Any defect in an optical system
The most common type of objective lens found in a refractor telescope.
An achromat has two elements made of two different types of glass with
different refractive indicies. A properly designed achromat will bring at least two different wavelengths (colors) of
light to a common focus. In practice, achromats are designed to minimize errors across all wavelengths.
Alt-Az is short for "Altitude-Azimuth"
refers to a type of telescope mount that allows the telescope to be
moved in Altitiude (up and down) and Azimuth (left to right).
Compare this with an Equatorial Mount.
Altitude When describing a type of telescope mount (alt-az), altitude refers to the the y-axis (up-down) motion of a mount.
Aperture The size of the main lens or mirror (the objective) that collects light in your telescope. Common amateur telescopes might have a range of apertures from about 2.4" (60mm) to about 12 inches or larger.
Aperture Fever is the mythical disease that causes otherwise rational amateur astronomers to purchase larger and larger telescopes in the hope of seeing more. The cure is to become a more careful observer!
Apochromat A type of compound lens that uses either three or more elements, or special glasses, like fluorite or ED glass, to bring at least three colors to a common focus. In practice, errors are minimized across all wavelengths in order to achieve a very high degree of color correction. Most camera lenses are apochromatic. Compare this with an achromat.
Astigmatism Optical abberation or imperfection that renders stars as crosses instead of points. Difficult to correct.
Aspheric A lens or mirror whose curve is not the surface of a sphere.
Astrograph A lens or telescope intended for photographic, rather than visual, use.
Astrophotography Process of taking pictures of the sky.
The x-axis (left-right) motion of an alt-az
Barlow Lens A lens assembly with a negative focal length placed between an
and telescope that
effectively increases the focal length (and magnification) of a
telescope. Barlows commonly come in magnifications of 1.5x, 2X, and 3X.
Catadioptric Telescope Any design using a combination of mirrors and lenses. Catadioptics are typically smaller than other designs of similar focal length as the fold the light path or a telescope by reflecting it back and forth.. The popular Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain designs are common examples of catadioptic designs.
Telescope A pure cassegrain telescope uses a parabolic primary
mirror, just like a Newtonian telescope, but instead of a flat diagonal
mirror, the cassegrain has a hyperbolic convex mirror that reflects
light back through a hole in the primary mirror. Cassegrains typical
have a very long focal length, which results in high
Cassegrains are popular with some planetary observers.
CCD Camera Digital camera using a Charge Coupled Device to form
images. Most digital cameras use CCDs, but in astronomy the term "CCD
Camera" is often used to differentiate it from a film camera.
Chromatic Aberration Refractor telescopes cannot bring all colors to a single point of focus, and consequently there is always some amount of secondary color seen in a refractor, especially at high magnifications. Think of how a prism sperates light into a spectrum; simple lenses do this, too, and so complex multi-element lens designs are needed to minimize this. The inability to focus all colors to a common point is called chromatic abberation. Purely reflecting telescopes have zero chromatic abberation.
Classical Cassegrain is a term used to describe simple cassegrain designs, as opposed to Schmidt-Cassegrains.
Coating One or more thin layers of a mineral compound
vacuum-deposited on a lens or mirror in order to increases it light
transmission (in the case of a lens) or reflectance (in the case of a
The process of aligning the optics within a telescope. Most refractors
are permanant collimated, but Newtonian reflectors
regular collimation for optimal performance.
Coma A common aberration in short focal length reflectors that
results in increased distorion of star images that increases closer to
the edge of the
field of view. Coma can be reduced with auxiliary optics, like the
TeleVue Paracorr adapter.
Coma corrector An optical device used to reduce coma.
Dawes' Limit William Dawes wrote that a telescope can resolve (that is, see as two sperate points) a pair of double stars separated by 4.56 arc seconds divided by the aperture of the telescope in inches. This is commonly called " Dawes' Limit", and is generally accepted as the limit of what a very well made telescope can resolve.
Equatorial mounts have two axis of movement referred
to as Declination
and Right Ascension. Right Ascension refers to the
axis in line with
the Earth's axis of rotation, and Declination is the axis 90 degree
from Right Ascension.
Diagonal In a refractor or catadioptric telescope, a mirror or prism used with that is used to turn the viewing axis 90 or 45 degrees to make it easier to veiw from behind the scope. In a Newtonian telescope, a diagonal is the mirror placed near the front of the telescope that reflects the collected light out the side of the tube, where the focuser and eyepiece are located.
A simple type of alt-az Newtonian telescope
mounting designed to
be both stable and inexpensive.The name comes from telescope builder
John Dobson, who shuns the name "dobsonian" and prefers to call his
telescopes "sidewalk telescopes".
ED glass. A type of optical glass with exceptionally low dispersion often used to make apochromatic telescopes.
Equatorial Mount A type of telescope mount in
which one axis of
movement (called Right Ascension) is aligned with the earth's polar
axis. An equatorially mounted telescope can be moved on just one axis,
by hand or by motor, to keep the telescope aimed at the same path of
sky as the earth rotates.
Erfle A five-element eyepiece design
yielding a wide field of
Eyepiece A (typically) 1.25" or 2" tube containing one or
more lenses that is inserted in the focuser of
the telescope and
magnify the image formed by the objective lens.
f/ratio A number determined by dividing the focal length of an objective lens or mirror by its diameter. Typical f/ratios of amateur telescopes range from f/5 to f/15.
