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DEFINITION OF TERMS (GLOSSARY): A

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PART X. DEFINITION OF TERMS (GLOSSARY)

1/4 Wave: a meaningless term unless the form of the measurement, and the wavelength of the light used for the units of measurement, are specified. In general, 1/4 wave refers to 1/4 wave deviation in the highest and lowest points of a given wavefront in the d-line of the spectrum, and is referred to as the "peak-to-valley" wavefront error. But this measure is not useful, because it does not indicate whether this deviation is across a large area of the wavefront or a small area. For example, if half of the wavefront is 1/4 wave advanced that of the other half, the images provided are likely to be quite horrible. However if only a percent or two of the wavefront is advanced by 1/4 wave, and the rest of the wavefront is perfect, the image will be nearly indistinguishable from perfect. Because wavefront errors like this are not weighted as to their severity, the usefulness of the measurement is almost zero. This applies to 1/8 wave, 1/10 wave and other similar claims.

A

AAVSO--The American Association of Variable Star Observers (AAVSO) is a non-profit worldwide scientific and educational organization of amateur and professional astronomers who are interested in stars that change in brightness - variable stars. The AAVSO was founded in 1911 at Harvard College Observatory to coordinate variable star observations made largely by amateur astronomers. In 1954, the AAVSO became an independent, private research organization headquartered in Cambridge, Massachusetts. Today, with members in 45 countries and over 12.5 million observations, the AAVSO is the world's largest association of variable star observers. Membership in the AAVSO is open to anyone - professionals, amateurs, and educators alike - interested in variable stars and in contributing to the support of valuable research. Professional astronomers have neither the time nor the telescopes needed to gather data on the brightness changes of thousands of variables, and amateurs make a real and useful contribution to science by observing variable stars and submitting their observations to the AAVSO International Database.

Abbe orthoscopic: an eyepiece, in a 3-element field group and one-element eye group configuration, designed by Ernst Abbe (Aka aplanar eyepiece) in the 19th Century. It is orthoscopic in the optical sense (see Orthoscopic, below). In addition, this design is resistant to internal reflections (regardless of coatings) because of the curves on the interior facing elements. The design exhibits almost no astigmatism out to about 40 apparent degrees of field; at any given distance from the center, astigmatism in a Plossl is 1.5x and in a "five-element Pl?ssl" 1.2x (with some variation due to different lens recipes) that of the Abbe orthoscopic. The Abbe orthoscopic has about as much eye relief as focal length (i.e., a 10mm Abbe orthoscopic will have about 10mm of eye relief), or a little more. Originally, Abbe orthoscopics were made with crown and flint elements. In modern times, the recipe has been slightly modified to often include a rare earth element.

Abell Catalog - Either a catalog of faint planetary nebulae or a catalog of galaxy clusters.

Aberration In an optical system, any departure from a spherically converging final wave front.

Absolute Magnitude - the brightness of a celestial object at a set distance. Since this distance is identical for each object, absolute magnitude describes the object¡¯s intrinsic brightness rather than its apparent magnitude. The set distance for a star is different than that for a galaxy, but the principle is the same.

Absorption lines/spectrum - When light passes through intervening materials, like clouds of gas or dust, the light is absorbed by the intervening material. This creates gaps in the light received by the object, and the gaps, or lines, in the spectrum of the object helps us determine what the intervening material is.

Absorption Nebula - a cloud of gas that is emitting light that is caused by the absorption of energy from a nearby star or stars. Most gas in space is Hydrogen, and hydrogen emits light at discrete frequencies. If the nebula is thick, the nebula itself blocks whatever light it emits, and the nebula appears dark, blocking (absorbing) the light of whatever lies beyond it.

