, 0 hours of right ascension equates to Earths Greenwich Meridian. The RA of any celestial body indicates when it will first appear in the heavens. With the knowledge that each hour of RA equals 15 degrees of arc, it is easy to calculate the best times for observation.

right ascension circle   The setting circle on the polar axis of an equatorial mount which is divided into 24 segments indicating the hours of Right Ascension.

ring, planetary Matter orbiting a planet which reflects light to gives the appearance of a disk-like ring.  Saturns rings are the only planetary rings visible from an Earth telescope, but astronomers detected rings around Neptune and Uranus when they occulted stars that should have been visible. Space probes verified the presence of these rings.

rising  The instant a heavenly body becomes visible above the eastern horizon.

RMS  Royal Microscopical Society. The 33mm parfocal distance for objectives.

RMS Screw Thread The standard microscope objective thread set by the Royal Microscopical Society (RMS) for universal compatibility. Threads that are 20.32 mm in diameter with a pitch of 0.706, conform to the RMS standard also used on DIN objectives. LOMO objectives conform to this thread specification.

scattering  The random deflection of photons when light passes through an irregular medium, or a medium with obstructions.

Schmidt-Cassegrain Telescope (SCT) A catadioptric (uses both lenses and mirrors) telescope with a triple-folded light path that has become very popular among amateur astronomers. While SCTs are compact and deliver excellent images when not pushed near their design limitations, they have the drawback of a relatively large secondary mirror obstruction. This reduces image contrast. Combined with other necessary design compromises, it cannot match the performance of Maksutov telescopes (Maks) and Newtonian reflectors of equal manufacturing quality. The relatively thin, light weight corrector plate makes SCTs easier to manufacture in large apertures than Maks. Maks have thick, spherical miniscus lens corrector plates and superior sharpness and contrast.

secondary mirror  The mirror in a reflecting optical system which receives the image from the primary mirror.

seeing  The rating of observational conditions depending on light pollution, air turbulence, temperature changes, humidity. The five-point Anontiadi Scale rates seeing as I. Perfect, II. Good, III. Moderate, IV. Poor, V. Appalling.

setting  The moment a heavenly body disappears below the western horizon

setting circles  The graduated scales on the polar and declination axes of a telescope mount.  The declination axis setting circle is usually set and locked in place while the polar axis may be moved to match the RA of a heavenly body.

sidereal time  Twenty-four hour time measured with reference to Earths rotation relating to the stars. A sidereal day has 24 hours, designated from 0 to 24, but is infinitesimally shorter than a true Earth day. Telescope drives are set to match the sidereal rate of Earths rotation.

Standard Time The time in any of the 24 time zones agreed upon by international convention.

star  A luminous celestial body composed primarily of hydrogen and helium gas that emits energy generated by nuclear fusion in its core, or has at one stage in its existence maintained a fusion reaction, or will eventually maintain a fusion reaction. Although stars are massive, they are limited to about 120 Solar masses. Beyond that enormity, the star would self destruct with enormous explosions. Stars which become smaller than 1/12th the size of our Sun cool into brown dwarfs.

stereo microscopy  Three-dimensional microscopy made possible by the use of matched dual optical systems, one for each eye, which gives separate views of an object through the two eyepieces. It differs from a binocular compound microscope in that stereo microscopes have matched pairs of objectives, while binocular compound microscopes split the image from a single objective between two eyepieces.

substage condenser  The vertically movable, or fixed, lens beneath the stage that focuses the cone of light striking the specimen. The four most common types are abbe, aplanatic, achromatic and aplanatic achromatic.

substage diaphragm  Also known as aperture diaphragm, it is the adjustable opening beneath the substage condenser of a microscope. Its proper function is to limit the size of opening through which light passes from the illuminator to optimize resolution and contrast for the numerical aperture of the objective not to adjust light intensity.

telescope  An optical instrument to collect light from distant objects, magnify the image and enable viewing of the object.

tube length (fixed)  The design criterion for a microscope optical system describing in millimeters the distance from the objective shoulder to the seat of the eyepiece (usually set at 160 or 170mm) where the focused image forms. Microscope objectives engraved with 170mm, for example, perform poorly on optical systems designed for 160mm tube length.

tubelength, infinity corrected-- see infinity corrected optics.

visual magnitude  The apparent magnitude of a star estimated by eye.

wave front  A descriptive term for the movement of a light wave from the light source, through the optical system and specimen to the eye of the observer.

wave plate  A transparent optical device which retards the phase of one plane of lights vibration to emphasize contrast.

x-axisThe forward and back direction on a microscope mechanical stage.

y-axis The left-right directions on a mechanical microscope stage.

zenith  The point on the celestial sphere directly overhead from an observer from which, if a line were drawn through the observer and the center of the Earth, would meet Nadir the point directly below the observer. The zenith is 90 from all points on the observers horizon.

Zernike Phase contrast system and technique named after Dutch microscopist and optical designer Frits Zernike, who first developed phase contrast technique. Phase contrast excels for examining low-contrast specimens, usually in liquid substrates with a similar index of refraction to the phase objects being sought. See Phase Contrast.

Zodiac   An 18-wide band overlaying the ecliptic through which the Sun, the Planets (excepting Pluto) and the Moon orbit. Ancient Greeks divided the band into 12 constellations, each covering 30 and each identified by one of the signs of the Zodiac.