A Glossary for Telescope Buyers and Users

This glossary is for people concerned with buying and using astronomical telescopes, and so is restricted mostly to terms related to telescope design, manufacture, and use. A glossary including astronomical terms as well, would be too large for me to create and maintain.

I have included a generous handful of manufacturers, brand names, and product-line names. I do not mean to imply product endorsement by my selection.

Note:


I welcome corrections, comments, and suggestions for additional words to include. I am particularly eager to hear from beginners who tried to find a term here, and failed.

A. Jaegers

Former United States supplier of telescope components and accessories, mid to late twentieth century. Jaegers refractor objectives are still in widespread use.

achromat

A type of refractor objective made from lenses -- usually two -- of different materials, selected to bring two colors of light -- usually blue and yellow -- to the same focal point, thereby bringing the rest of the colors of visible light to very nearly the same focal point, thus making a decent stab at being in focus for all colors of visible light at the same time. That is, an achromat attempts to reduce longitudinal chromatic aberration.

An achromat will also have spherical aberration corrected at one wavelength of light -- usually green.

actual field of view

A characteristic of eyepiece/telescope combinations. The actual field of view is the angle, measured on the sky, from one edge of the eyepiece field of view to the other. It is approximately equal to the apparent field of view divided by the magnification.

Short for altazimuth mounting.

altazimuth mounting

A kind of telescope mounting in which one of the two axes the telescope turns about is pointed vertically, and the other horizontally. The first axis is the azimuth axis, and the second is the elevation axis.

A telescope on an altazimuth mounting must usually rotate around both axes simultaneously, to follow celestial objects across the sky as the Earth rotates. For telescopes not too large or too powerful, a human being can usually operate an altazimuth mounting well enough for visual observations. Several modern commercial amateur telescopes have altazimuth mountings which are motor-driven and computer controlled.

aluminizing

Telescope mirrors used to be made of metal, laboriously worked to the right shape. Metals tarnish, however, so that the labor -- or most of it -- had to be repeated periodically: Polishing off the tarnish changed the shape enough to matter. It was a big deal, in the nineteenth century, when new chemical processes made it possible to deposit a film of silver on glass: A tarnished mirror could be repaired simply by removing the remains of the old coating chemically, and depositing a new one. In the 1930s, the new process of aluminizing allowed deposition of a coating of aluminum on a mirror in a vacuum chamber, essentially by boiling aluminum vapor off a heated piece of that metal.

Aluminum is much more tarnish-resistant than silver, hence its advantage. Other metals and materials may similarly be deposited.

Alvan Clark and Sons

Former United States manufacturer of high-quality classic refractors from mid nineteenth to the early twentieth centuries. Made instruments for both professionals and amateurs. These telescopes are thought by many to be the finest of their kind ever produced.

anastigmatic

Loosely, "corrected for astigmatism", but the more technically correct sense of this term is to describe an optical system which is corrected for spherical aberration, coma, and astigmatism. The point is, that in many common optical designs, spherical aberration and coma tend to be more serious problems than astigmatism, so there isn't much point in correcting astigmatism unless the other two are already dealt with.

aperture

Loosely, the working diameter of a lens or mirror; the diameter of the largest beam of light that can get through it. Also see clear aperture.

aperture fever

An amateur astronomer who is eager to the point of obsession, to have telescopes of ever larger clear aperture, is said to have aperture fever. The disease is expensive to cure, particularly if it is combined with refractoritis.

aplanatic

Loosely, "corrected for coma", but the more technically correct sense of this term is to describe an optical system which is corrected for both spherical aberration and for coma. The point is, that in many common optical designs, spherical aberration tends to be a more serious problem than coma, so there isn't much point in correcting coma unless spherical aberration is already dealt with.

APO

Allegedly, an abbreviation for apochromat, but often merely a meaningless marketing term.

apochromat

A type of refractor objective made from lenses -- usually three -- of different materials, selected to bring three colors of light to the same focal point, thereby bringing the colors that are in between to very nearly the same focal point, thereby permitting a decent stab at being well-focused for all colors of visible light at the same time. Thus, an apochromat attempts to reduce longitudinal chromatic aberration.

An apochromat will also have spherical aberration corrected at two wavelengths of light -- usually blue and yellow -- thus attempting to reduce chromatic variation of spherical aberration. It will probably have coma corrected at one wavelength of light -- usually green.

At any rate, what preceeds is the technical definition. There is an increasing colloquial tendency to label any telescope which is free, or almost free, from chromatic aberration an apochromat. The problem with that is that advertisers have picked up on "apochromat" as a high-tech term useful in suckering buyers: Thus as the years go by, what gets labeled "apochromat", gets worse, and worse, and worse...

The problem is compounded because there are telescopes which are apochromats in the technical sense, as discussed in the preceding paragraphs, which in fact show a lot of chromatic aberration when used visually. These early apochromats had one of their "three colors" in the near ultraviolet, so that they could be focused visually to take sharp photographs with early blue/ultraviolet-sensitive photographic emulsions.

apparent field of view

A characteristic of eyepieces. The apparent field of view is the angle through which your eyeball rotates when you look through an eyepiece and transfer your gaze from one edge of the field to the other.

aspheric

Not spherical, as in "the primary mirror of a Newtonian telescope has an aspheric surface".

astigmatism

An aberration of optical systems. If you try to focus on a star with a telescope whose only aberration is astigmatism, you will find that the best-focus image is notably larger than in a similar telescope with no astigmatism. Moreover, you will find that when you move the focuser a little way one side of best focus, the star image straightens out into a short straight-line segment. If you then move the focuser the same distance the other side of best focus, you will find the image to be a similar short straight-line segment, but oriented at ninety degrees to the first one.

Pure astigmatism is relatively rare in telescopes. Astigmatism is most often seen off axis, mixed with other aberrations -- often with coma -- sometimes resulting in bewilderingly complicated images.

Astro-Physics

United States manufacturer of high quality refractors, including many apochromats, as well as equally high quality Maksutov-Cassegrains, together with mountings and other accessories.

averted vision

When you look squarely at something, you are using a part of the retina of your eye that is not as sensitive to low light levels as the parts that are off to the side. Thus to see faint objects, don't look straight at them. Center them in the field of view of your telescope, but fix your stare part way out to the edge of the field.

People sometimes ask which way to avert -- that is, which way away from the center of the field to move their gaze. Try several.

back focal length

Classically, the distance from the last optical element of a system encountered by the light passing through it, to the focal plane. Opinions differ on whether flat mirrors and diagonal prisms count as "optical elements", for determining back focal length. I think they do not.

With classical all-lens optical systems, like refractor objectives and many camera lenses, back focal length is approximately what you get if you put a measuring stick against the lens cell and measure the distance to the focal plane. But with things like Schmidt-Cassegrains, the term is more confusing -- for that system, back focal length is measured from the secondary, which is way up inside the tube.

There is an increasing tendency to measure the back focal length of folded systems, like the various Cassegrain types, from the point where the beam emerges from within the assembly of optical parts -- e.g., from where it comes out of the hole in the primary. Be careful of which meaning is in use.

Barlow lens

A negative (diverging) lens that fits into your telescope's focuser, a little way inside the focal plane. It acts as a focal-length multiplier for the telescope. Thus, suppose you have a telescope with a focal length of 1000 mm. If you use a 10 mm focal-length eyepiece, you will obtain a magnification of 1000/10 = 100x. With a 2x Barlow lens used in front of the eyepiece, the telescope's focal length will be increased to 2000 mm, and the same eyepiece will give a magnification of 2000/10 = 200x.

The multiplication ratios of commercially available Barlow lenses vary from 1.75 to 5. Barlow lenses are sometimes also called telextenders.

Not all Barlow lenses work with all eyepieces and all telescopes. Try before you buy.

barrel distortion

An aberration of optical systems, in which magnification decreases with distance away from the optical axis. With such a system, squares are imaged with their sides bulged out, looking sort of like barrels. (Actually, the points of the squares are imaged a little bit less far out than where they ought to be, because of the reduced magnification in the outer part of the field.) Also see pincushion distortion.

binocular

Two telescopes held side by side, so you can look at something with both eyes at once. Binoculars make quite good instruments for beginning astronomers, and even advanced amateurs find them fun and useful.

Binoculars are traditionally labeled with two numbers, separated by the letter "x", as in "7x50". The "x" is read as "by"; hence a binocular labeled "7x50" is called a "seven by fifty". In such a pair of numbers, the first one is how many times the binocular magnifies, and the second is the clear aperture of its front lenses, in millimeters. Thus a 7x50 magnifies seven times, and has front lenses 50 mm in diameter.