Accretion disk - when material falls into a gravitational body, it tends to form a disk of material around the equatorial plane of that object. If that object is of sufficient mass, that disk may form bodies within it (such as planets). If the gravitational body is sufficiently massive to accelerate the infalling particles to high velocities (like a black hole or neutron star), the disk may emit quite a bit of energy as the particles jostle each other at relativistic speeds. These disks we detect by their energy signatures. Ultimately, the material in the disk falls into the object.

Achro - Achromatic Refractor, or Achromat

Achromat: a two-element lens which brings two colors to a common focus. Typically refers to a crown and flint together, but achromats may be made of more exotic glasses. A simple achromat may be made by selecting a positive lens of half the absolute focal length of a negative lens with twice the dispersion of the positive lens, and mounting them very close together. In order to control spherical aberration, the negative lens will usually have a higher refractive index than the positive lens. A classical recipe is a positive crown of index 1.5 and an Abbe number of 60, and a negative flint with an index of about 1.6 and an Abbe number of about 35. Astigmatism and curvature of field cannot be completely corrected in a normal achromat.

Achromatic Lens--A refracting optical system without color correction. literally, "no color." In optical work, refers to a lens system that brings two wavelengths of light to a common focus. A lens with two or more elements designed to produce an image substantially free from false color.

Adaptive optics - When a device is added to the optical chain of a telescope that corrects for turbulence in the atmosphere by rapidly changing the image equal in amplitude but opposite in phase to the incoming signal degradation, the image of the telescope will come closer to the theoretical ideal for resolution of the instrument. The correctional device is referred to as an adaptive optics device. This is critical for high-resolution through large observatory telescopes.

Aerosols - Any form of airborne dust, water vapor, or atmospheric pollutants that cause a loss of transparency in the sky.

Afocal: refers to an optical system that does not form an image. A telescope with an eyepiece is afocal, because it does not form an image of its own (the optics of the eye must be used as a "re-imager").

Afocal Photography A type of eyepiece projection photography in which the camera lens is placed very close to a telescope's eyepiece

AFoV - Apparent Field of View. Refers to the visual size of the field seen through an eyepiece, not the actual angular sky coverage. See also TFoV.

AG ¨C Autoguider - A camera connected to a computer that also controls the telescope mount. It will make the minor corrections to prevent the wandering of a star image in a photograph due to inaccuracies in the drive, and it does so automatically. This enables the observer to take long-duration astronomical photographs on an equatorial mount without having to guide the image by looking through an eyepiece.

Airglow - When the atoms in the molecules in our sky absorb energy from the Sun, they re-emit that energy in certain discrete wavelengths, and they do so for periods of time after the Sun has gone down. This produces a glow in the sky that lasts all night. This airglow is one of the reasons the night sky is not black, but more of a silver-gray.

Airy Disk The central, brightest part of a normal Airy pattern. Its linear diameter is equal to 2.44?f/D where ? is the wavelength of light in microns, f is the telescope's focal length and D is the telescope objective's diameter. The Airy disk is the brightest spot formed by a star image as seen through a telescope. It is surrounded by alternating rings of light and dark (these are due to diffraction - any light passing through an aperture is diffracted, and the effect is inversely proportional to the size of the aperture.) An optical system of higher quality increases the relative brightness of the central Airy disk compared to the surrounding diffraction rings.

Airy Pattern The diffraction pattern of a telescope with central Airy Disk and surrounding diffraction rings. An ideal telescope objective produces a diffraction-limited Airy pattern from an infinitely small, luminous object point, with 84% of the energy of the image in the central disc and 16% in the rings.

Albedo The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%. Our Moon is about 4%.

ALPO - The Association of Lunar and Planetary Observers is an organization dedicated to the recording of and sharing of information about the Solar System

Alt - Altitude

Altitude The angular distance of an object above the horizon.

Alt/Az The name given to a telescope mount that is movable in Altitude and Azimuth.

Alt-Azimuth Mount A type of telescope mount designed to move up and down (altitude) and left and right (azimuth).

