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.
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
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
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+
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
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
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
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.