Note that what most people call "a pair of binoculars" is in fact just one binocular: That is, a binocular is sufficient for two eyes; a pair of binoculars will do for four.

bino-viewer

An optical gadget that splits the single beam of light coming out of a telescope into two parts, so you can observe with both eyes at once. Bino-viewers generally do not work with all telescopes or all eyepieces.

Good bino-viewers are quite expensive.

A specific eyepiece design. Brandon is a "house brand" of Vernonscope, and is applied to a variety of products, including a well-established line of eyepieces that are generally known as just "Brandons". Several of the short to medium focal-length Brandon eyepieces are advertised as orthoscopics, yet they appear upon disassembly to resemble Plossls. (There are in fact several eyepiece designs that are properly labeled "orthoscopic": Brandons likely are one of the less common ones.) They have moderately wide apparent fields of view, of about 45 degrees.

Brashear

Former United States telescope manufacturer, late nineteenth and early twentieth centuries. Made equipment for both professionals and amateurs, including several large observatory classic refractors.

Carl Zeiss

German manufacturer of telescopes, other optical equipment, and accessories. Zeiss has sold to professionals regularly, but has been in and out of the amateur market. Zeiss Abbe orthoscopic eyepieces are well-regarded for planetary work.

Cass

Short for "Cassegrain".

Cassegrain

In the strictest sense, a Cassegrain telescope is a reflecting telescope that uses two curved mirrors of particular shapes to form the image: The primary mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller convex hyperboloid, positioned a little way toward the primary from the primary's focal plane. The secondary reflects the converging beam of light back toward the primary, which typically has a hole in the center, so that the focal plane of the combined mirrors is easily accessible behind the primary.

More loosely, the term "Cassegrain", or "Cassegrain configuration", or "Cassegrain system", has come to be applied to any telescope in which a large primary mirror and a small convex secondary mirror are among the elements used to form the image. Common Cassegrain systems include the Schmidt-Cassegrain, in which a corrector plate which appears flat to the unaided eye is mounted just skyward of the secondary mirror, and several varieties of Maksutov-Cassegrain, in which a correcting lens that resembles a thick watch glass is similarly mounted. In some Maksutov-Cassegrains, the secondary mirror is an aluminized spot on the correcting lens.

There are many other Cassegrain systems besides the ones just mentioned. In some, the only difference from the classical Cassegrain is that the two mirrors are not the traditional paraboloid/hyperboloid combination.

catadioptric

A catadioptric optical system is one that uses both lenses and mirrors to form the image. Flat mirrors, panes of flat glass, and prisms which are used to reflect the light beam about, do not count in making this determination. Common catadioptric systems include Schmidt-Cassegrains and Maksutovs, but there are lots of others.

Cave

Former United States telescope manufacturer, mid twentieth century. Made mostly amateur instruments. Particularly noted for the fine optics of the "Astrola" product line, of which the Newtonians are most famous.

Celestron

United States telescope manufacturer and importer, founded in the 1960s. Celestron pioneered techniques for manufacturing Schmidt corrector plates at low cost, and successfully introduced relatively inexpensive Schmidt-Cassegrain telescopes to the amateur market. Current product line includes a variety of Schmidt-Cassegrain, Newtonian, Maksutov-Cassegrain, and refractor telescopes, as well as numerous accessories.

cell

The mechanical support that grips a lens or mirror, and which is in turn fastened to the telescope tube, or to some other piece of telescope structure.

cemented doublet

See doublet.

chromatic aberration

Catch-all term for aberrations of optical system in which the problem is that different things happen to different colors of light. See chromatic difference of magnification, chromatic variation of spherical aberration, lateral color, longitudinal chromatic aberration, longitudinal color, and spherochromatism.

chromatic difference of magnification

An aberration of optical systems, in which the magnification is different in different colors of light. One might think of the image of an object seen with such an aberration, as composed of superimposed red, green, and blue images of slightly different sizes.

Chromatic difference of magnification is often detectable as colored fringes at the edges of objects of different colors or brightness, near the edges of the field of view. It is particularly common in binoculars.

Chromatic difference of magnification is also called lateral color.

chromatic variation of spherical aberration

An aberration of optical systems, in which spherical aberration is different in different colors of light.

Chromatic variation of spherical aberration is also called spherochromatism.

Clark

See Alvan Clark and Sons.

classic refractor

A well-made refractor with an achromat objective, dating from before optical technology permitted manufacture of such instruments with short focal ratio: Classic refractors generally have focal ratios of f/14 or longer.

By extension, any modern refractor patterned after the classic model.

clear aperture

The diameter of the beam of light that goes into a telescope. In most cases, that is the unobstructed diameter of the front lens, or the corrector, or the primary mirror.

clock drive

See sidereal drive.

coated

"Coated" is most commonly used to describe a lens or prism in which the surfaces where the beam of light goes from air into glass, or from glass into air, have been coated with something that reduces the amount of light that gets reflected. That reflected light is lost to the image, and may cause glare, so reducing it is desirable. The coatings that do this are more correctly called low-reflection coatings -- for after all, mirrors are coated, too, but with something that is supposed to increase reflectivity rather than reduce it. But the term "coated" is common usage for "coated with low-reflection coatings."

There is another meaning, that has to do with marketing terminology. If you bought a binocular that had "coated optics", you might naively think that all the surfaces where a beam of light went from air into glass, or from glass into air, were coated with low-reflection coatings, right? Wrong! The common usage in optical advertising is that "coated optics" merely means that some of those surfaces are so coated -- perhaps only one. To get what you thought you were buying, you would have to seek a binocular that was "fully coated". Also see multicoated and fully multicoated.

collimation

A telescope is said to be collimated when all its optical parts are correctly aligned with respect to one another -- everything is square, centered, and spaced correctly. Some instruments have collimation set at the time of manufacture, and with luck, the collimation will be correct for the life of the instrument; these instruments include binoculars and many small refractors. Others require frequent tweaking, which is sometimes a vexing task; these include most Newtonian reflectors.

coma

An aberration of optical systems. The image of a star, in a telescope whose only aberration is coma, resembles a short, stubby comet with a broad tail, or a side view of a badminton birdie.

Pure coma is relatively rare in telescopes. Coma is most often seen off axis, mixed with other aberrations -- often with astigmatism -- sometimes resulting in bewilderingly complicated images.

compound telescope

A telescope in which a secondary mirror makes a substantial change in the focal length. Compound telescope designs include all variants of Cassegrain and Gregorian.

contrast

I include this common word because the amateur astronomy community often uses it in two very different senses without being clear which is which. To someone concerned with the measurement of light, contrast is merely the ratio of brightnesses of two parts of an image. You can determine it by actually measuring the brightnesses with a photocell, or a light meter, or some such thing, and dividing. But to someone concerned with the processing which the brain and eye do when light falls upon the retina, contrast might more often mean how well two areas of different brightness are distinguished; for example, can you see such-and-such a faint galaxy on the black sky beyond it?

The relationship between contrast as measured by light meters and contrast as reported by visual observers, is complicated and poorly understood. How well two areas of different brightness are distinguished by the eye and brain has to do with lots more than the ratio of brightnesses.

corrector plate

Short for Schmidt corrector plate.

Coulter

Former and resurrected United States manufacturer of telescopes and accessories, late twentieth century. Coulter was the first successful mass producer of inexpensive Dobson-mounted Newtonian telescopes. Following the death of its founder, the company filed for bankruptcy in the mid 1990s. Its assets and name were bought by another company -- Murnaghan -- and telescopes under the Coulter name returned to production.

Crayford focuser

A fancy type of telescope focuser in which the focus tube is moved by a roller, and may well slide on rollers, as well. Crayford focusers are noted for smooth, slack-free motion, and for precisely adjustable friction.

Criterion

Former United States manufacturer of amateur telescopes and accessories, mid twentieth century. Criterion was particularly noted for quality Newtonians, notably the relatively inexpensive Dynascope line. Criterion also produced Schmidt-Cassegrains, though not as successfully as Celestron and Meade.

curvature of field

An aberration of optical systems, in which the surface on which images in sharp focus are formed is not flat, but curved. Curvature of field is also called field curvature.

Your eye does not reach its full power of detecting low light levels -- its full dark adaptation -- until you have been in a dark environment for quite a long time -- perhaps several hours. To obtain full dark adaptation, the darkness of the surroundings must be very deep. Many observing sites have enough stray light from nearby towns and cities to prevent full dark adaptation.