Aluminizing The process of coating a telescope mirror with a thin layer of vaporized aluminum. The aluminum coating is usually over-coated with a ? wavelength-thick layer of silicon monoxide.

Anastigmatic: a term used to refer to an optical system that exhibits no astigmatism.

Angular Magnification Distortion: magnification of an object for rays parallel to the axis that varies with the distance of the ray bundle from the axis. This aberration affects the shape of the object and the apparent separation between two objects, but not the sharpness of the image. An easier way to think of this is to consider that the magnification of the eyepiece (or the scale of the camera lens, etc) varies across its field of view.

Annular eclipse - When the Moon passes in front of the Sun, but is at a point a little too far away from the Earth in its orbit to completely cover the sun, the eclipse produces a ring of sunlight around the Moon¡¯s shadow. This annulus, or ring, creates an ¡°annular eclipse¡±.

Anti-Light Pollution (ALP) Filter A type of nebular filter that eliminates light frequencies from mercury and sodium vapor street lights. This reduction in light is quite gentle, and the contrast enhancement is also gentle.

Anti-Reflection Coatings Interference coatings deposited on a lens surface to increase light transmission. These are typically transparent layers, but interfere with each other to reduce reflection from the surface of the lens underneath.

Antoniadi scale-- The Antoniadi scale was invented by a Eug¨¨ne Antoniadi, a Turkish astronomer, who lived from 1870 to 1944. The scale is on a 5 point system, with one being the best seeing conditions and 5 being worst. The actual definitions are as follows:1.Perfect seeing, without a quiver 2.Slight quivering of the image with moments of calm lasting several seconds 3.Moderate seeing with larger air tremors that blur the image 4.Poor seeing, Constant troublesome undulations of the image 5.Very bad seeing, hardly stable enough to allow a rough sketch to be made

Aperture The measurement of the diameter of the main lens or mirror of an optical system. The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a larger opening while a lower number represents a smaller opening.

Aphelion - the greatest distance any body in orbit around the Sun reaches in its orbit. The Earth¡¯s aphelion occurs in July.

Aplanatic: Simultaneously free of coma and spherical aberration. This can be done with a single lens if the front and back surface power are in the ratio of F1 = 6F2. It can be done with two or more lenses with more flexibility.

APO - Apochromatic Refractor, or Apochromat

Apochromatic: Any optical system without color aberration (free of secondary spectrum): formally, a lens design that brings three wavelengths to a common focus. For various reasons, the formal definition is not very useful by itself. If color correction is plotted by wavelength against distance from focus, the lines will cross three times for an apochromat; but it is at least as important to keep the lines from diverging very much where they do not cross. Apochromatic refractors have three elements in their objectives, sometimes air spaced and sometimes oil spaced. Several optical designers have suggested that the best definition of "apochromatic" is driven by what is actually seen at the focal plane, and this is more useful than the formal definition (some consider that a true apochromatic lens ought to eliminate spherical aberration also.).

Apogee - the greatest distance from the Earth that any body in orbit around the Earth reaches in its orbit.

Apparent Field of View (AFOV) The angular size of the field as seen through a particular eyepiece without a telescope. (Varies with different designs, and is often specified: roughly, Orthoscopics 40 degrees; Pl?ssls 50 degrees; Superwides 60-70 degrees; Ultrawides 75+ degrees.).

Apparent Magnitude The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a small apparent magnitude number while dim objects will have a larger apparent magnitude number due to the magnitude scale¡¯s reversed approach (see Magnitude).

Arc Second - 1/648,000 the distance from horizon to horizon. It is 1/60th of an arc-minute, which is, in turn, 1/60th of a degree, which is, in turn, 1/180th of the distance from horizon to horizon.

Ashen light - the appearance of apparent lightening of the atmosphere on Venus that is on the dark side (not illuminated by the Sun). It is probably due to the bending of sunlight in Venus¡¯ dense atmosphere, but some of the effect could be due to the visual proximity of the brighter image of Venus¡¯ sunlit side in the eyepiece.

