Every field of scientific study- every hobby, for that matter- has its own lexicon.
A complete astronomy vocabulary could fill an entire book, but the following list will familiarize the novice with the most important and commonly used terms, particularly those terms used in reference to the equipment and the most commonly sought astronomical targets.
- Astronomy Terms & Names
- Absolute magnitude
- Air-spaced or Air-spaced Doublet
- Altitude and Azimuth
- Apparent magnitude
- Apparent Field of View
- Aperture Fever
- Apochromatic lens
- Arc minute
- Arc second
- Classical Cassegrain
- Coated, fully multi
- Coated optics, Coating
- Collimation cap
- Crepe ring
- Compound telescope
- Crown Glass
- Ecliptic –
- ED glass –
- Electronic drive
- Element –
- Equatorial mount –
- Emission nebula –
- Erfle eyepiece –
- Eyepiece –
- Exit Pupil –
- Eyeguard –
- Eyepiece, 2″
- Eyepiece, illuminated –
- Eyepiece, Kellner –
- Eyepiece element –
- Eyepiece, Huyghenian
- Eyepiece, Lanthanum –
- Eyepiece, Orthoscopic
- Eyepiece, Plossl
- Eyepiece Ramsden –
- Eyepiece, Zoom –
- Field-lens –
- Field of View (FOV) –
- Field stop –
- Finderscope –
- Field of view, true –
- Filter, color –
- Filter, light-pollution –
- Filter, moon –
- Filter, planetary –
- Filter, solar –
- Filter, Variable-polarizing
- Flint Glass –
- Flashlight, red –
- Fluorite –
- Focal length, telescope –
- Focal ratio (f-ratio)-
- Focal plane –
- Focuser –
- Focal length, eyepiece –
- Globular cluster –
- Guide scope –
- (IC) Index Catalog
- Local Group
- Maksutov (MAK)
- Maksutov-Cassegrain telescope (MCT)
- Meniscus lens – The front lens in Maksutov telescope.
- Messier objects
- Mirror cell
- Messier objects –
Astronomy Terms & Names
Here are definitions or explanations of terms from “Altazimuth” to “Zenith”
A measure of a star’s true or intrinsic brightness by determining how bright stars would appear if viewed from a distance of 10 parsecs, or 32.6 light-years. Therefore, Alnitak, the easternmost star in Orion’s belt, with an apparent magnitude of 2.05 would have an absolute magnitude of -5.9, which is how bright it would appear if it were10 parsecs away.
refractor telescope with an achromatic lens– the most common lens found in refractor telescopes. Achromats have two elements made of two different types of glass with different refractive indicies. A properly designed achromat will bring at least two different wavelengths (colors) of light to a common focus minimizing errors across all wavelengths.
Air-spaced or Air-spaced Doublet
This refers to the type of achromatic lens where the two components are just pressed together, not glued (‘cemented’) together. The air space between the two components of the lens can be so thin that the two components are touching.
a telescope mount with two perpendicular rotational axes– one horizontal, the other vertical.It allows movement in altitude (up and down) and azimuth (side to side). A camera tripod is an example.
Altitude and Azimuth
Coordinate terms used in the horizontal coordinate system. When describing a type of telescope mount (alt-az), altitude refers to the the y-axis (up-down) motion of a mount.
Measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness.
Apparent Field of View
The apparent field of view of any eyepiece is the angle through which the observer would need to move his eye in order to see the entire width of the view. Good quality eyepieces, such as Plossls, have an apparent field of view greater than fifty degrees. Erfle eyepieces can have an apparent field of view up to eighty degrees.
The diameter of the primary mirror or lens of a telescope.
Aperture Fever is the mythical disease that causes otherwise rational amateur astronomers to purchase larger and larger telescopes in the hope of seeing more.
A type of compound lens that uses either three or more elements, or special glasses, like fluorite or ED glass, to bring at least three colors to a common focus. In practice, errors are minimized across all wavelengths in order to achieve a very high degree of color correction. Most camera lenses are apochromatic. Compare this with an achromat. Apochromatic Literally means “without color”. A refractor lens that virtually eliminates chromatic aberration. This is done by using exotic glasses or by using two or more lens elements for the objective lens.
