Airy Disk This is the central spot of the image of a star, surrounded by diffraction rings, formed when viewed in a telescope. Because of the large distances of stars, it is impossible to resolve a star into a disk, no matter what the magnification. This is due to diffraction. The disk formed by the diffraction of stellar images is known as an Airy Disk, after the English Astronomer Royal, George Airy.

Altazimuth Mount This is the simplest type of telescope mount, with only up/down (altitude) and left/right (azimuth) motions. This type of mount is mainly used 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 This is 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.

Aperture Mask Large telescopes often have a problem when looking at the Moon or planets. They gather too much light. The Moon and planets are bright enough on their own. You don't need to gather a lot of light to see them. If an image is too bright, a property called irradiation comes into play within your eye. Irradiation causes bright areas to bleed over into adjoining dark areas and soften or eliminate the border between them. For example, when two bright areas are separated by a thin dark area, such as a narrow rille on the Moon, the irradiation of the two bright areas can cause the thin dark rille to disappear. With a big scope, then, trying to see faint details in a planet's atmosphere or on its surface is often like trying to read the brand name on a car headlight at night when it's lit. There's just too much glare to see any detail. In addition, big scopes look through more of our atmosphere than small scopes and are often limited in their ability to resolve small details by conditions in our turbulent atmosphere. An aperture mask is a circular piece of cardboard the same size as the front cell of your telescope. A 3" to 4" circular hole is cut into the cardboard off to one side, not directly in the centre of the cardboard circle. The cardboard is painted flat black and taped over the front of your scope. If properly sized and positioned, the circular hole will line up with an unobstructed portion of your scope's optical tube. It will not be blocked by the edge of the tube or the secondary mirror, and none of the secondary mirror's spider vanes or support structure will cut across the circular hole. By masking down the aperture of a large scope you reduce the brightness and glare, thereby reducing irradiation, You also eliminate the diffraction effects of the secondary mirror and its supports, thereby improving contrast. And you also reduce the image-blurring effects of atmospheric turbulence, thereby sharpening the image. It is true that an aperture mask reduces the resolution of a large scope to that of an unobstructed refractor with the same aperture as the size of the hole cut in the mask, but most observers would rather have a sharp steady image with reduced resolution than a blindingly bright image that's too fuzzy and washed out to see any details. An aperture mask gives them a sharp steady image with a big scope.

Apochromat A refractor telescope with a two to four lens optical system that uses one or more elements of costly ED (Extra-low Dispersion) or SD (Special Dispersion) glass and/or calcium fluorite crystal to virtually eliminate chromatic aberration. Technically, any lens system that is corrected for spherical aberration at two wavelengths, or colours of light, and for chromatic aberration at three.

Astigmatism An optical defect in which star images are elongated into ovals which change from a radial orientation (pointing towards the centre of the field) to a tangential one (at right angles to the centre) as the observer moves the telescope focuser from one side of the best focus to the other. Should not be confused with the star images of an out-of-collimation telescope, which may also be oval but which will not change orientation as the observer passes through best focus.

Astronomical Binoculars For the faint fuzzies outside the solar system, large aperture binoculars offer something a telescope can almost never give you -- a wide field of view. They let you see the big objects all at once, such as the three degree width of the Andromeda galaxy that has to be seen in segments in the one degree field of the average telescope. 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 have the same light gathering capacity as a 70mm refractor at the same power. 80mm binoculars have the same light gathering 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). Exit pupil is found by dividing the binocular aperture by the magnification. Binoculars with 6mm to 7mm exit pupils (a 7 x 50mm or 8 x 50mm binocular, for example) are best suited for teenagers and those in their early 20's, who have eyes with pupils that can still expand wide enough in dark skies to take in all the light of the large binocular exit pupils.

Averted Vision At night, the periphery of the eye's retina is more sensitive to faint light than the centre (which is more specialized for observing colour 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.

Barlow lens A diverging lens which has the effect of increasing (usually doubling) the effective focal length of the telescope.