Do not confuse this hours-long process with the opening of the pupils of your eyes to their maximum diameter: That takes at most tens of seconds. Dark adaptation has to do with slower changes in the sensitivities of light-detecting cells on the retinas of the observer's eyes.

D & G

United States manufacturer of optical and mechanical components for amateur astronomy, particularly noted for their large doublet objectives for classic refractors.

Dawes limit

The Dawes limit is an empirically determined standard of how well an excellent small telescope can be expected to perform, in excellent conditions, when observing double stars that are not too faint, in which the two components are of the same brightness, or very nearly so. The empirical result is that double stars can be distinguished as separate when they are as closely separated as 4.56 arc-seconds, divided by the telescope clear aperture in inches. Thus a four-inch telescope can expect to resolve such double stars down to a separation of 4.56 / 4.00 = 1.14 seconds of arc. In metric units, the formula is 116 arc-seconds divided by the telescope clear aperture in millimeters. To be just barely distinguished means there will be a barely perceptible dark line between the images of the two components. Note that the ability to perceive such a dark line may vary from observer to observer: The Dawes limit has to do with the physiology of human vision as well as the optical quality of telescopes. (And Dawes was noted for having particularly acute vision.)

Dawes performed his experiments, on which the formula is based, using refractors. It turns out that telescopes in which a secondary mirror or diagonal mirror obstructs part of the incoming beam, can do ever so slightly better than the limit, for stars of equal brightness, but they don't do as well as refractors when the brightness of the stars is considerably different.

One of two coordinates for the celestial sphere, which are analogous to latitude and longitude for the Earth's surface. The declination of an object is how many degrees it is north or south of the celestial equator. The other coordinate is called right ascension, and it is measured eastward from a somewhat arbitrary "prime meridian" on the sky.

The "prime meridian" passes through the position of the Sun at the time of the vernal equinox. Thus its position changes slowly over the years, due to the precession of the equinoxes. The position of the celestial poles also changes with precession. Thus, to locate an object from its right ascension and declination, you must also know the date for which those coordinates are valid; that date is called the epoch of the coordinates.

deep sky

Deep sky astronomical objects are generally considered to be those which lie beyond the solar system, except that some people do not include double stars and variable stars. That's not because these objects do not lie beyond the solar system -- they certainly do -- but because the term "deep sky" evolved to describe what many people do with large-aperture telescopes at low to medium magnification. What they do is look at galaxies, star clusters and nebulae. It is a mouthful to say "I look at galaxies, star clusters and nebulae", and much easier just to say "I look at deep-sky objects".

Veteran deep-sky observers can routinely detect extremely faint objects in circumstances when less experienced observers see nothing. They use terms like "faint fuzzy nothing", "lumpy darkness", and "the elusive Elvis Nebula", a lot.

dew cap

A cylindrical extension of the telescope tube beyond the upper end, whose purpose is to retard the formation of dew on optical elements at or near that end. Dew caps are common on refractors, but often are useful on Schmidt-Cassegrains and Maksutovs. Many Newtonians also might benefit from extensions of the tube, to keep dew off the diagonal mirror.

diagonal mirror

There are two meanings. The small flat mirror used near the upper end of a Newtonian telescope, to direct the converging beam of light over to the side of the tube, is called the diagonal mirror. (Now and then you see a Newtonian that uses a prism instead of a mirror.)

Alternatively, star diagonals, that use mirrors and not prisms, are sometimes also called diagonal mirrors.

diffraction limited

A perfect telescope, used in perfect conditions, does not deliver a perfectly sharp image. A detailed explanation of why requires a modest amount of physics, and has to do with the wave nature of light. The relevant physical phenomenon is sometimes called "diffraction" (not quite correctly, if you are a purist). Thus an optical system is said to be "diffraction limited" when it is so good, that when used in excellent conditions (seeing is steady, telescope has cooled down to the local air temperature, telescope is well collimated...), then only the wave nature of light itself prevents the telescope from delivering perfectly sharp images.

To say that a telescope is diffraction limited, is to say that no conceivable improvement in its optical quality can make its images noticeably sharper. (You are not allowed to increase the aperture or change the telescope design, however.) That is an extremely strong statement: Think "a perfect ten" -- or maybe a 9.8; not merely "good enough", but "superb".

Marketeers frequently abuse the term "diffraction limited". To read some ads, you'd think it meant "any telescope manufactured by us". But in my opinion, only a very small proportion of the astronomical telescopes produced for the commercial market are truly diffraction limited, and only a few limited-production manufacturers produce such systems for their entire product line.

Also see Rayleigh criterion and Dawes limit.

digital setting circles

Electronic apparatus to achieve the function of setting circles; that is, to indicate where in the sky your telescope is pointed. These devices are generally capable of performing trigonometry to correct for imperfect polar alignment of your telescope, or to allow them to be used with an altazimuth mounting.

distortion

An aberration of optical systems, in which magnification changes systematically with distance away from the optical axis, causing distortions of images. See barrel distortion and pincushion distortion.

Dob

An atrocious abbreviation for the last name of noted telescope maker and inventor John Dobson. See Dobson mounting, for example. If I ever invent something that gets my name attached to it, I hope they never call it a "Freem"...

Dobson mounting

An ingenious modern altazimuth mounting, invented by San Francisco telescope maker John Dobson, which uses such slippery plastics as Dupont's "Teflon" to achieve very smooth bearing operation. Dobson-mounted telescopes generally require no slow-motion controls: They are guided by hand, even when observing with substantial magnification. Telescopes as large as a meter in clear aperture have been so mounted. I personally have operated a 45 cm Dobson-mounted Newtonian at a magnification of 1200x, which was pushing the limit for hand guiding, but which did work, and which provided a view of the lunar surface normally reserved for astronauts.

Dobson-mounted telescopes are almost always Newtonians, but other telescope types have been so mounted.

Dobsonian

Another name for a telescope with a Dobson mounting.

Dobsonians are most commonly Newtonians, but strictly, the term applies only to the mounting type, not the optical system.

doublet

Two simple lenses used in combination, placed close together or in contact. If they are cemented together, they constitute a "cemented doublet". If they are merely closely adjacent, they are a "separated doublet".

DSCs

Abbreviation for digital setting circles.

dust cap

A lid for one end of a telescope tube, to keep dust from settling on the optics.

ED doublet

An achromat made with ED glass. The most common commercially available ED doublets are better than achromats made with conventional glass, by a factor of about four, at bringing all colors of visible light to the same focal position, but not as good as fluorite doublets.

ED glass

A special kind of optical glass, recently introduced, of particular value in making achromats and apochromats. See also ED doublet. Many ED glasses have a high content of fluorite.

Edmund

See Edmund Scientific.

Edmund Scientific

United States supplier of diverse scientific paraphernalia, including telescopes, accessories, and related components. Edmund was founded in the mid-twentieth century, and for many years did a substantial business that featured military surplus material in great variety. Its present astronomical offerings include the AstroScan, which is a very portable fast 4.25-inch Newtonian, as well as the RKE line of eyepieces.

epoch

The coordinates commonly use for the celestial sphere, which are analogous to latitude and longitude for the Earth's surface, are called right ascension and declination. The "prime meridian" of this system passes through the position of the Sun at the time of the vernal equinox. Thus its position changes slowly over the years, due to the precession of the equinoxes. The position of the celestial poles also changes with precession. Thus, to locate an object from its right ascension and declination, you must also know the date for which those coordinates are valid; that date is called the epoch of the coordinates.

equatorial mounting

A telescope mounting in which one of the axes about which the telescope rotates is pointed parallel to the Earth's axis of rotation. Such a telescope can be made to follow celestial objects across the sky, as the Earth rotates, merely by turning this one axis at a constant rate of one revolution per day. A simple, geared-down motor can be used to do so. See sidereal drive.

Erfle eyepiece

A specific eyepiece design. The Erfle has a rather wide apparent field of view -- perhaps 68 degrees or more. The image quality at the edges of the field, at small focal ratios, is not as good as for more modern wide-field eyepieces.

It used to be said that Erfles were "Erfle big and Erfle expensive", but no longer -- modern wide-field eyepieces are more massive and more expensive, by far.