– Literally means “without color”. A refractor lens that virtually eliminates chromatic aberration. This is done by using exotic glasses or by using two or more lens elements for the objective lens.
A lens that consists of three components. This enables lenses with short focal lengths to enjoy the same freedom from colour distortions as the longer focal length achromatic lenses.
A unit of angular measurement. One arc minute is 1/60th of a degree. Just as there are 60 minutes in one hour of time, there are 60 arc minutes in a one-degree angle.Arc minute – A measure equal to one-sixtieth of a degree (there are 360 degrees in a circle).
A unit of angular measure consisting of 1/60th of an arc-minute. A dime seen from a distance of 2 miles transects 1 arc second. There are 3,600 arc-seconds in a one-degree angle. It is used to measure the separations of double stars or small deep-sky objects, like planetary nebulas. The full moon is about 30 arc minutes in diameter. See also
DEGREE.Arc second – A measure equal to one-sixtieth of an arc-minute.
Asterism – A group of stars that appear to make a recognizable shape, such as the Big Dipper.
Asteroid Belt – The asteroid belt is the concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter.
Astigmatism – Optical abberation or imperfection that renders stars as crosses instead of points. Difficult to correct.
A lens or telescope intended for photographic, rather than visual, use.
A telescope whose optical construction is designed to minimise light loss within the optical path. Such telescopes often produce an inverted image.
A method of detecting very faint objects using the sensitive low-light rod cells of your eyes that are located around the sides of your eyes. To see faint objects, don’t look straight at them but rather look at them out of the corners of your eyes.
The x-axis (left-right) motion of an alt-az telescope mount– the angle of an object from an observer’s north point projected onto the horizon. If an object is due north, its azimuth is 0 degrees. If it is due east, its azimuth is 90 degrees.
Baffles A series of rings or thin metal plates with a central hole that are fitted within the main tube, along the light path of a telescope. The purpose is to increase contrast. With apertures that gradually “step down,” baffles prevent internal light scatter reflected from the inside surfaces of the telescope, which could otherwise enter the eyepiece and reduce the quality of the image.
A lens placed in front of a telescope eyepiece that increases focal length and magnification.
Also known as “borosilicate” glass. Most optical prisms are made of BK-7 glass.
Also known as “barium crown” glass. BAK-4 is a superior-quality glass used to make optical prisms. It yields bright images and high contrast.
This is more correctly called Canada balsam. It is a natural, colourless resin that is used to glue optical components, such as achromatic lenses, together.
Barred spiral galaxy (SB) – A spiral galaxy whose center is elongated or bar-shaped.
A lens assembly with a negative focal length placed between an eyepiece and telescope that effectively increases the focal length (and magnification) of a telescope. Barlows commonly come in magnifications of 1.5x, 2X, and 3X.
Two (or more) stars that rotate around a common center of mass.
Bino-viewer – An optical device that splits the beam of light from a telescope into two parts, so you can observe objects with both eyes at once.
Blower bulb – A pliable hand-held bulb that, when squeezed, produces brief gusts of air. Used to blow dust off of optical surfaces.
Cassegrain Telescope A pure cassegrain telescope uses a parabolic primary mirror, just like a Newtonian telescope, but instead of a flat diagonal mirror, the cassegrain has a hyperbolic convex mirror that reflects the light back through a hole in the primary mirror. Cassegrains typical have a very long focal length, which results in high magnification. Cassegrains are popular with some planetary observers. Schmidt-Cassegrains (SCTS) have been the most popular telescope design sold in the United States over the last 25 years.
that uses the benefits of both refraction and reflection to form an image. A typical catadioptric consists of a primary spherical mirror, a secondary mirror, and a correcting plate or lens to counter the aberration introduced by the spherical primary. It can be either Cassegrain or Newtonian in configuration.
The main division between Saturn’s largest rings (A and B rings).