Bird-Jones This is a modified Newtonian with a spherical primary mirror. Before the light strikes the flat after being reflected, it passes through a Barlow-like achromatic lens that corrects the spherical aberration of the primary mirror. The system is also corrected for coma.

Cassegrain Any telescope that folds the light path and directs it through a hole in the centre 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.

Collimation The alignment of the optical elements of a telescope at the correct angles to the light path. If not properly collimated, a telescope will deliver distorted images ( lopsided or elongated stars, hazy planetary images, an inability to split close binary stars, etc.) Atmospheric turbulence ( seeing ), thermal currents within the telescope, and dirty or defective eyepieces can often mimic poor collimation.

Colour correction How free an eyepiece is from coloured halos around stars at the edge of the field, false colour in planetary images, or stars that change colour as they move across the field.

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 millimetres 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 centre 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.

Coude This is an set of auxiliary mirrors so arranged as to always direct light to a fixed position, usually the polar axis, irrespective of the actual position of the telescope. It is used with instruments, such as spectrographs, which are too large to mount on the telescope itself.

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 centre, 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.

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.

Dew Our atmosphere always contains evaporated water, or humidity, released into the air by the heat of the sun. Warm air can hold more of this moisture than cool air, so as the atmosphere cools down after sunset it becomes less and less able to hold the moisture picked up during the day. At some point, called the dew point, the amount of moisture in the air becomes greater than the air's capacity to hold that moisture, and the excess water condenses on the nearest cool surface in the form of dew.You can easily see dew form on a cold pop bottle when you take it out of the refrigerator. As warm air passes over the cold glass, the air around the glass is cooled below the dew point and condensation (dew) forms on the sides of the bottle. Telescope lenses and eyepieces collect dew in the same way. Air currents passing over their surfaces produce convection cooling (the same cooling that the breeze from a fan produces on a hot day), drawing heat away from the mirrors and lenses. This lowers the temperature of the glass below the dew point, allowing the formation of dew that blurs visual images and ruins long exposure astrophotographs. How can you prevent dewing? Easy. Keep your telescope and eyepieces warmer than the surrounding air. You can keep eyepieces warm by keeping them in your pocket, on the warm hood of your car, in an accessory case with a pocket hand warmer, or on top of an operating drive corrector.You can't keep your telescope in your pocket, though, and that's where a dew shield (dew cap) or dew heater comes in. A dew shield is simply an extension of the telescope tube that restricts the air flow across the exposed lens of the telescope, slowing convection cooling. This helps to hold heat near the lens to keep it above the dew point. It does not work by keeping dew from "falling" onto the lens. Dew shields are standard equipment with refractors, but are optional with most catadioptrics. Reflector mirrors, being at the bottom of telescope tubes that essentially act as giant dew shields, rarely dew up. Exceptionally humid conditions might overcome a dew shield's ability to prevent dewing. In that case, mild heating of a catadioptric's corrector lens or refractor's objective lens might be needed. In the backyard, a short blast of warm air from an ordinary hand-held hair dryer will leave the corrector dew-free for many minutes at a time. Away from the house, an automobile windshield defroster gun (the type that plugs into a car's cigarette lighter and available from most auto supply stores), works quite well for temporary dew removal. For a more permanent solution, Electronics heater strips (dew heaters) that wrap around the objective lens of the telescope (as well as the finderscope and eyepieces, if desired) are available. These warm the lens above the dew point to prevent the formation of dew. Unlike dew shields, dew heaters don't merely slow the formation of dew, they prevent it entirely.

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.

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 the Rayleigh 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 which causes uneven magnification of an object in different directions. Makes straight lines appear curved and is more visible in terrestrial observing, as there are few straight lines in space.

Dobsonian Telescope A conventional Newtonian reflector optical tube on an inexpensive plywood or fibreboard 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.

Doublet A lens system comprising two elements, used to reduce chromatic aberration.

Drive The means of automatically compensating a telescope for the Earth's rotation so that it always points to the same place in the sky.