Erfles are generally composed of five or six simple lenses, grouped into two doublets and a singlet, or three doublets.

exit pupil

More or less, the beam of light that comes out of the eyepiece of a telescope. (More nearly correctly, the image of the objective formed by the eyepiece.) The diameter of the exit pupil is equal to the clear aperture divided by the magnification in use. If that diameter is greater than the diameter of the pupil of the observer's eye, then not all of the light entering the telescope will reach the observer's retina: Some will be wasted.

eyepiece

Eyepieces are glorified magnifying glasses used to view the image formed by the objective of a telescope. That image could in principle be cast onto a sheet of paper, placed at the right place in the telescope's optical path (or perhaps captured on a piece of photographic film at the same position). If you will imagine using a magnifying glass to examine the image, on that same piece of paper, you will get an idea of what an eyepiece does -- except, of course, that it does it without the paper.

Eyepieces come in various types. Every eyepiece has a focal length. The magnification that results when a given eyepiece is used with a given telescope, is equal to the focal length of the telescope divided by the focal length of the eyepiece. Thus if the telescope has a focal length of 1000 mm and the eyepiece has a focal length of 25 mm, the magnification will be 1000 / 25, or 40.

eye relief

The distance from the surface of the rearmost lens of the eyepiece, to the exit pupil. When the eyepiece is in use, that distance should be the distance from the rearmost lens of the eyepiece to the iris of the observer's eye. Note that the iris is a few mm behind the front surface of the eye. The remaining distance is the space between the observer's eye and the eyepiece: It is the clearance available for moving the observer's head without bumping the telescope, and is also the place where the observer's spectacles must fit, if they are worn while observing.

f-number

Another label for focal ratio.

Fecker

See J. W. Fecker.

field curvature

An aberration of optical systems, in which the surface on which images in sharp focus are formed is not flat, but curved. Field curvature is also called curvature of field.

field flattener

Field curvature can be corrected by a simple lens placed at or close to the focal "plane" of the instrument in question. Such a lens is called a field flattener. If the field curvature is concave toward the incoming beam of light, the field flattener must be a negative lens. If the field curvature is convex toward the incoming beam of light, the field flattener must be a positive lens. The size of the field flattener must be equal to or slightly greater than the size of the desired flat field.

field glass

See Galilean telescope. Do not see field glasses.

An instrument rather resembling a binocular, but which uses a special eyepiece design, instead of prisms, to achieve an upright image. Field glasses are thereby generally lighter and less expensive than binoculars, but -- a disadvantage of the particular type of eyepiece used -- they have an extremely narrow apparent field of view.

In essence, this instrument consists of two Galilean telescopes mounted side by side, as for a binocular. However, contemporary usage does not apply the term "binocular" to two Galilean telescopes so mounted.

It is uncommon to see field glasses of sufficient optical quality to use for astronomy, though their light weight may make them desirable for use by very young beginners. Field glasses are also in common use as opera glasses, for concerts and the theater.

Note that although two of any other kind of telescope mounted side by side are called a "binocular" -- not a "pair of binoculars"; nevertheless, two Galilean telescopes mounted side by side are called "field glasses" (plural). That is because "field glass" is another name for Galilean telescope. It is perhaps best not to think about what you would do if you needed a short phrase to describe a collection of Galilean telescopes.

finder

A small, low-magnification telescope or sight, equipped with a cross-hair or similar device, to be mounted upon a larger telescope as an aid to locating objects for viewing.

flourite

Library paste used as an optical material. I'm sorry, that was a joke: "Flourite" is a common misspelling of "fluorite". I do not think that there are any commercial optics made from library paste, but some times I wonder.

fluorite

A special kind of optical material, of particular value in making achromats and apochromats. See also fluorite doublet.

fluorite doublet

An achromat in which one lens is fluorite. Fluorite doublets are better than achromats made with conventional glass, by a factor of about eight, at bringing all colors of visible light to the same focal position.

focal length

The focal length of a thin, simple lens is the distance from the lens to the image it forms of an object that is very far away. For more complicated optical systems, the focal length generally does not correspond to any easily measurable distance. It is instead the focal length of a simple lens which would give the same size image of an object that is very far away.

focal plane

The surface on which a lens or objective forms an image: Where you would have to put a piece of paper to catch an image in sharp focus. The term "focal plane" is often a misnomer, for the focal surface is often not flat, but curved.

focal ratio

The ratio of focal length to clear aperture, generally written as the letter "f", followed by a slash, followed by the ratio, like this: f/8. Such an "f/8" system (pronounced "eff eight") has a focal length equal to eight times its clear aperture.

Focal ratios which are large numbers are said to be long or slow; those which are small numbers are said to be short or fast. There are no sharp lines for determining when to use these terms, but among contemporary astronomical telescopes, any telescope with a focal ratio slower (bigger number) than f/12 would probably be said to be slow, and any telescope with a focal ratio faster (smaller number) than f/6, would be said to be fast.

Fast systems often have more aberrations away from the center of the field of view, than do slow ones. Fast systems are generally more demanding of eyepiece designs, than are slow ones: In practical terms, fast systems generally require more expensive eyepiece designs than do slow ones.

focuser

The part of a telescope into which you put the eyepiece, together with whatever arrangement it has for focusing the image.

fork mounting

A telescope mounting in which the optical tube assembly is carried between the tines of a stubby, two-armed fork. The OTA rotates about the bearings which attach it to the arms, and the "handle" of the fork also rotates, thus providing the telescope with the two motions it needs to point in any direction.

If the handle of the fork points parallel to the Earth's axis of rotation, the system is an equatorial fork mounting. If it points vertically, it is an altazimuth fork mounting. A Dobson mounting is a particular type of specialized altazimuth fork mounting, made of specific materials.

fully coated

A marketing term for an optical system in which every optical surface where light passes from glass into air, or from air into glass, is coated with one of the early low-reflection coatings, almost certainly magnesium fluoride. Also see coated, multicoated, and fully multicoated.

fully multicoated

A marketing term for an optical system in which every optical surface where light passes from glass into air, or from air into glass, is coated with one of the newer, high-tech low-reflection coatings, which transmit more light than such earlier ones as magnesium fluoride. Also see coated, fully coated, and multicoated.

Galaxy

United States manufacturer of relatively large optics for amateur-sized Newtonians.

Galilean telescope

The kind of telescope built by Galileo featured a singlet objective and a singlet eyepiece. The eyepiece -- a Galilean eyepiece -- consisted of a so-called "negative" lens, which is what most people think of as the opposite of a magnifying glass. This kind of lens gives a very narrow apparent field of view, but it gives an image that is upright and has the left and right sides correctly positioned; most astronomical telescopes give an image that is upside down, or has left and right reversed.

In Galileo's original telescope, neither of the lenses was an achromat, so the instrument had considerable chromatic aberration. By extension, however, any telescope which uses a negative lens for the eyepiece is called a Galilean telescope, even if it uses achromatic lenses.

Because of their simplicity, Galilean telescopes are very inexpensive to manufacture. They are thus common as toys. Two Galilean telescopes mounted side by side, as for a binocular, constitute field glasses.

Note that although two of any other kind of telescope mounted side by side are called a binocular -- not a "pair of binoculars"; nevertheless, two Galilean telescopes mounted side by side are called "field glasses" (plural). That is because "field glass" is another name for Galilean telescope. It is perhaps best not to think about what you would do if you needed a short label for a collection of Galilean telescopes.

GEM

Abbreviation for German equatorial mounting.

German equatorial mounting

A telescope mounting in which the optical tube assembly is attached at right angles to one of two long, rotating shafts, which are also at right angles to one another. The first shaft -- the right ascension axis or polar axis -- points parallel to the Earth's axis of rotation. The first shaft carries the bearings within which the second shaft rotates. The second shaft -- the declination axis, which carries the telescope -- points toward the celestial equator. Rotation of the polar axis, and of the declination axis about its attachment point to the polar axis, allows the telescope to point in any direction. The end of the declination axis opposite to the telescope carries a counterweight.

ghost images

Ghost images are spurious images caused by unwanted reflections in an optical system. The are sometimes in focus, or nearly so. Several common eyepiece designs are plagued by ghosts. To look at Jupiter, for example, with such an eyepiece, is perhaps to see a second, dimmer, image of the planet in the field of view, changing its relative position with respect to the real image as the telescope is moved.

glasses

An old, and not quite correct, name for a binocular. Also see field glasses.

Goto

Japanese optical manufacturer whose amateur-sized telescopes and accessories were imported into the United States in the mid 20th century. Goto is perhaps better-known for planetarium projectors. There is occasional confusion between "Goto", the manufacturer, and "goto", meaning a computer-controlled telescope with a command interface in which the observer tells the telescope what object to "go to" next.

goto, or go-to

Shorthand for the kind of command interface to a computer-controlled telescope, in which the observer tells the telescope what object to "go to" next. Not to be confused with the optical manufacturer, "Goto".