Aspheric - referring to an optical surface that is not spherical (describing part of the surface of a sphere). This usually is the term given paraboloidal and hyperboloidal optical surfaces used in telescope mirrors or lenses.

Asteroid A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets. A farther belt, the Kuiper Belt, exists beyond the orbit of Neptune, and contains the dwarf planets of Pluto, Eris, and others.

Asterism - a visually interesting grouping of stars that form a pattern. The stars are not necessarily related at all, but if the pattern is noticeable, a name might be given for the group. In a sense, constellations are merely larger examples of the same thing.

Astigmatism: An optical aberration that causes the image of a point light source to appear as an ellipse with the long axis of the ellipse shifting by 90¡ã on opposite sides of the focal plane. Astigmatism exists when there is a difference between the optical power of the optical system in the tangential (horizontal) plane and that in the sagittal (vertical) plane. This can be better envisioned by two planes passing at right angles through the lens or mirror. The first of these passes through the object being viewed and the optical axis of the lens, and is called the tangential plane (or the meridianal plane). The second is at right angles to this plane. Rays traveling along one of these planes are focused to a specific point in the focal plane. Rays along the other plane, however, are focused at a slightly different point. This produces two focal surfaces. These two focal surfaces touch each other on the optical axis, i.e., there is no astigmatism on axis and astigmatism is therefore an off-axis aberration (unless cylinder is ground into the lens or mirror). The two focal surfaces (the tangential focal surface and the sagittal focal surface) may be convex or concave, depending on the design of the system, so there are four possible configurations for these surfaces. It is impossible to produce a sharply-focused image in the presence of astigmatism; on one side of focus the star will appear as a line; on the other side of focus it will appear as a line rotated 90 degrees from the first line, and in between the star will appear boxy. Well out of focus the star may appear oval. The diameter of the astigmatic star image at the point midway between the two focal surfaces bears an inverse relationship to the focal ratio, so that at f/5 the diameter is three times that at f/15. In almost all cases, the astigmatism of the eyepiece dominates astigmatism of the objective, which plays only a small part in perceived astigmatism in the field. Eyepiece astigmatism strongly dominates coma in a Newtonian.

Astronomical Unit (AU) A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.

Astrophotography - the process of taking pictures of the night sky through cameras and telescopes.

Atlas - a map of the positions of the stars and celestial objects in the night sky. Atlases are usually described by the faintness of the faintest stars on the charts, e.g. a 7th magnitude atlas.

ATM - Amateur Telescope Making (or Maker)

Atmosphere A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases. Every planet in our solar system has an atmosphere, though a few of them have very thin atmospheres nearly indistinguishable from the vacuum of space.

Aureolae - the area of glow surrounding any bright object. The most common use of the word in astronomy describes the area of lighter sun surrounding a sunspot (also called the penumbra of the spot).

Aurora A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora Australis in the southern.

Autocollimator - a collimation tool containing a mirror at right angles to the optical axis and a peep hole on that axis. When the optical system is completely aligned, no extraneous light gets in and the field of the autocollimator turns black. Any centermark on the primary mirror will also reflect 4 times in this tool: when all 4 images coincide, or ¡°stack¡±, the telescope is collimated to a small number of thousandths of an inch.

Averted Vision - An observing technique in which a faint object image may be made to appear brighter to the eye by not looking directly at it. This is useful because the most sensitive parts of the retina lie outside the central area of vision.

Axis Also known as the poles, this is an imaginary line through the center of rotation of an object.

Az - Azimuth

Azimuth The angular distance of an object around or parallel to the horizon from a predefined zero point, usually North.

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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
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Moon Observer's Guide, Firefly Books
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Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
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Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
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This essential reference features an alphabetical listing of every deep sky object plotted in Sky Atlas 2000.00.
 
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Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
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