Focal length The distence from a mirror or lens at
which the light rays being focused are brought to a common point, and
(in a telescope) an image is formed. If you've ever focused the sun's
rays with a
magnifying glass, the distance from the lens at which the rays are most
concentrated is the focal length of the lens.
Focuser The device that holds the eyepiece, and allows a viewer to move it back and forth in order to bring the scope into focus.
Filter As with camera filters, telescope filters are used to selectively block or pass certain wavelengths or colors. oOlored filters are commonly used by planetary viewers to bring out detail. Deep sky viewers often use "light pollution filters", which admit only a narrow band of light in order to see faint images and block the light from streetlights and signs.
Finder or Finderscope A
low powered auxiliary telescope mounted on a telescope used to
make it easier to point the main telescope at some object.
Natural fluorite mineral crystal or artifically grown fluorite crystal
material used to make lenses with exceptionally high refractive index
and low dispersion. Fluorite glass is very expensive but can be used to
make telescopes with exceptionally low chromatic
Goto The capability of some computer-controlled, motorized
telescopes to automatically point at some desired object or location.
Also the name of an former Japanese maker of quality telescopes.
Huygenian An eyepiece composed of two simple (i.e., non-achromatic) lenses. In the past there were some quality Huygenian designs from makers like Unitron, but today, with automation making reducing the cost of lenses, Huygenian eyepieces are only found on cheap telescopes.
Kellner A good, inexpensive eyepiece design that uses one simple (single element) lens, and one two-element achromatic lens.
Magnification is calculated simply by by dividing the focal
length of the objective lens
or mirror by the focal length of the eyepiece.
Maksutov Cassegrain A catadioptric telescope using a spherical
primary mirror and secondary mirror, and a spherical corrector plate.
Common Maksutov Cassegrain designs include the Questar and the popular
Meade ETX series.
Mount A device that supports a telescope and allows it to move and point to different locations in the sky.
Newtonian Reflector A type of Reflecting Telescope named arfter
Sir Issac Newton, its inventor. Most reflecting
telescopes are of a
Objective The main mirror or lens of a telescope that collects and focuses light.
Ocular Another name for eyepiece.
A four-element eyepiece design
consisting of one triplet
(three-element) lens, and one simple lens. Well made orthoscopic eyepieces
deliver excellent image quality over a fairly wide field of view.
An abbreviation for "optical tube assembly", i.e., a telecope tube
without a mount.
Parfocal Commonly refers to a group of eyepieces
that can be swapped without
refocusing. Some makers produces sets of eyepieces that are designed to
Piggyback Photography A method of photography in which a camera
is mounted on the side of a telescope. The telescope is used to guide
Polar Alignment Equatorial mounts must have
one axis- the Right
Ascension, or polar, axis aligned with the Earth's axis of rotation
order to be able to track the rotation of the earth. This is called
Plossl A four-element eyepiece design consisting of two achromatic doublets facing each other. A similar design often mistakenly referred to as a plossl is the symmetrical eyepiece, which has two identical doublets.
Ramsden Another simple two-element eyepiece generally found today only on cheap telescopes.
Reflecting Telescope A
telescope which uses a curved mirror to focus the light rays entering
telescope. All the largest telescopes in the world, as well as the
Space Telescope, are basically reflecting telescopes.
Refractor A telescope design which uses a lens at the front of
the tube to focus the incoming light.
Resolution A measure of how fine a detail a given telescope can pick out, usually specified as an angle. A telescope might be described as being able to resolve 1.5 seconds of arc.
Right Ascension The axis of rotation of an Equatorial
mount aligned with the earth's axis of rotation. It's calibrated in in
24 hour segments, since the Earth takes 24 hours to complete one
Schmidt-Cassegrain A catadioptric
design, commpnly referred to as an "SCT" that uses two mirrors and an aspheric
"corrector plate" on the front of the tube to focus incoming light.
Secondary Mirror The second, smaller mirror in the light path of a reflecting or catadioptric telescope.
Term used to indicate how relatively steady or turbulent the atmosphere
above a telescope is. Turbulent air means blurred or smeared images, as
the air itself can bend (or refract)
the light passing through it.
Setting Circles Calibrated disks on an equatorial mount used to point the telescope at some desired celestial coordinates.
Spherical aberration A common form of optical abberation
caused when the light from the periphery of a mirror or lens doesn't have the same
focal point as light from the center. A mirror or lens can be otherwise perfect, but suffer
from spherical abbertion from being improperly figured. The original space telescope
mirror suffered from spherical abberation that was corrected by later installing an additional set of corrective lenses.
If a mirror used to form an image has a spherical shape, it will not bring all the light reaching it to a common focus; telescope mirrors must be given a parabolic curve in order to focus light properly.
Star diagonal See diagonal.
Star Test A method of evaluating optics by pointing a telescope
at a bright star under high magnification, and observing the image as
the telecope is focused and defocused. This technique can reveal a
number of common abberations.
Strehl Ratio. A measure of optical quality. The Strehl ratio is calculated as the percentage of the light in a focused image of a point source that is contained within the center of the image, when compared to a theoretically perfect lens or mirror. The best high quality, hand-figured lenses and mirrors typically have Strehl rations of 97-99%. A typical mass-produced SCT or reflector might have a Strehl ration in the 80s. A higher ratio indicates more light brought to a common focus; that translates into sharper images and higher contrast.
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