Gregorian

In the strictest sense, a Gregorian telescope is a reflecting telescope that uses two specific shapes of concave mirrors to form the image: The primary mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller concave ellipsoid, positioned a little way beyond the primary's focal plane. The secondary reflects the beam of light diverging from the focus, back toward the primary, which typically has a hole in the center, so that the focal plane of the combined mirrors is easily accessible behind the primary.

More loosely, the term "Gregorian", or "Gregorian configuration", or "Gregorian system", has come to be applied to any telescope in which a large primary mirror and a small, extra-focal, concave secondary mirror are among the elements used to form the image. Gregorian systems are uncommon among contemporary astronomical telescopes: Celestron briefly manufactured a 66 mm aperture Gregorian Maksutov, but I cannot think of any other recent commercially available units.

It is common to confuse Gregorian telescopes with the Maksutov designs by John Gregory. The latter are correctly called "Gregory Maksutovs", but they are not Gregorian configurations; they are Cassegrain configurations.

Gregory Maksutov

Any one of several Cassegrain configuration Maksutov designs by John Gregory. These all feature a secondary mirror which is merely an aluminized spot on one surface of the Maksutov corrector.

It is common to confuse Gregory Maksutovs with Gregorians: They are not the same.

Intes

Russian manufacturer of telescopes and components. Actually, there are two closely related businesses, Intes and Intes Micro -- I lump them here together because of the similarity in names. Both are noted for high-quality Maksutov telescopes.

helical focuser

A telescope focuser in which adjustment of focus is made by screwing in and out either the eyepiece itself, or a focus tube which contains it. Many binoculars have a helical focuser for one or both eyepieces.

Huygenian eyepiece

A specific eyepiece design. Same as Huygens eyepiece.

The Huygenian is among the oldest of eyepiece designs. It has a narrow apparent field of view -- perhaps only 30 degrees -- and works well only with telescopes which have quite long focal ratios. Many inexpensive small refractors come equipped with Huygenian eyepieces, which not only have the faults just listed, but are also often not very well made. However, a decent Huygenian -- if you can find one -- will work reasonably well with focal ratios of perhaps f/15 or more.

The simplest form of Huygenian eyepiece is composed of two simple lenses, each flat on one side and convex on the other. The lens nearest your eye is smaller and has a shorter focal length than the other. The flat sides of both lenses are toward your eye. Even though neither of the lenses is an achromat, Huygenian eyepieces do an excellent job of correcting lateral color.

Huygens eyepiece

A specific eyepiece design. Same as Huygenian eyepiece.

J. W. Fecker

Former United States manufacturer of telescopes, mid twentieth century. Fecker sold both to amateurs and professionals.

Jaegers

See A. Jaegers.

Jason

Major United States importer of low-end telescopes and other optical goods. Jason's low-end products are inadequate for serious astronomy, and are often overpriced by unscrupulous retailers. Some of its better products are perhaps acceptable for beginner use.

Kellner eyepiece

A specific eyepiece design. In essence, the Kellner is an achromatized Ramsden. It has a slightly larger apparent field of view than the Ramsden, and works at slightly faster focal ratios. Kellners tend to have rather prominent ghosts.

Kellner eyepieces consist of a small achromat -- a cemented doublet -- near your eye, and a simple lens at the far end of the eyepiece.

kidney-bean effect

A popular name for an image defect seen when using an eyepiece which (1) has an exit pupil almost as large as the pupil of the user's eye, and (2) is afflicted with an uncommon aberration called "spherical aberration of the exit pupil". The presence of that aberration, in essence, means that you must hold your eye a distance from the eyepiece that varies, depending on whether you want to look at things near the center of the field, or near the edge. Having your eye in the wrong place, and a little off center, as well, results in the appearance of a dark, kidney-bean shaped patch, part way out to the edge of the field. The only common eyepiece type in which this effect is noticeable is the early version of the Tele Vue Nagler.

Koenig eyepiece

A family of eyepiece designs. Koenigs have a rather wide apparent field of view -- perhaps as much as 70 degrees.

The various eyepieces commonly labeled "Koenig" contain anywhere from four to seven simple lenses, grouped into various combinations of cemented doublets and singlets.

"Koenig" is sometimes spelled "Konig".

Lanthanum LV eyepiece

A specific eyepiece design. Lanthanum LV is a "house brand" of the large Japanese optical manufacturer, Vixen. The Lanthanum LV eyepiece series is particularly noted for having a long eye relief of 20 mm, even for the shortest focal lengths.

At least some of the Lanthanum LV eyepieces are composed of seven simple lenses, cemented together into three doublets and a singlet. The frontmost doublet consists in essence of a built-in Barlow lens. The rest of the eyepiece comprises two doublets separated by a singlet.

lateral color

Another name for chromatic difference of magnification.

light-pollution reduction filter

A filter specially prepared to reflect wavelengths which are prominent in light pollution, and transmit other wavelengths. Such a filter improves the contrast of deep-sky objects, when light pollution is present.

What makes good light-pollution reduction filters possible is that much light pollution comes from mercury-vapor and sodium-vapor street lights, which emit preferentially in narrow spectral ranges.

limiting magnitude

The magnitude of the faintest stars which can be seen with a given optical system. It depends on the aperture, magnification, sensitivity of the observer's eye, sky transparency, light pollution, and steadiness of the air.

longitudinal chromatic aberration

An aberration of optical systems, in which the focal length of a system is different for different colors of light.

If you were to try to focus the image of a white star, in a telescope in which the only aberration was longitudinal chromatic aberration, your eye -- being most sensitive to green light -- would lead you to position the focuser so that green light was in focus. At that point, the red and blue light from the star would not be in focus, but would merge to form an out-of-focus violet halo about the star.

longitudinal color

Another name for longitudinal chromatic aberration.

Losmandy

United States manufacturer of well-machined, high-quality, German equatorial mountings, in several sizes, with diverse accessories and attachments.

low-reflection coating

Any of several coating materials whose purpose is to reduce unwanted reflections from optical elements which are supposed to transmit light. These optical elements include lenses and prisms.

When a beam of light passes from air into glass, or from glass into air, most glasses transmit only about 96 percent of the light, and reflect the rest. The first coating material that was widely used commercially, magnesium fluoride, increased the transmission to 98 percent. More modern coatings, comprised of multiple layers of material (see multicoated), transmit 99 percent or more.

There is a great deal of marketing gibberish obscuring the kind and quality of coatings in use on commercially-available optics. See coated, fully coated, multicoated, and fully multicoated.

LPR filter

Abbreviation for a light-pollution reduction filter.

MA eyepiece

A specific eyepiece design. MA is a "house brand" of Meade. "MA" perhaps means "Modified Achromatic". These inexpensive eyepieces work reasonably well with longer focal ratios.

These eyepieces are similar in construction to Kellners.

Mag 1 Instruments

United States manufacturer of medium-sized amateur-size truss-tube Newtonian telescopes, on an innovative and very portable mounting.

magnification

The magnification that results when a given eyepiece is used with a given telescope, is equal to the focal length of the telescope divided by the focal length of the eyepiece.

Mak

Short for Maksutov.

Maksutov

Any optical system using a Maksutov corrector.

There are many commercially available Maksutovs, most commonly of Cassegrain or Newtonian configuration.

Maksutov corrector

A particular kind of lens used in Maksutov optical systems. Maksutov correctors are thick, and have strongly curved, nearly concentric, surfaces. They resemble large, thick watch-glasses, or certain styles of art-deco ash trays.

Masuyama eyepiece

A five-element eyepiece sometimes described as a "hybrid Plossl", with better performance away from the center of the field. I believe the design consists of two doublets with a singlet in between, but I am not sure.

Meade

Major United States manufacturer and importer of telescopes and accessories. Meade got its start as an importer of small telescopes and components in the 1970s, then expanded into Newtonians, and finally took off big-time as a competitor to Celestron in the Schmidt-Cassegrain market. Current product line includes a variety of Schmidt-Cassegrain, Newtonian, Maksutov-Cassegrain, and refractor telescopes, as well as numerous accessories.

mirror diagonal

A star diagonal which uses a mirror.

monocentric eyepiece

A class of related eyepiece designs. Monocentrics tend to have very narrow apparent fields of view, and prominent ghost images, but they have only two air/glass interfaces, hence very little scattered light. Good ones therefore make nice eyepieces for Lunar, planetary, and double-star work.