Catadioptric Telescope Any design using a combination of mirrors and lenses. Catadioptics are typically smaller than other designs of similar focal length as the fold the light path or a telescope by reflecting it back and forth.. The popular Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain designs are common examples of catadioptic designs.
A telescope that uses both lenses and mirrors to form an image. Common catadioptric telescopes include Schmidt-Cassegrain and Schmidt-Maksutov telescopes.
Camera or T-adapter
An adapter, either a T-ring, , or tele-extender used to attach a 35mm SLR camera to a telescope for astrophotography.
CCD camera or Imager – Short for “charge-coupled device.” A CCD is a light-sensitive electronic detector widely used in making astronomical images. Sensitive over a wide range of wavelengths, and much more efficient than emulsion in gathering light, CCDs are often used to image extremely faint objects.
Pairs of numbers (right ascension and declination) used to locate celestial objects.They are similar to longitude and latitude on Earth.
The components of most small achromatic lenses are glued together (‘cemented’ together) with a transparent adhesive. Canada balsam was the traditional adhesive, but it has been somewhat replaced by modern, UV-curing adhesives.
This is the phenomenon of rainbow colours surrounding a view seen through a telescope. It is most noticeable when the object being viewed is in sharp contrast to its surroundings. Chromatic aberration is caused by the fact that light of differing colour is brought to a focus at slightly differing distances from the lens. Achromatic and apochromatic lenses attempt to correct this defect.
Chromatic Aberration Refractor telescopes cannot bring all colors to a single point of focus, and consequently there is always some amount of secondary color seen in a refractor, especially at high magnifications. Think of how a prism sperates light into a spectrum; simple lenses do this, too, and so complex multi-element lens designs are needed to minimize this. The inability to focus all colors to a common point is called chromatic abberation. Purely reflecting telescopes have zero chromatic abberation.
Classical Cassegrain is a term used to describe simple cassegrain designs, as opposed to Schmidt-Cassegrains.
Coated, fully multi
All air-to-glass surfaces receive multiple anti-reflection coatings.
Coated optics, Coating
an antireflection coating in lenses and mirrors. Coatings often appear blue, purple or green when seen in reflected light.
the process of adjusting the optical components of a telescope so they are aligned with each other.The process of aligning the optics within a telescope.
Most refractors are permanant collimated, but Newtonian reflectors generally require regular collimation for optimal performance. Schmidt-Cassegrain telescopes (SCTs) particularly need carefull and often frequent collimation in order to perform optimally.
An eyepiece insert with a center hole that aids in quick optical alignment of a Newtonian reflector’s primary and secondary mirrors.
One of the most common aberrations inherent in large-aperture, short-focal-length reflectors. Star images become increasingly comet-like or pear shaped away from the center of the field. Good optical alignment can mitigate some coma. Coma can be reduced with auxiliary optical adapters, such as TeleVue’s Paracorr.
The Crêpe ring, or C ring, is the inner ring of Saturn’s three major rings. It is visible in small telescopes.
Compound telescopes include Schmidt-Cassegrain and Schmidt-Maksutov telescopes.
A weight that is placed on an equatorial, dobsonian, or other mount, to counterbalance the weight of the telescope tube assembly.
Glass used to make one of the two elements of an achromatic objective lens, or doublet. The other element is a plano-concave lens composed of flint glass. The two elements work together to focus light of different colors due to the dispersion of light through glass.
Dark apadptation The process of the eyes’ pupils gradually dilating to their widest opening. Under dark skies, the average dilation aperture of the pupil is about 7mm, though this usually decreases slightly with age. On average, the process takes between 20 and 30 minutes. Red goggles can be worn prior to observing to achieve dark adaptation.
Dawes’ Limit – William Dawes wrote that a telescope can resolve (that is, see as two sperate points) a pair of double stars separated by 4.56 arc seconds divided by the aperture of the telescope in inches. This is commonly called ” Dawes’ Limit”, and is generally accepted as the limit of what a very well made telescope can resolve.
Deep sky – Any celestial object beyond the Solar System.
One of the two celestial co-ordinates, the other being right ascension, declination is based off the celestial equator, an imaginary line from the Earth’s equator extended into space. Values are given in degrees from -90 to +90. The negative values are located in the Southern Celestial Hemisphere; the positive values are in the Northern Celestial Hemisphere.
