A Short Glossary of Common Telescope Terms



Altazimuth Mount: The simplest type of telescope mount, with only up/down (altitude) and left/right (azimuth) motions. Primarily for low to medium power Dobsonian telescopes and terrestrial (spotting scope) use, as it cannot easily follow at high powers the seemingly curved paths taken by celestial objects as they cross the sky and cannot be used for photography.

Aperture: The diameter of the main mirror or objective lens of a telescope. In general, the larger the aperture, the better the resolution and the fainter the objects you can see.

Astronomical Binoculars: Large aperture binoculars offer what a telescope seldom can -- a wide field of view. Binoculars will encompass celestial objects covering relatively broad areas of sky, such as the Andromeda Galaxy (M31), without having to 'sweep'; while the one-degree field of the average telescope will show them only in segments. Generally speaking, binoculars with apertures of 50mm and larger, and magnifications of 10x and higher, are best suited for astronomical observing outside the solar system. 50mm binoculars possess a light-gathering capacity equivalent to a 70mm refractor at the same power. 80mm binoculars will gather as much light as a 4.5" reflector. You should aim for a binocular exit pupil of 4mm to 5mm if you're in your 40's or older (10 x 50mm, 16 x 70mm, or 20 x 80mm binoculars, for example). Binoculars with 6mm to 7mm exit pupils (a 7 x 50mm or 8 x 50mm binocular, for example) are better suited to late adolescents, whose eye-pupils generally can still expand enough in dark skies to take in all the light from these larger binocular exit pupils.

Averted Vision: At night, the periphery of the eye's retina is more sensitive to faint light than the center (which is more specialized for observing color and detail in brightly lit objects). Looking slightly to one side of a faint object (averting your vision), so that the faint light falls on the more sensitive outer part of the retina, usually reveals the object more clearly than looking directly at it.

Cassegrain: Any telescope that folds the light path and directs it through a hole in the center of the primary mirror (called the Cassegrain focus) at the bottom of the telescope.

 

Catadioptric Telescope: A telescope that uses a combination of mirrors and lenses to increase the effective focal length of the telescope while allowing it to be folded into a more convenient and compact size. The use of a full-aperture correcting lens in these scopes virtually eliminates spherical aberration, chromatic aberration, and coma. The word catadioptric is derived by combining the term for an optical system that forms images by using mirrors (catoptric) with the one for a system that uses lenses (dioptric). The most popular catadioptric designs are the Schmidt-Cassegrain and Maksutov-Cassegrain.

Chromatic Aberration: An optical defect in refractor telescopes in which all colors of light do not come to a focus at the same point, causing stars and planets to have a faint violet halo which masks planetary details and makes it difficult to split faint binary stars. Also known as spurious or secondary color.

Coma: An optical defect in reflector telescopes in which in-focus star images appear progressively more triangular or comet-like the closer they get to the edge of the field of view. The faster the focal ratio, the more prominent the coma.

The visually coma-free field of a telescope in millimeters is roughly equal to the square of the scope's focal ratio - an f/5 focal ratio scope has a 25mm field (5 squared = 25), an f/6 scope has a 36mm field (6 squared = 36), etc. Since a 1.25" eyepiece barrel only about 29mm in internal diameter, and a 35mm film negative or slide measures 44mm across its diagonal, it can be seen that even a 25mm coma-free field is more apparent in photos than it is in most visual observing. Coma can superficially appear similar to a star's image in a poorly collimated telescope. With coma, however, the brightest portion of the comatic wedge (actually the Airy disk) always points toward the center of the field. This differs from an out-of-collimation telescope, where the Airy disks are all offset to the same side of the diffraction rings, no matter where in the field the star image is located.

Curvature of Field: An optical defect in which objects at the edge of the field of view can't be brought into sharp focus at the same time as objects in the center, and vice versa.