All the types of monocentric eyepiece that I know of consist of three simple lenses, cemented together into a triplet.

monocular

A spotting telescope, which resembles half a binocular. Monoculars are often hand-held, and are used in much the same way, and for the same purposes, as binoculars.

mounting rings

Circular clamps which fasten around an optical tube assembly and are threaded, machined, or otherwise prepared to fasten to a mounting. Also called tube rings.

multicoated

"Multicoated" describes a lens or prism in which the surfaces where the beam of light goes from air into glass, or from glass into air, has been coated with a high-tech, many-layered coating that decreases the amount of light that gets reflected. That reflected light is lost to the image, and may cause glare, so reducing it is desirable. The coatings that do this are more correctly called low-reflection coatings -- for after all, mirrors are coated, too, but with something that is supposed to increase reflectivity rather than reduce it.

There is another meaning, that has to do with marketing terminology. If you bought a binocular that had "multicoated optics", you might naively think that all the surfaces where a beam of light went from air into glass, or from glass into air, were coated with fancy low-reflection coatings, right? Wrong! The common usage in optical advertising is that "multicoated optics" merely means that some of those surfaces are so coated -- perhaps only one surface! To get what you thought you were buying, you would have to seek a binocular that was "fully multicoated". Also see coated and fully coated.

Nagler eyepiece

A specific eyepiece design. Nagler is a "house brand" of Tele Vue. These eyepieces are noted for a very wide apparent field of view -- 82 degrees -- and for excellent correction at fast focal ratios. Naglers are big, heavy, and expensive, and consist of seven or eight simple lenses grouped together into four singlets or doublets.

Newtonian

A simple and common design of reflector. The objective is a concave paraboloid at the lower end of the tube. A flat diagonal mirror near the upper end of the tube reflects the converging beam over to the side of the tube, where the focuser is mounted.

Nikon

Japanese manufacturer of optical goods. Products for amateur astronomers include several lines of high-quality eyepieces.

United States manufacturer of relatively large optics for amateur-sized Newtonians.

object glass

An old name for the objective, particularly of a refractor.

objective

Loosely, the big mirror or big lens of a telescope. Some times it is a little hard to tell what optical components make up the objective and what are accessories or auxiliaries. Is a Schmidt corrector plate or a Maksutov corrector part of the objective?

Obsession

United States manufacturer of relatively large amateur-sized truss-tube Dobson-mounted Newtonians.

obstruction ratio

In telescopes which have a diagonal mirror or a secondary mirror so positioned as to obstruct part of the incoming beam of light, the presence of that obstruction reduces the contrast of the image somewhat. The amount of contrast degradation depends on the relative size of the obstruction, compared to the clear aperture of the telescope. The question is, what number do we use to quantify that proportion? The big issue is, do we report the ratio of areas, or the ratio of diameters?

In a certain sense, it doesn't matter. Thus, a telescope with a clear aperture of 150 mm, having an obstruction that is 50 mm in diameter, experiences the same loss of contrast whether we report the obstruction as 0.33 (ratio of diameters) or 0.11 (ratio of areas). The important thing is to make sure to specify which of the two ratios the reported number actually is.

Historically, the usual quantity to report was the ratio of diameters. Then various manufacturers started reporting the ratio of areas, as a marketing gimmick: After all, an obstruction ratio of 0.11 sounds smaller than one of 0.33, even if we are describing the same 150 mm aperture telescope with its 50 mm obstruction, and particularly if we forget to specify which ratio we are talking about.

It is probably simpler to report the ratio of diameters, if only to save readers the possible effort of doing a square root if they are given the ratio of areas and want to know the ratio of diameters. Furthermore, most of the technical literature that describes the effect of obstructions on contrast uses the ratio of diameters as a parameter in things like equations and graphs.

off-axis

Said of a telescope whose optical elements (not counting simple flat mirrors and prisms) are not mechanically symmetric about a single optical axis. The common types are reflectors, in which the primary mirror is tilted, so as to deflect the reflected beam off to the side, so that other optical components, which might otherwise partially obscure the incoming beam, will not do so.

Many off-axis telescope designs are hard to make.

See, for example, the Schiefspiegler or the tri-Schiefspiegler.

opera glasses

Small field glasses for use at concerts or the theater.

optical tube assembly

The tube that holds the optics, also called an OTA. An optical tube assembly generally consists at minimum of all the optics it takes to form an image (but not necessarily any eyepieces or star diagonal), plus mechanical parts to hold them in place, correctly positioned with respect to one another, plus some sort of focuser. Optical tube assemblies often also come with finders, tube rings, and perhaps other gadgets.

The optical tube assembly for a refractor of the traditional style resembles a spy-glass. For a Newtonian, it resembles a water heater. For a Schmidt-Cassegrain or a Maksutov-Cassegrain, a waste basket.

Orion

United States manufacturer and importer of telescopes, mountings, and accessories, many of which are sold under their own "Orion" label. Current products include Maksutovs, refractors, Newtonians, and diverse accessories.

orthoscopic eyepiece

A specific eyepiece design. Orthoscopics have moderate apparent fields of view -- 40 or 45 degrees -- and work pretty well at fast focal ratios. Many consider them the best eyepieces for Lunar, planetary, and double-star work.

There are actually several designs called "orthoscopic". The most common kind has a simple lens nearest your eye, and a cemented triplet further away. Another kind resembles a Plossl.

OTA

Abbreviation for optical tube assembly.

Panoptic eyepiece

A specific eyepiece design. Panoptic is a "house brand" of Tele Vue. These are well-corrected eyepieces with large apparent fields of view (68 degrees), that work well at fast focal ratios.

I have never taken a Panoptic apart, but they are rumored to consist of six or seven simple lenses, grouped into singlets and cemented doublets.

Parks

Former United States manufacturer of amateur-sized telescopes and accessories, mid to late twentieth century. Particularly noted for Newtonians.

Pegasus

United States manufacturer of relatively large optics for amateur-sized Newtonians.

pincushion distortion

An aberration of optical systems, in which magnification increases with distance away from the optical axis. With such a system, squares are imaged with their sides curved inward, looking sort of like pincushions. (Actually, the points of the squares are imaged a little bit further out than where they ought to be, because of the increased magnification in the outer part of the field.) Also see barrel distortion.

Pentax

Japanese manufacturer of optical goods. Products for amateur astronomers include a line of high-quality refractors, and several lines of high-quality eyepieces.

Plossl eyepiece

A specific eyepiece design. Plossls have moderate apparent fields of view -- 50 degrees is typical -- and work pretty well at fast focal ratios.

Plossls consist of four simple lenses, grouped as two cemented doublets.

polar alignment

The process of getting the axis of an equatorial mounting that is supposed to point parallel to the Earth's axis of rotation, to do so.

By extension, the process of orienting a telescope with a computer-controlled altazimuth mount to the sky around it. Such a telescope doesn't need to have particular mechanical parts pointing in particular directions, but it does need to know which way is north and which way is up, and can usually figure those things out by being pointed to several known and specified stars in succession.

polar-alignment telescope

A small telescope permanently attached to an equatorial mounting, aligned with the mounting axis that is supposed to point parallel to the Earth's axis of rotation. The polar-alignment telescope probably has markings visible through its eyepiece which can be aligned with stars near the north or south celestial poles, to aid in performing a polar alignment.

primary mirror

The big concave mirror of one of those telescope types that uses a big concave mirror.

prime focus

The meaning of this term is changing. It used to apply only to reflecting telescopes in which the primary mirror was a paraboloid, and thus capable of forming an image all by itself. Some such telescopes were built so that film or some other equipment could be placed inside the tube, or perhaps a bit beyond the upper end, to capture light that had reflected only off the primary mirror. That location was called the "prime focus".

In some very large professional telescopes, there was room inside the tube for an enclosed space where an observer could sit, to guide the telescope when the prime focus was in use. Yet even much smaller telescopes were sometimes equipped to use the prime focus, using extra optics and what-not to allow guiding from the side of the tube.

Other focal positions were named by the design of the optical system that illuminated them -- the Newtonian focus for a Newtonian, the Cassegrain focus for a Cassegrain, and so on. One also heard the term "prime focus" used for refractors, when film or equipment was placed in the focal plane of the objective.