Similar to latitude on the Earth’s surface. The distance in degrees north or south of the celestial equator (similar to the Earth’s Equator). Declination is measured in degrees, minutes and seconds.Equatorial mounts have two axis of movement referred to as Declination and Right Ascension. Right Ascension refers to the axis in line with the Earth’s axis of rotation, and Declination is the axis 90 degree from Right Ascension.
Degree – An angular unit of measurement equal to 1/360th of a circle. Hence, a great circle drawn across the sky from due east to due west would contain 180 degrees. From overhead to any point on the horizon is 90°. One degree is roughly equivalent to the diameter of two full moons side-by-side in the sky. Each degree is divided into 60 arc minutes and each minute further subdivided into 60 arc seconds.
Dew shield, Dew Cap – A covering of ABS plastic wrapped snuggly around the tube assembly and extending beyond the aperture of a telescope to prevent dew from forming on the objective lens of a refractor or correcting plate of a Schmidt-Cassegrain or Maksutov telescope.A tube attached to the forward end of a telescope used to prevent dew from forming on the lens as it cools down.
A flexible heating strap wrapped around the tube near the correcting plate or lens of a Schmidt-Cassegrain or Maksutov telescope that keeps the temperature of the corrector above the dew point of the ambient air.
In a refractor or catadioptric telescope, a mirror or prism used with that is used to turn the viewing axis 90 or 45 degrees to make it easier to veiw from behind the scope. In a Newtonian telescope, a diagonal is the mirror placed near the front of the telescope that reflects the collected light out the side of the tube, where the focuser and eyepiece are located.
A 45° or 90° diagonal used primarily for terrestrial viewing because it renders images as the unaided eye sees them — upright and left-to-right. Some resolution is lost when using a correct-image diagonal, so it is generally not recommended for astronomical viewing.
An accessory that fits into a telescope’s focuser and diverts incoming light at a right angle. This is for viewing at a more comfortable angle when using a refractor or catadioptric telescope. Prisms are used to redirect the light within the diagonal.
Dial-sight eyepiece – See ‘Plossl eyepiece’.
The point at which optical quality is good enough that the limits of viewing detail are determined by the physical properties of light, and not any optical defects in the telescope. See also RESOLUTION.
Diffuse nebula – A wide, irregularly-shaped cloud of gas that can be up to 100 light-years across.
Digital setting circles (DSC) – Electronic equivalent of a mechanical setting circles. These devices are used with an altazimuth telescope mounts and aid in aligning the telescope.
DSC – Abbreviation for Digital Ssetting Circles.
Dobsonian, Dob, Dobsonian telescope
A simple type of alt-az Newtonian telescope mounting designed to be both stable and inexpensive.The name comes from telescope builder John Dobson, who shuns the name “dobsonian” and prefers to call his telescopes “sidewalk telescopes”.
An altazimuth telescope system developed by John Dobson in the 1970s. A Newtonian reflector telescope is mounted in a box-like cradle that allows the tube to move smoothly up or down (in altitude) and to pivot in azimuth. Known for its stability, ease of use, and quick set up.
A Newtonian reflector telescope that sits in a special type of mount, called a Dobonsian mount. Dobsonian mounts sit on the ground and have a swiveling base. Dobsonian mounts are very stable, usually are made of wood, and reduce the cost of the telescope.
Double star/Binary Star
Two or more stars that appear very close in position. True double stars are are gravitationally bound and orbit about one another, while optical double stars simply appear close together as seen from the Earth.
An objective lens made of two elements of glass of different refractive qualities (or indices) to counteract the effects of chromatic aberration. Also known as an achromatic lens.
Dovetail bracket – A holder for finder scopes designed to interlock with a dovetail base on the optical tube and secured with a single thumbscrew.
Drawtube – The sliding tube of a telescope that permits the telescope eyepiece to be moved to such a position that the telescope is in focus.
A dust lane is lane of dark interstellar dust in a galaxy or emission nebula. Dust lanes obscure light.