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).

There are several types of eyepiece designs. The most popular are:

Kellner: This is a three element design eyepiece, it gives sharp bright images at low to medium powers. It is best used on small to medium sized telescopes, kellner's have an apparent fiel of about 40 degrees and good eye relief, though short at high powers. A good low cost performer. There is also a variant called a Reverse Kellner (RK). It has similar or slightly better parameters than the Kellner, but it is less popular.

Orthoscopic: This is a four element design and was once considered the best all-round eyepiece, but is no longer as popular due to its narrow field of view compared to newer designs. It has excellent sharpness, colour correction and contrast. Longer eye relief than Kellner's and is especially suitable for planetary and lunar observations.

Plossl: This four element design is todays most popular. It provides excellent image quality, good eye releif and an apparent field of view of about 50 degrees. High quality Plossl's exhibit high contrast and pinpoint sharpness out to the edge of the field. An excellent all round performer.

Erfle: The five or six element Erfle is optimised for a wide field of view, 60 to 70 degrees. At low powers it provides impressive deep sky views. At high powers the image sharpness suffers at the edges.

Field curvature An inability to bring the centre and edge of the field into focus at the same time, with the edge out of focus when the centre is sharply focused and vice-versa.

Finderscope A small telescope, with a wide field of view, mounted on the main telescope tube to enable an observer to easily locate celestial objects, and place them within the field of view of the main telescope.

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 millimetres.

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.

Focuser The mechanism which holds the eyepiece and allows adjustment for focussing the image.

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 un-powered 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 180 degrees 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.

Ghosting A flare of unwanted light around bright objects, or multiple faint images of bright objects, due to internal eyepiece reflections.

Graticule Reference marks, or measuring scale, placed at the focal plane of a telescope to aid object centring, or to make measurements.

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 nebulas 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 stabilise enough for them to use such very high powers.

Klevtsov A telescope design that has a spherical primary mirror and secondary optical train consisting of mangin lens and a meniscus. This combination is designed to correct spherical aberration, chromatic aberration, coma and field curvature.

Light Gathering Power A telescopes ability to collect light. It is directly proportianal to the area of the objective lens or primary mirror, the larger the area, the greater the light gathering power. As an example to calculate the LGP of a 24cm telescope compared to a 4cm telescope you simply square (24 divided by 4) which is 6 squared = 36. So a 24cm telescope has 36 times more light gathering power than a 4cm telescope.

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 Magnification is 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 125 power (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).
The magnification of a telescope is determined by dividing the focal length of the telescope (usually in millimetres) by the focal length of the eyepiece used (again, usually in millimetres; but in all cases by the same unit of measurement used for the telescope focal length). For example, a 2000mm focal length telescope and a 10mm focal length eyepiece will give you a magnification of 200 power (2000 ÷ 10 = 200). The same 2000mm telescope with a 20mm eyepiece will give you 100x (2000 ÷ 20 = 100).

Magnitude A number indicating the brightness of a star or extended object. The larger the positive number, the fainter the star or object; while the larger the negative number, the brighter the star or object. A one digit magnitude change indicates 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. The Sun has an apparent magnitude of -26.5.
On extended objects (galaxies and nebulas), 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 .

Nagler Eyepiece An eyepiece having a very wide field of view, typically greater than 80 degrees. Particularly suitable for comet hunting.

Newtonian Reflector This classic 300-year old Sir Isaac Newton design uses a large primary mirror at the bottom of the telescope tube, with a flat diagonal mirror at the top that brings the light out to the Newtonian focus at the side of the tube. Totally colour-free, for excellent planetary observing. Offers more light-gathering aperture per dollar than any other telescope design, as well, for very good deep space performance.

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

Parfocal Eyepieces These are eyepieces that can be interchanged without the need to re-focus your telescope. This is desirable but not necessary while switching eyepieces when looking at the same object. Usually eyepieces of the same design from the same manufacturer are parfocal, but the same design from different manufacturers will likely not be parfocal.