Dawes' Limit: If two equally-bright stars are so close together that their Airy disks overlap, they will be seen as one star, although perhaps as an elongated one. If, however, the Airy disk of one star falls in the first dark diffraction ring of the second, each star can be seen - not as two distinct points, but as a Figure 8 as shown below, in which the intensity of light between the two touching disks drops by a clearly visible 30%. The smallest separation between two stars which shows this 30% drop was empirically determined by English astronomer William R. Dawes (1799-1868, and known as the "eagle-eyed" for his acute vision) to be 4.56 arc seconds divided by the aperture of the telescope in inches. The larger the telescope aperture, the smaller the separation that can be resolved. Dawes' limit (determined by testing the resolving ability of many observers on white star pairs of equal magnitude 6 brightness) only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as planets. For example, Cassini's Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5" telescope - which has a Dawes' limit of 1.8 arc seconds! The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer's visual acuity, than it is by the optical quality of the telescope. The illustration below simulates the visual appearance of a pair of equal magnitude stars separated by the Dawes' limit for the telescope being used to make the observation. As you can see, even though the telescope is resolving to Dawes' limit, the binary pair is not cleanly split into two separate stars, but appears as a Figure 8 in shape.

Declination: The angular distance of a celestial object north or south of the celestial equator, measured in degrees. One of the two coordinates (right ascension is the other) that let you find celestial objects with the aid of a star chart and telescope setting circles. Called declination because stellar positions in degrees "decline" or decrease in numerical value from 90 degrees at the north and south celestial poles (around which everything in the sky appears to rotate) down to zero degrees at the plane of the celestial equator. Declination is in positive degrees if the object is between the celestial equator and the north celestial pole, and in negative degrees if it is between the celestial equator and the south celestial pole.

Diffraction: An optical interference effect due to the bending of light around obstacles in its path (the edges of a telescope tube or its internal light baffles, for example), similar to the way ocean or lake waves are bent or deflected around dock pilings or the edge of a jetty. All telescopes show faint light and dark diffraction rings around a star's Airy disk at high power, as the diffracted light waves alternately cancel out and reinforce each other. Diffraction rings are very faint and an observer's inability to see them should not be a cause for concern. For example, in a perfect refractor about 84% of the light would be imaged in the Airy disk, with half of the remainder falling in the first diffraction ring and the balance scattered among the second, third, fourth rings, etc. Since the first diffraction ring is about six times the area of the Airy disk itself, its fainter light is spread over a much larger area, so that the brightness of the first diffraction ring is actually less than 2% that of the Airy disk. The other rings are dimmer still. It is easy to see how the beginning observer can have difficulty separating the very faint diffraction rings from the much brighter Airy disk. Catadioptric and reflector diffraction rings start out about twice as bright as those of a refractor due to the additional diffraction caused by their secondary mirror obstructions, but their brightness is still low in relation to their Airy disk (only 4% as bright in the case of the first ring). A catadioptric's higher diffraction ring brightness shows itself as lower contrast and some loss of sharpness on planets, binary stars, and star clusters when compared with a refractor. The spider vanes holding a reflector's diagonal mirror create additional contrast-lowering diffraction spikes radiating out from each star's image, an effect particularly visible on long exposure photos. A catadioptric telescope also has a circular secondary mirror shadow, but does not have diffraction spikes and spider vane shadows.

Diffraction-Limited: As mentioned above, a star appears in a telescope as a small Airy disk surrounded by faint diffraction rings. A telescope is said to be "diffraction limited" if its optics are made with enough accuracy so that all the light rays from a star fall within that star's Airy disk and diffraction rings, with no excess light being scattered out of the disc and rings by defects in the mirrors. Optics that bring all light rays to a focus within 1/4th of a wavelength of light of each other at the final focus are considered to be diffraction limited. Technically, a telescope is diffraction limited if it meets the Rayleigh limit - which specifies the separation in arc seconds of two equally-bright binary stars which appear to be just touching as being equal to 140 divided by the aperture in mm. The Rayleigh limit, which deals with a telescope's ability to separate closely-spaced stars, should not be confused with theRayleigh criterion, which deals with how accurately an optical system is made. Note that the visual Rayleigh limit for an 8" (203mm) aperture telescope is 0.69 arc seconds (140 / 203), a less-stringent specification than the Dawes' limit of 0.57 arc seconds. Telescopes meeting either limit can resolve more detail than the Earth's atmosphere will allow us to see under average seeing conditions, as our atmosphere typically limits the seeing to no better than one arc second resolution (the resolution of a 6" scope) on even a very good night. Five arc second resolution or worse is more typical of an average night.

Distortion: An optical defect causing uneven magnification of an object in different directions. Distortion causes straight lines to appear curved and is more apparent during terrestrial observation (there being few straight lines in space!).