– Faint light reflected from the Earth onto the dark part of the moon.
The plane defined by the Earth’s orbit around the Sun. Most planets in our solar system appear close to the ecliptic plane. The Earth’s axis is tilted 23.5° from the ecliptic, which causes our seasons.
ED glass –
A type of optical glass with exceptionally low dispersion often used to make apochromatic telescopes.
ED glass – A high-quality optical glass found primarily in apochromatic telescopes. Many ED (extra-low dispersion) lenses have a high content of fluorite, which reduces aberration.
ED glass Short for “Extra-low dispersion”, an optical glass that has superior refractive properties compared to standard optical glass. Lenses made with ED glass typically exhibit less chromatic aberration than lenses made with standard glass.
A motorized system incorporated into the telescope mount that enables a telescope to track celestial objects electronically. Drive systems can operate on the right ascension or declination axes, or both. With a hand controller the user can move (slew) the telescope in either direction. The motors also move the scope along the right ascension at the same rate as the east-west drift of the stars (See SIDEREAL RATE) so stars or other deep space objects can be continuously tracked in the eyepiece without continually having to adjust the aim of the scope. Without tracking, long exposure photography would be impossible.
Each of the components of a lens or other optical system is called an element of that system. Achromatic lenses, for example, consist of two elements – a convex lens and a concave lens.
Encke Division – The Encke Division of Saturn splits the A Ring into two parts.
Equatorial mount –
A type of telescope mount with an axis parallel to the axis of the earth(Right Ascension) . An Equatorial (or EQ) mount provides tracking of celestial objects for astrophotography when combined with a motorized drive,
keeping the telescope aimed at the same path of sky as the earth rotates and at the same speed. Also called German equatorial mount.
Emission nebula –
A nebula that emits light and glows An interstellar cloud of gas and dust in which hot embedded stars ionize much of the cloud’s gas atoms causing the nebula to emit its own light. The name is derived from the pattern of emission lines in the spectra of these nebulas. Examples include the Orion and Lagoon nebulas.
Erfle eyepiece –
A specific type of telescope eyepiece wich offers a wide field of view of 68 degrees or higher.
Invented by Heinrich Valentin Erfle (1884-1923). This is a type of eyepiece that consists of five or even six elements. It can easily be identified by the fact that its field lens is invariably concave. It has the widest field of view of all the eyepieces and gives a breath-taking view. It suffers from a rather short eye-relief and is always expensive.
Enke division –
This is the lens within an eyepiece that is nearest to the observer’s eye.
A (typically) 1.25″ or 2″ tube containing one or more lenses that is inserted in the focuser of the telescope and used to magnify the image formed by the objective lens.f/ratio A number determined by dividing the focal length of an objective lens or mirror by its diameter. Typical f/ratios of amateur telescopes range from f/5 to f/15.
Exit Pupil –
The diameter of light from the eyepiece that reaches the pupil of the eye.
A pliable rubber cup for eyepieces and binoculars that improves viewing comfort and helps block extraneous peripheral light.
The part of the telescope that actually magnifies the image. Using different focal length eyepieces will change the resulting magnification of the telescope. Typically, an observer will own several eyepieces in order to accommodate a variety of different viewing situations. Eyepieces usually consist of three or more lens elements. A wide variety of designs, each with their own characteristics, are available. Eyepiece A lens, or more usually a combination of lenses, whose function is to magnify the image produced by the primary lens or mirror of a telescope. There are many patterns of eyepiece, ranging from two-element designs such as the Ramsden and the Huyghenian, to four element (the Plossl) and six element (the Erfle) eyepieces.
An eyepiece with a 2″ barrel rather than the standard 1-1/4″. Two-inch eyepieces can provide exceedingly wide fields of view and are favored by deep-sky observers.