Phase Coatings An optical technique used with roof prism binoculars to increase colour fidelity. Due to a roof prism's optical design, the light entering a binocular's image-erecting roof prism is split in two. The two halves travel through the prism independently and are rejoined before entering the eyepiece. Because the two light paths are slightly different lengths, one half of the light takes a little longer to travel through the prism than the other. When the two halves of the image are rejoined, the longer light path half is slightly out of phase with the light that took the shorter route. This can reinforce some colours of light and cancel out others, affecting the colour balance and fidelity. Phase correcting coatings are optical coatings that are applied to one surface of the shorter light path half of the prism. The coating slightly slows down the short light path half of the incoming light that passes through that surface, causing it to once again be in phase with the light that travelled the longer path when they halves are rejoined. With phase-corrected prisms, no colours are reinforced or cancelled, giving a more accurate colour reproduction. The effect is particularly visible when looking sunward at a back-lit or silhouetted bird, where more colour and detail can clearly be seen in the shadowed areas of the bird.

Rayleigh Criterion Lord J. W. S. Rayleigh (1842-1919), the Nobel Prize-winning English physicist, empirically determined that telescope optics that yield 1/4th wave accuracy at the final focus (so that all light gathered by the system comes to a focus within 1/4th of a wavelength of the green light to which the eye is most sensitive) will produce results on stars that are visually indistinguishable from an optically perfect system. This is known as the Rayleigh Criterion and is a handy yardstick by which telescope quality can be measured. To achieve a 1/4th wave accuracy overall, each mirror in a reflector must be finished to 1/8th wave smoothness. When observing extended deep space objects (such as nebulas and galaxies), most amateur astronomers find it difficult to see any visible difference between optics made to 1/4th wave accuracy and those made to 1/10th wave accuracy - although experienced observers usually find the higher accuracy to be beneficial on planets. Large optics polished to higher levels of accuracy than 1/10th wave usually gain the observer little additional benefit visually, however, as the performance of the telescope will be limited more by atmospheric conditions than it will be by mirror accuracy. Inexpensive scopes can have mirrors polished to 1/4th wave accuracy and still have a rough surface marred by micro-ripple whose errors might be 1/50th wave or less. Such rough mirrors will have visibly lower contrast and less-sharp images than a well-finished mirror.

Recollimation The need to realign the optical elements of a telescope for best performance after the telescope has been disassembled, frequently moved, or given rough treatment. Usually required relatively frequently with reflector telescopes (particularly with large fast focal ratio systems), very rarely with catadioptrics, and almost never with a refractor. Recollimation is very easy with a catadioptric; still easy, but a little more time consuming, with a reflector; and difficult and best left to the manufacturer with a refractor.

Refractor A telescope that uses 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, a measure of how much detail a telescope can reveal. In theory, resolution equals 4.56 arc seconds divided by the aperture of the scope (in inches), so that an 8" scope has a resolution of 0.6 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, nebulas, 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.

Ritchey-Chrétien telescope (RCT) A specialized Cassegrain telescope with a hyperbolic primary and secondary mirror. It was invented in the early 1910s by American astronomer George Willis Ritchey (1864–1945) and French optician Henri Chrétien (1879–1956).

The Ritchey-Chrétien design is free of first-order coma and spherical aberration, although it does suffer from third-order coma, severe large-angle astigmatism, and comparatively severe field curvature (Rutten, 67). When focused midway between the sagittal and tangential focusing planes, stars are imaged as circles, making the RCT well suited for wide field and photographic observations. As with the other Cassegrain-configuration reflectors, the RCT has a very short optical tube assembly and compact design for a given focal length. The RCT offers good off-axis optical performance, but examples are relatively rare due to the high cost of hyperbolic primary mirror fabrication; Ritchey-Chrétien configurations are most commonly found on high-performance professional telescopes.

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 useable 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 centre, 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 moulded 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.

Wedge A device used to attach a fork mounted telescope to a tripod.