 

Dobsonian Telescope: A conventional Newtonian reflector optical tube on an inexpensive plywood or fiberboard altazimuth mount. Nylon or Teflon bearings allow smooth telescope motion at a finger's touch, with no vibration or unsteadiness. The scope is moved by hand from object to object (there are no manual slow motion controls or motor drives) using a technique called star-hopping to locate objects. Usually it's a large aperture, fast focal ratio scope designed for visual deep space observing - although 6" and 8" medium f/ratio Dobsonians also suitable for planetary observing are becoming increasingly popular. Cannot be used for astrophotography. The Dobsonian is an economical way to get into large aperture astronomy at a fraction of the cost of an equatorially-mounted scope.

Equatorial Mount: A telescope mount designed for astronomical use. It aligns the axis of rotation of a telescope with the axis of the Earth, allowing the scope to follow the seemingly curved paths taken by the stars and planets. When equipped with a motor drive, it automatically tracks celestial objects without the need for constant manual corrections, as is the case with an altazimuth or Dobsonian mount. This is particularly important at high magnification, where objects drift across the field of an unmoving scope in a minute or less. Usually supplied with setting circles that help locate objects by their right ascension and declination coordinates. Convenient for visual observing and essential for astrophotography. Two types are commonly available with commercially-made amateur telescopes -- the German equatorial and the fork mount.

Exit Pupil: The circular image or beam of light formed by the eyepiece of a telescope. To take full advantage of a scope's light-gathering capacity, the diameter of an eyepiece exit pupil should be no larger than the 7mm diameter of your eye's dark-adapted pupil, so that all of the light collected by the telescope enters your eye. (The eyepiece exit pupil diameter is found by dividing the eyepiece focal length by the telescope focal ratio.) Your eye's ability to dilate declines with increasing age (to a dark-adapted pupil of about 5mm by age 50 or so). For those in this age group, eyepieces with exit pupils larger than their eyes can dilate to simply waste their telescope's light-gathering capacity, as some of the scope's light will fall on their iris instead of entering their eye.

Eye Relief: The distance between the lens of an eyepiece and the point behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. This is where your eye should be positioned to see the full field of view of the eyepiece. If you must wear glasses because of astigmatism, you'll need at least 15mm of eye relief if you want to see the full field of view with your glasses on.

Eyepiece: A telescope collects light and forms a small fixed-size image at a point (called the prime focus) that's determined by the focal length of the optical system. You can see this image by aiming your telescope at something bright, such as the Moon, taking out the eyepiece and star diagonal, and holding a piece of paper behind the focuser. Move the paper back and forth. At some point, you will find a small, but sharp, image of the Moon projected onto the paper. This is the prime focus image formed by the telescope. Unfortunately, human eyes typically cannot focus sharply on an image unless it's more than eight inches from the eye. This makes it difficult to see detail in the small prime focus image formed by the telescope if it's examined solely with the unaided eye. An eyepiece is a small microscope that allows you to get closer than eight inches from that small fixed-focus image -- and the closer you can get to an object, the bigger it appears. A 25mm eyepiece, for example, lets you focus on the scope's prime focus image from an effective distance of only 25mm (one inch away from your eye); a 12mm eyepiece puts you half an inch away; etc. The magnification of an eyepiece is found by dividing the telescope focal length by the eyepiece focal length. A 25mm eyepiece used with a 2000mm focal length scope therefore provides 80 power (2000 / 25 = 80x), making objects appear 80 times larger than they do to the bare eye (or 80 times closer, to put it another way).

Focal Length: The length of the effective optical path of a telescope or eyepiece (the distance from the main mirror or lens where the light is gathered to the point where the prime focus image is formed). Typically expressed in millimeters.

Focal Ratio: The `speed' of a telescope's optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera. Fast f/4 to f/5 focal ratios are generally best for wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either. An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio - so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

Fork Mount: A type of equatorial mount used on short tube catadioptric telescopes in which the telescope tube is mounted between two arms connected to a motor drive. It does not need a counterweight to balance the tube, as with a German equatorial mount. An equatorial wedge and field tripod are used tilt the scope over to align it on the celestial pole for proper tracking. Setting circles are provided to locate celestial objects by their right ascension and declination coordinates. The r. a. setting circle is usually driven by the scope's motor drive to move across the sky at the same speed as the stars, following their apparent motion. This makes fork mount setting circles more convenient to use than the unpowered circles on most German equatorial mounts, as the latter must be readjusted periodically to keep pace with the motion of the stars. Photography near the north celestial pole is difficult with a fork mount.