Eyepiece, illuminated –
An eyepiece with a red-illuminated crosshair, or reticle, that can be adjusted for brightness. Illuminated eyepieces are used in astrophotography to keep a faint guide star exac
Eyepiece, Kellner –
A good, inexpensive eyepiece design that uses a basic three-element design. one simple (single element) lens, and one two-element achromatic lens. It produces acceptably bright images, though with somewhat narrow fields of view. Kellners works best on long-focal-length telescopes and exhibit slight chromatic aberration.Kellner eyepiece Invented by Carl Kellner (1826-1855). This is another low-cost eyepiece, often marked ‘K’. It consists of three elements – an achromatic eye-lens spaced apart from a simple (usually planoconvex) field lens. Kelner eyepieces generally work well in telescopes.
Eyepiece element –
A single glass lens in an eyepiece. Various eyepiece designs employ various elements to increase contrast, flatten viewing fields, or provide sharp views at high magnifications.
Invented by Christian Huygens (1629-1695). This is a low-cost eyepiece, often marked with ‘H’ or ‘HM’. It consists of two simple lenses that are separated in the eyepiece by more than their focal lengths. The effect of this is that the focal plane of the eyepiece lies between the two lenses. The consequence is that the eyepiece cannot be used as a magnifying glass. Huyghenian eyepieces have a short eye-relief and a narrow field of view. They are perfectly acceptable in microscopes, but generally perform poorly in telescopes.
Eyepiece, Lanthanum –
An eyepiece design in which one of the eyepiece elements is made of Lanthanum, a rare-earth element that eliminates visual aberrations.
A four-element ocular with less chromatic aberration than a Kellner but a narrow field of view. Still highly regarded by many amateur astronomers as one of the best for lunar and planetary viewing.
Invented by Georg Simon Plossl (1794-1868). An eyepiece often marked with the letter P. This type of eyepiece consists of four elements, in the form of two achromatic, doublet lenses. The two doublets are disposed with their most-curved surfaces almost touching. The Plossl eyepiece is probably the easiest eyepiece for the home constructor to make. It is characterised by a long eye-relief and a wide, distortion-free field of view. It is also known as the ‘dial-sight’ or ‘symmetrical’ eyepiece.
A four-element eyepiece consisting of two nearly identical pairs of lenses. Plössls provide sharp, high-contrast images and flat fields of view. One of the best all-purpose eyepiece designs.
Eyepiece Ramsden –
Invented by Jesse Ramsden (1735-1800). An eyepiece often marked with the letters ‘R’ or ‘SR’. The eyepiece consists of two elements, in the form of two identical planoconvex singlet lenses. The two lenses are disposed with their curved surfaces facing one another, separated by a distance equal to the focal length of the lens. Ramsden eyepieces perform well but have a short eye-relief.
Eyepiece, Zoom –
Provides a continuous magnification range and hence the option of using a single eyepiece versus switching from one to another. The less expensive zooms sometimes suffer from inteField of view, apparent
The edge-to-edge angular diameter of the light emerging from the eyepiece as seen by the eye. It is an inherent specification for a given eyepiece type or design. The apparent field of view of an eyepiece is directly related to the true field of view seen through the telescope; for a given focal length eyepiece, the greater the eyepiece’s apparent field of view, the greater area of sky will be seen.
tly positioned relative to the crosshairs or in eyepieces with an illuminated micrometric scale for making small angular measurements.
The distance between the eyepiece and the eye where you can see the entire image through a telescope or binoculars. Eyeglass wearers need longer The distance, measured in millimeters, between the observer’s eye and the eyepiece lens in which the entire field of view remains visible. Eye relief varies with the optical design but generally increases with decreasing magnification. Long eye relief is advantageous for observers who wear glasses, as they don’t have to put their eye directly on the eyepiece to see the entire field of view, nor do they have to remove their eyewear.
This is the lens within an eyepiece that gathers the light from the objective lens or mirror. It is the eyepiece lens that is nearest to the objective lens or the primary mirror.
Field of View (FOV) –
The maximum view angle of a lens. The number supplied by a manufacturer is the Apparent FOV. The True FOV (or Actual FOV)is found by dividing the Apparent FOV by the magnification.
Filter As with camera filters, telescope filters are used to selectively block or pass certain wavelengths or colors. oOlored filters are commonly used by planetary viewers to bring out detail. Deep sky viewers often use “light pollution filters”, which admit only a narrow band of light in order to see faint images and block the light from streetlights and signs.