In many modern amateur telescopes, the primary mirror is not capable of forming an image all by itself, or even if the Schmidt corrector plate or Maksutov corrector is included, assuming there is one. Besides, "Gregory-Maksutov-Cassegrain focus", and the like, is a mouthful. Perhaps for such reasons, the term "prime focus" is increasingly being used in a more general manner: In systems in which there is only one place where there is a focuser -- where an image is formed and you can get at it with an eyepiece or some other equipment -- that location is called the "prime focus".

Note that there are systems in which the image can be accessed at more than one location. For example, reflecting telescopes are still built with paraboloidal primary mirrors, which have interchangeable flat diagonal mirrors and hyperboloidal secondary mirrors, with a focuser installed both at the Newtonian focus and the Cassegrain focus. For these systems, the term "prime focus" often indicates the Newtonian focus.

prism

An odd-shaped hunk of glass, often more or less triangular in one cross-section, used to bend or reflect light internally.

prism diagonal

A star diagonal which uses a prism.

Quantum

Line of Maksutov telescopes, manufactured by OTI in the late 1970s and 1980s. Noted for aesthetics, fit and finish.

quarter-wave optics

See the discussion under Rayleigh criterion.

quartz

In telescope optics, quartz generally does not refer to the crystalline material found in nature, but to a glassy substance, fused quartz, formed by melting down lots of quartz crystals (e.g., very pure sand) in an oven. Quartz has thermal properties that make it valuable for lenses and mirrors alike, and optical properties that are of particular use in certain kinds of systems, even those which don't need its thermal properties. In particular, very fine achromats can be made from quartz and fluorite.

Questar

United States manufacturer of Maksutov telescopes, founded in the 1950s. The 3.5-inch Questar has been known for high mechanical and optical quality, immaculate fit and finish, and elegant aesthetics, for some fifty years.

rack and pinion focuser

A focuser in which the tube containing the eyepiece has a straight row of gear teeth (the rack) running along one side of it, which are engaged by a more conventional-looking gear (the pinion), attached to a knob, in order to adjust the focus.

Radian eyepiece

A specific eyepiece design. Radian is a "house brand" of Tele Vue. These are well-corrected eyepieces with fairly large apparent fields of view (60 degrees), that work well at fast focal ratios. Radians were originally intended to have apparent fields of view of one radian -- 57.3 degrees -- but that parameter was increased a little before they were introduced.

I have never taken a Radian apart, but they are rumored to consist of six or seven simple lenses, grouped into singlets and cemented doublets.

Ramsden eyepiece

A specific eyepiece design. The Ramsden is a very old design, with a rather narrow apparent field of view -- perhaps as little as 30 degrees. Ramsdens do not work well at focal ratios shorter than about f/9, but good ones make surprisingly nice eyepieces for Lunar, planetary, and double-star observation, at longer focal ratios. Ramsdens often have prominent ghosts.

The simplest form of Ramsden consists of two identical simple lenses, each flat on one side and convex on the other, with convex sides facing each other, spaced apart by a distance equal to or slightly less than their focal length. Even though neither of the lenses is an achromat, Ramsden eyepieces do an excellent job of correcting lateral color.

Rayleigh criterion

The famous physicist, Lord Rayleigh, asserted that it would be difficult to tell that an optical system was less than perfect, if (1) it was well-enough figured that as a converging wavefront of light approached focus, at most a small part it departed from the theoretically correct shape, (2) the maximum difference in deviation from perfection, measured between any two points on the wavefront, did not exceed a quarter of the wavelength of light in question, and (3) the deviation was smooth.

Rayleigh was a smart man: Optics which meet this criterion are indeed difficult to distinguish from optics which are truly diffraction limited. However, optics good enough to meet the Rayleigh criterion are far less common than many marketers of telescopes would have you believe. The business about "only a small part", often gets forgotten, as does the business about "smooth", and whereas Rayleigh specified a measurement of error which in modern terms is called "peak to valley, on the wavefront", optical accuracies are often reported on the optical surfaces, or as plus/minus (+/-) or root-mean-square (RMS) deviations. All of these other kinds of report result in a smaller numerical value of the deviation, than would be measured peak-to-valley, on the wavefront. The discrepancy can be as much as a factor of six. Thus the common term "quarter-wave optics" means exactly nothing -- and probably, not what you hope it means -- unless it also includes some indication of what measurement is reported.

reflector

A telescope in which only mirrors are used to form the image. The eyepiece doesn't count, and if there are any prisms used to redirect the beam of light, they don't count either.

reflex finder

See unit-magnification finder.

reflex sight

See unit-magnification finder.

refractor

A telescope in which only lenses are used to form the image. If there are any mirrors or prisms used in the system, whose sole purpose is to redirect the beam of light, they don't count: It's still a refractor.

refractorholism

Acute and disabling refractoritis. E.g. ...

"I'm Richard, and I'm a refractorholic."

"Hi, Richard!"

refractoritis

An amateur astronomer who is fond of refractors to the point of obsession, is said to have refractoritis. The disease is expensive to cure, particularly if it is combined with aperture fever.

resolution

Loosely, the ability of a telescope to show detail. Also known as resolving power. One common way to describe the resolution of a telescope is to state the minimum angular separation at which a double star, whose two components are fairly bright and have very nearly the same brightness, can be distinguished as two separate stars. See Dawes limit.

resolving power

Loosely, another word for resolution.

right ascension

One of two coordinates for the celestial sphere, which are analogous to latitude and longitude for the Earth's surface. The right ascension of an object is how far it lies east of a rather arbitrarily chosen "prime meridian" in the celestial sphere.

Because the Earth rotates in twenty-four hours, right ascension is not measured in degrees, but in hours and fractions thereof: An hour of right ascension corresponds to fifteen degrees of "longitude" upon the celestial sphere. The other coordinate is called declination.

The "prime meridian" passes through the position of the Sun at the time of the vernal equinox. Thus its position changes slowly over the years, due to the precession of the equinoxes. The position of the celestial poles also changes with precession. Thus, to locate an object from its right ascension and declination, you must also know the date for which those coordinates are valid; that date is called the epoch of the coordinates.

Ritchey-Chretien

A telescope closely resembling a classical, two-mirror-only Cassegrain, except that the primary mirror is an hyperboloid -- slightly more strongly figured than the Cassegrain's paraboloid -- and the secondary is a slightly stronger hyperboloid than that of the Cassegrain. Ritchey-Chretiens are corrected for coma as well as for spherical aberration; thus they can deliver relatively sharp images across a wider field of view than can Cassegrains. For that reason, many of the large reflectors used by professional astronomers have been made as Ritchey-Chretiens, and at focal ratios suitable for photographic work.

Schiefspiegler

A particular type of off-axis compound telescope: Specifically, an off-axis Cassegrain.

There are actually several kinds of Schiefspiegler, but they are usually described with additional adjectives or nomenclature, as in the tri-Schiefspiegler.

The original Schiefspiegler has a primary mirror with a relatively long focal ratio. Schiefspieglers are awkward and odd-looking -- some common ways to mount the optical parts result in telescopes that resemble "washtub bass" musical instruments, or perhaps certain kinds of hand-held garden sprayers. (For that reason, they have occasionally been waggishly referred to as "Sheepsprinklers".) They are sometimes hard to keep in collimation, too. Notwithstanding, they can deliver excellent images.

Schmidt camera

One of several related types of photographic telescope. The simplest design consists of a concave spherical primary mirror with a Schmidt corrector plate at its center of curvature. Schmidt cameras usually have extremely fast focal ratios, and provide very high-quality images. The focal "plane" of the simplest design is not flat, but strongly curved, and is furthermore positioned well inside the body of the camera, where it is difficult to reach with an eyepiece. Hence the system is almost always used as a camera.

Schmidt corrector plate

The key piece of any Schmidt-type optical system, such as a Schmidt camera, a Schmidt-Cassegrain, or any of many others. To the naked eye, a Schmidt corrector plate looks like a flat piece of glass, but its surface is actually figured in a subtle manner, to affect the spherical aberration of the system in which it is used.

Schmidt-Cassegrain

A Cassegrain configuration telescope whose optics consist of a concave primary mirror, which is spherical in most of the common commercial designs, a full-aperture Schmidt corrector plate mounted near the focal point of the mirror, and a small, convex secondary mirror positioned in the converging beam, as in a true Cassegrain.

The development of techniques to manufacture Schmidt corrector plates of reasonable quality (not everyone would call the quality "reasonable"), in the 1960s, revolutionized amateur astronomy, by making available at not too high a price telescopes which were were relatively compact and portable: A Schmidt-Cassegrain optical tube assembly is lots more compact than that of a Newtonian or refractor of the same aperture.