German Equatorial Mount:
A mount used primarily with refractors and reflectors. A counterweight on one side of the polar axis balances the weight of the optical tube on the other. Not as convenient as a fork mount when sweeping from horizon to horizon, as the tube can bump the legs or pedestal mount as the scope passes the zenith, requiring that the tube be "tumbled" or rotated 180o to continue its tracking of objects down to the western horizon. Its setting circles usually are operated manually. Somewhat more difficult to use and transport than a fork mount telescope, but stable, relatively inexpensive, durable, and capable of astrophotography near the celestial pole.

Highest Useful Magnification: The highest visual power at which a telescope can realistically be expected to perform before the image becomes too dim for useful observing (generally about 50x to 60x per inch of telescope aperture). However, turbulence in our atmosphere usually limits the number of nights in which this power is obtainable. Very high powers are best reserved for planetary observations and binary star splitting, as faint nebulae and galaxies appear at their best at relatively low powers (8x to 12x per inch of aperture). On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture) is often more enjoyable than attempting to push a telescope's magnification to its theoretical limits. Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.

Limiting Magnitude: The magnitude (or brightness) of the faintest star that can be seen with a telescope. An approximate formula for determining the visual limiting magnitude of a reflector is 7.5 + 5 log aperture (in cm). However, this formula does not take into account light loss within the scope, seeing conditions, the observer's age (visual performance decreases as we get older), etc. The limiting magnitudes specified by manufacturers for their scopes assume very dark skies, trained observers, and excellent atmospheric transparency - and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

Magnification: The ability of a telescope to make a small, distant object large enough to examine in detail. If you look at the Moon (250,000 miles away) with a 125x telescope, it's essentially the same as looking at it with your bare eyes from 2000 miles away (250,000 / 125 = 2000). The same telescope used terrestrially will make an object one mile away appear to be only 42 feet away (5280 feet / 125 = 42).

Magnitude: A number indicating the brightness of a star or extended object. The larger the positive number, the fainter the star or object, with a one digit magnitude change indicating a 256% difference in brightness. 4th magnitude stars are often the faintest visible to the naked eye from a light-polluted suburb. 14th magnitude stars, by comparison, are a mere 1/10,000th as bright! 6th magnitude stars are typically the faintest naked eye stars visible from a reasonably dark sky observing site. On extended objects (galaxies and nebulae), the magnitude is the one the object would have if all its light was gathered into a single point, like a star. A 9th magnitude galaxy, therefore, will appear dimmer than a 9th magnitude star because its light is spread over a larger area than the star. A good example is M33, the face-on spiral galaxy in Triangulum. It's a 6th magnitude object, but is often difficult to see in even an 8" telescope (whose visual limiting magnitude is 14), because its mag 6 brightness is spread over nearly one square degree of sky. Such an object is said to have low surface brightness and is quite often masked by light pollution when observing from city or suburban sites.

Maksutov-Cassegrain: A catadioptric telescope that uses a thick and deeply-dished spherical corrector lens to correct for the spherical aberration of its spherical primary mirror - an all-spherical design that keeps its collimation virtually indefinitely. Its typically long focal ratio and small secondary obstruction yield higher contrast and resolution than any other catadioptric or reflector design .


Newtonian Reflector: This classic design, first used by Sir Isaac Newton 300 years ago, employs a large primary mirror at the bottom of the telescope tube, with a flat diagonal mirror at the top bringing the light out to the Newtonian focus in the tube's side. Totally color-free for excellent planetary observing and with more than adequate deep space capability, the Newtonian offers more light-gathering aperture for the money than any other telescope design, particularly in its Dobsonian guise.

Objective: The main light-gathering lens or mirror of a telescope.

Rayleigh Limit: Similar to the Dawes Limit, but a somewhat less stringent measure of a telescope's theoretical resolution. In the case of an 8" (203mm) reflector, applying the Rayleigh limit formula yields a maximum resolution of 0.69 arcseconds. The Dawes Limit for the same instrument works out at 0.53", a less likely attainment in even ideal conditions of terrestrial seeing.

Recollimation: The need to realign the optical elements of a telescope for peak performance after the instrument has been disassembled, handled roughly, or moved over long distances. Recollimation is required relatively often with reflector telescopes (particularly those with large fast focal ratios), very rarely with catadioptrics, and almost never with a refractor. Recollimating a catadioptric is a simple procedure; still easy, but a little more time consuming, for a reflector; in the case of the refractor, a difficult operation best left to the manufacturer.