Field stop –
The field stop is the aperture that limits the field of view of a telescope eyepiece lens system. Most field stops consist of a metal ring inside an eyepiece barrel that limits the field of view of the eyepiece lens system. A telescope eyepiece field stop appears as a circle surrounding the visible field of view when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) of the eyepiece and is a fixed property for each eyepiece design. Zoom eyepieces with an adjustable focal length range may employ an adjustable field stop, the aperture of which shrinks and expands in correlation with focal length setting adjustments.
A small, low-power telescope attached parallel to the main telescope that aids in locating celestial objects and in pointing a telescope.
Field of view, true –
The actual field of view of the eyepiece when inserted into a telescope. The true field of an eyepiece may be determined by dividing the eyepiece’s apparent field of view (provided by the manufacturer) by the magnification. For example, an eyepiece with an apparent field of view of 40° at a magnification of 30 yields a true field of view of 1.3°. Finder More usuallyFinder or Finderscope A low powered auxiliary telescope mounted on a telescope used to make it easier to point the main telescope at some object.
Filter, color –
Glass filters, each of a specific color, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various color filters reduce other interfering or scattered wavelengths that blur certain wavelength-specific features. Red filters, for example, bring out Martian surface detail while green increases contrast of Jupiter’s Red Spot. Also called planetary filters.
Filter, light-pollution –
A filter that threads on to an eyepiece or rear cell of a Schmidt-Cassegrain that blocks wavelengths of light pollution sources such as mercury vapor and high-pressure sodium, but pass wavebands specific to deep-sky objects, such as hydrogen alpha, hydrogen beta, and oxygen III.
Filter, moon –
A glass filter in an aluminum cell that threads onto an eyepiece barrel and reduces the Moon’s glare so that it can be comfortably observed. Without the eye being overwhelmed by moonlight, more lunar detail becomes apparent.
Filter, planetary –
Glass filters, each of a specific color, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various color filters reduce other interfering or scattered wavelengths that blur certain wavelength-specific features. Red filters, for example, bring out Martian surface detail while green increases contrast of Jupiter’s Red Spot. Also called color filters.
Filter, solar –
A glass filter that fits snugly over the aperture of a telescope and allows the photospheric surface of the sun — sunspots and solar faculae — to be observed comfortably and safely. A good solar filter blocks some 99.99% of the sun. Observing the sun without a solar filter may cause serious damage to the eye.
Variable-polarizing filters act as dimmer switches to bright celestial objects, including the Moon or a planet. The filter, which threads on to 1.25″ eyepiece barrels, consists of two pieces of polarized glass mounted in an aluminum cell that, depending on how much it is rotated, varies light transmission from 1% to 40%.
Flint Glass –
One of the optical components of a doublet or achromatic objective lens of a refractor. Combined with crown glass, the elements work together to bring the colors inherent in white light into the same focus.
Flashlight, red –
A flashlight that emits only pure red light, so as not to impair the night vision of the observer. The human eye is insensitive to red light.
This is a solid, transparent chemical substance that is used to make one of the elements in a triplet, apochromatic objective lens.Natural fluorite mineral crystal or artifically grown fluorite crystal material used to make lenses with exceptionally high refractive index and low dispersion. Fluorite glass is very expensive but can be used to make telescopes with exceptionally low chromatic abberation.
Focal length, telescope –
The distance from the center of a curved mirror or lens at which light rays converge to a single point. Light rays are assumed to come from an effectively infinite distant pinpoint light source (like a star) and so are parallel when they reach the mirror or lens. The focal length is an inherent specification of a mirror or lens and is one of the factors in determining resultant magnification for a telescope (along with the focal length of the eyepiece being used).
Focal length The distence from a mirror or lens at which the light rays being focused are brought to a common point, and (in a telescope) an image is formed. If you’ve ever focused the sun’s rays with a magnifying glass, the distance from the lens at which the rays are most concentrated is the focal length of the lens.Focal length The focal length of a lens is the distance from the lens to the place at which light from a distant object is brought to a focus. The focal length of a lens is decided by the curvature of its glass surfaces – the more curved the surfaces, the shorter the focal length.