SCT

Abbreviation for Schmidt-Cassegrain telescope.

secondary mirror

In telescopes which use more than one curved mirror to form the image, the one which the light beam reaches second is called the secondary mirror. It is usually smaller than the primary mirror.

semi-APO

Abbreviation for semi-apochromat.

semi-apochromat

Marketing term intended to convey the impression that a telecope is particularly well corrected for longitudinal chromatic aberration. There is no generally accepted technical definition for semi-apochromat. It is probably wisest to assume that the phrase is meaningless.

separated doublet

See doublet.

setting circles

Calibrated discs -- in essence, protractors -- attached at the axes of an equatorial mounting and reading in celestial coordinates (right ascension and declination), to facilitate pointing the telescope to a particular place in the sky. Also see digital setting circles.

sidereal drive

"Quick, Chewie, the jump to lightspeed!" Sorry, that was a joke. Actually, a mechanism used to make a telescope follow stars across the sky as the Earth rotates.

Modern sidereal drives for small telescopes are generally electrically powered and electronically regulated; older variants have used (a) synchronous motors, with regulation provided by the number of cycles per second of alternating-current line voltage, (b) clockwork mechanisms with a pendulum or governor for regulation, (c) clepsydra-style devices, with regulation provided by the slow leakage of water, sand, or compressed air from a container, and (d) hired help, turning a crank on the sidereal drive in precise synchronization with the ticking of a clock. And that last was not a joke.

singlet

One simple lens used all by itself. Contrast with doublet and triplet.

SMA eyepiece

A specific eyepiece design. SMA is a "house brand" of Celestron. "SMA" perhaps means "Super Modified Achromatic". These inexpensive eyepieces work reasonably well with longer focal ratios.

These eyepieces are similar in construction to Kellners.

Speers-Waler eyepiece

A specific eyepiece design, noted for a very wide apparent field of view -- almost 80 degrees -- and for excellent correction at fast focal ratios. I have heard that "Waler" is an acronym for "Wide Angle, Long Eye Relief".

Speers-Waler eyepieces are reported to be similar to Naglers.

spherical aberration

An aberration of optical systems, in which the focal length of a lens or mirror varies with distance sideways from its center. That is, if you were to place different annular masks -- discs of paper with a ring cut out -- squarely in front of such a lens or mirror, you would find that the focal length you measured depended on the size of the ring.

spherochromatism

An aberration of optical systems, in which spherical aberration is different in different colors of light. That is, if you performed a knife-edge or star test of a system with spherochromatism, you would get different results in different colors of light.

Spherochromatism is also called chromatic variation of spherical aberration.

spider

The arms or struts, if any, which support a diagonal mirror or a secondary mirror in position. That "if any" is there because in some telescopes, the auxiliary mirror is attached to a Schmidt corrector plate or to a Maksutov corrector, or is an aluminized spot on a Maksutov corrector.

star diagonal

A gadget resembling an plumbing "elbow" fitting, containing a mirror or prism, attached to a downward-pointing focuser to redirect the beam of light upward for more convenient viewing. That is, a star diagonal fits into a focuser, has an eyepiece fitted into it, and bends the light path through a right angle.

star test

An optical test of a telescope performed using a star as the light source.

StarMaster

United States manufacturer of relatively large amateur-sized truss-tube Dobson-mounted Newtonians.

Starsplitter

United States manufacturer of relatively large amateur-sized truss-tube Dobson-mounted Newtonians.

Super Wide Angle eyepiece

A specific eyepiece design. Super Wide Angle is a "house brand" of Meade. These eyepieces are well corrected, with large apparent fields of view (67 degrees).

Super Wide Angle eyepieces are reported to be generally similar in design to Panoptics or Erfles.

symmetrical eyepiece

A specific eyepiece design. The optics of a symmetrical eyepiece comprise two cemented doublets oriented face-to-face. Many designs sold as Plossl eyepieces are actually symmetrical: True Plossls are not symmetrical.

Takahashi

Japanese manufacturer of amateur-sized telescopes and accessories. Takahashi is particularly noted for its refractors featuring fluorite doublets, and for its no longer produced 225 mm Schmidt-Cassegrain. Takahashi eyepieces are well-regarded for planetary work.

Tasco

Major United States importer of low-end telescopes and other optical goods. Tasco's low-end products are perhaps inadequate for serious astronomy, and are often overpriced by unscrupulous retailers. Some of its better products are perhaps acceptable for beginner use. In years gone by, Tasco has upon occasion imported very good small refractors of 50 mm to 75 mm clear aperture.

Tele Vue

United States manufacturer of telescopes and accessories. Tele Vue is noted for several lines of eyepieces, including Naglers, Panoptics, and Radians, and for several small, high-quality refractors.

telecompressor

The opposite of a telextender or a Barlow lens. A telecompressor multiplies the focal length of a telescope by a factor which is less than one, thus reducing the focal length. Its purpose is generally to improve the photographic speed of a system, so that the time exposures required to take pictures are not so long.

TelRad

A particularly popular brand of unit-magnification finder.

telextender

Another name for a Barlow lens.

Tinsley

Former United States manufacturer of telescopes, mostly of professional size, mid to late twentieth century. Tinsley also produced refractors and Cassegrain telescopes in size ranges used by amateurs.

tri-Schiefspiegler

A common name for several related designs of off-axis telescope, all related to the Schiefspiegler, but using three mirrors instead of two.

triplet

Three simple lenses used in combination, placed close together or in contact.

truss-tube Dobson

A Dobson-mounted telescope in which the telescope tube is a truss assembly, perhaps looking like part of a bridge, or a bird cage, or the skeleton of a rather baroque telephone booth. Truss tubes are often designed for quick and easy assembly and disassembly in the field, and the disassembled parts are not very bulky. Thus a quite large telescope may be transported in modest space.

Other types of telescopes than ones with Dobson mountings can, and do, have truss tubes, but the truss-tube Dobson has become sufficiently popular -- and big ones are sufficiently eye-catching -- to deserve a separate entry in this glossary.

truss-tube Dobsonian

See truss-tube Dobson.

tube rings

Another name for mounting rings.

Ultima eyepiece

A specific eyepiece design. Ultima is a "house brand" of Celestron, who use it to label many other products besides eyepieces. Ultima eyepieces have moderate apparent fields of view, of about 50 degrees.

Ultima eyepieces are reported to consist of four to seven lenses, grouped into various combinations of singlets and cemented doublets.

Ultra Wide Angle eyepiece

A specific eyepiece design. Ultra Wide Angle is a "house brand" of Meade. These eyepieces are well corrected, with very large apparent fields of view, of 84 degrees.

Ultra Wide Angle eyepieces are reported to be generally similar in design to Naglers.

UltraScopic eyepiece

A specific eyepiece design. UltraScopic is a "house brand" of Orion. These eyepieces have moderate apparent fields of view, generally of 52 degrees.

UltraScopics are reported to consist of five or seven simple lenses, grouped into a singlet and either two or three cemented doublets.

unit-magnification finder

A finder which operates without magnification, as do many rifle 'scopes. Such a device is in essence a miniature head-up display: It projects a cross-hair, illuminated bulls-eye, or similar pattern on the sky, where the observer may view it easily with both eyes open. Unit-magnification finders make it very easy to find objects which are bright enough to be seen with the naked eye, or which are close enough to naked-eye stars.

Unitron

Japanese manufacturer of amateur-sized classic refractors and accessories. Unitron's heyday was in the 1950s and 1960s, but the company is still around.

Vernonscope

United States manufacturer of telescopes and accessories. Vernonscope is particularly noted for the Brandon product line, especially its eyepieces, but has also manufactured excellent small refractors from time to time.

Once upon a time, there actually was a separate company named Brandon, but Vernonscope bought them out. What's more, Vernonscope was recently bought by Gould Precision Optics, Inc., who continue to advertise its products under the "Vernonscope" name.

Vixen

Japanese manufacturer of telescopes, accessories, and other optical products. Vixen is particularly noted for refractors with fluorite doublets, and for Lanthanum LV eyepieces. Vixen has manufactured equipment for the Celestron and Orion labels.

Zeiss

See Carl Zeiss.

zero-power finder

A common misnomer for a unit-magnification finder (which has a magnification of unity, not zero).

zoom eyepiece

Any eyepiece whose focal length can be varied, perhaps by twisting a knurled ring on the barrel. Many do not work well at all, though several recent models are notable exceptions to this rule.