Refractor: A telescope employing two or three lenses to bring light to a focus at the end of a long tube.

Resolution: The ability of a telescope to separate closely-spaced binary stars into two objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25-cent coin at a distance of three miles! In essence, resolution is a measure of how much detail a telescope can reveal. In theory, resolution equals 5.45 arc seconds divided by the aperture of the scope (in inches), so that an 8" scope has a resolution of 0.69 arc seconds, and can show as two joined points binary stars separated by that small an angular distance - but see Dawes' Limit.

Rich Field Telescope (RFT): A fast focal ratio reflector that gives wide-angle views of star clouds, nebulae, large galaxies, etc. Most large Dobsonians are rich field telescopes.

Right Ascension: Technically, the angular distance of a celestial object east of the vernal equinox, measured in hours and minutes. Simply stated, one of the two coordinates (declination is the other) that let you find celestial objects by using a telescope's setting circles and a star chart or star atlas. If you face the north celestial pole, the stars will rise (ascend) on your right - hence the term "right ascension." The same point on the 360 degree celestial sphere passes overhead every 24 hours, making each hour of right ascension equal to 1/24th of a circle, or 15 degrees. Each degree of sky therefore moves past a stationary telescope in four minutes - a rapid rate when observing at high power.

Schmidt-Cassegrain Telescope (SCT): A catadioptric telescope that uses a thin aspheric corrector lens to compensate for the spherical aberration of its primary mirror.


Seeing: The steadiness of telescopic images due to conditions in the Earth's atmosphere. Seeing is bad when air currents and temperature differentials cause the image to twinkle or undulate, or appear blurred or distorted - typically when the barometer is low or falling. The seeing is good when the air is still and the image appears sharp and steady - as is the case when there's a high pressure ridge over the observing site. Poor seeing affects the resolution of a telescope, putting an upper limit on the maximum usable magnification on any given night. On most nights, seeing conditions limit the resolution of even large telescopes to no better than five arc seconds or so and bloat small Airy disks into "seeing disks" three or four arc seconds in diameter.

Setting Circles: Circular scales on an equatorial mount telescope that are used to point it at the position (in right ascension and declination) of a celestial object. Setting circles and a star chart make it possible to find objects even when they are too faint to see through the finderscope.

Spherical Aberration: An optical defect that causes light rays from an object, passing through an optical system at different distances from the optical center, to come to a focus at different points along the axis. On one side of focus, the Airy disk will virtually disappear and the outer diffraction ring will brighten. On the other side, the inner diffraction ring will be brightest. This may cause a slightly out of focus star, for example, to be seen as a discrete disk if the Airy disk and the inner ring blend together because of seeing conditions, but should not be confused with the star's normally smaller Airy disk.

Spherical aberration is most often seen in small inexpensive imported reflectors, which use molded spherical mirrors rather than the costly and more difficult to make hand-figured parabolic mirrors found in a quality reflector.

Spotting Scope: A small refractor or catadioptric telescope on an altazimuth mount or photo tripod for terrestrial observing. Usually has an image-erecting prism for correctly-oriented terrestrial views. (Astronomical reflectors have inverted and reversed images, while astronomical catadioptrics have upright mirror-image views.)

Star-Hopping: A way to locate celestial objects by moving to them in a series of small 4 or 5 degree steps or "hops" from a known star or object, using the 4 or 5 degree field of view of a conventional finderscope or non-magnifying illuminated finder to follow a path previously marked out on a star chart.

Transparency: A measure of how dark the sky is on a given night. Transparency is affected by the amount of humidity and dust in the atmosphere, as well as by the amount of light pollution. The four stars in the bowl of the Little Dipper are magnitudes 2.2, 3.1, 4.3, and 5.0. If all four can be seen most nights without using averted vision (after your eyes have had 10 minutes or so to become dark adapted), and you can clearly see the faint outline of the Milky Way, the transparency would be rated 5 and your observing site is probably dark enough to let you use a 10" scope without being overly affected by light pollution. If you have to use averted vision to see the fourth star, you may be limited to an 8" scope. If only three of the Little Dipper stars can be seen consistently (the faintest being magnitude 4.3), the transparency would be rated 4, and light pollution will probably limit you to a 6" scope. A transparency of 4 is only fair for deep sky observing. A transparency of 5 is much more satisfactory with an 8" or larger scope.