The Heretic's Telescope Glossary

[In progress. Coming: More words, and illustrations. If you can't find a word you're looking for, tell me, and I'll add a definition.]

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Abberation Any defect in an optical system

Achromat 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" and 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.

Azimuth The x-axis (left-right) motion of an alt-az telescope mount

Barlow Lens A lens assembly with a negative focal length placed between an eyepiece 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.

Cassegrain 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 the light back through a hole in the primary mirror. Cassegrains typical have a very long focal length, which results in high magnification. 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 mirror).

Collimation The process of aligning the optics within a telescope. Most refractors are permanant collimated, but Newtonian reflectors generally require 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.

Declination 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.

Check out Orion's Telescope Buying Guide

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.

Dobsonian 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 view.

Eyepiece A (typically) 1.25" or 2" tube containing one or more  lenses that is inserted in the focuser of the telescope and used to 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.

Fluorite 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 abberation.

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 Newtonian design.

Objective The main mirror or lens of a telescope that collects and focuses light.

Ocular Another name for eyepiece.

Orthoscopic 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.

OTA 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 be parfocal.

Piggyback Photography A method of photography in which a camera is mounted on the side of a telescope. The telescope is used to guide the camera.

Polar Alignment Equatorial mounts must have one axis- the Right Ascension, or polar, axis aligned with the Earth's axis of rotation in order to be able to track the rotation of the earth. This is called Polar Alignment.

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.

Reflector or Reflecting Telescope A telescope which uses a curved mirror to focus the light rays entering the 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 telescope 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 rotation.

Schmidt-Cassegrain A catadioptric telescope 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.

Seeing 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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