Focal ratio (f-ratio)-
This is found by dividing a telescope’s focal length by its aperture. Any telescope with a focal ratio larger than f/12 would probably be said to be “slow”, and any telescope with a focal ratio smaller than f/6, is said to be “fast”. his is the relationship between the diameter of a lens and its focal length. It describes the ‘speed’ of the lens. For example, a lens with a focal length of 60cm and a diameter of 4cm has a focal ratio of f/15, because 60cm divided by 4cm is 15.The ratio of a telescope’s focal length to its aperture. Short focal ratios (f/5, f/4.5) produce wide fields of view and small image scales, while long focal lengths produce narrower fields of views and larger image scales.
Focal plane –
When a lens brings light to a focus, the image produced by the lens lies on an imaginary plane, which is the same distance from the lens as the focal length.The imaginary plane suspended in space where the image is created by a telescope. The eyepiece magnifies a section of the telescope’s focal plane in order to produce the images seen. When prime-focus astrophotography or telephoto photography is attempted, the camera’s film is positioned at the precise position of the telescope’s (or camera lens’) focal plane.
Focal reducer – Shortens the effective focal length of long focal-length telescopes by nearly half, thus providing wider fields of view for visual or photographic use.
A device into which an eyepiece is inserted and adjusted to bring a telescopic image to focus. A focuser can be as simple as a manual drawtube, but the more efficient type is the “rack-and-pinion” design, whereby a threaded axle affixed with knurled knobs at each end meshes with a threaded drawtube, enabling it to be moved up or down through the focal plane.
Focuser (Crayford) –
Focal length, eyepiece –
The distance from the center of the field lens (where light passes through the first element of the eyepiece) to the focal point. The magnification of a telescope varies with the eyepiece focal length. Short focal-length eyepieces produce high magnifications; long focal-length eyepieces produce low magnifications.
German equatorial mounting A telescope mount consisting of two rotating axes at right angles. The axis, called the Right Ascension (RA) axis or polar axis, is parallel to the Earth’s rotational axis. The second axis, called declination axis, points toward the celestial equator. The end of the declination axis carries a counterweight.
Globular cluster –
Dense clusters of stars bound by gravity. Many globular clusters surround galaxies, including our own Milky Way galaxy.
Goto, or Go-To Telescope – telescope mount and software that automatically points a telescope to astronomical objects based on data base information listing celestial coordinates (right ascention and declination)
Guide scope –
A high-magnification telescope that is fixed to an astronomical telescope. It is used to keep the astronomical telescope exactly aligned with an astronomical object, such as a star, while the astronomical telescope is being used to photograph the object.
Illuminated reticle eyepiece An eyepiece with a red-illuminated crosshair, or reticle, that can be adjusted for brightness. Illuminated eyepieces are used in astrophotography to keep a faint guide star exactly positioned relative to the crosshairs or in eyepieces with an illuminated micrometric scale for making small angular measurements.
(IC) Index Catalog
A star classification system that supplements to the New General Catalog, which lists nebulae and star clusters with an IC number.
Limiting magnitude – The magnitude of the dimmest star you can see overhead on a given night.
Light Bucket- Slang term for a large aperture telescope, usually a dobsonian mounted reflector
Light pollution – Stray light generated by unshielded high-pressure sodium and mercury vapor street and security lighting, billboard uplighting, glaring porch lights, and poorly designed landscape illumination that radiates upward or into the eyes, lowering sky contrast and significantly degrading the visibility of the stars.
Light grasp – A measure of a telescope’s ability to gather light. Light grasp is determined almost solely by the diameter of the aperture of a telescope’s objective lens: the larger the aperture, the greater the light grasp and the greater the resolution or amount of detail that may be observed. Measured in square inches.
The cluster of 30 galaxies to which we belong. The largest of the galaxies are the Andromeda galaxy, Triangulum, and the Milky Way galaxy.
Local sideral time (LST) – Local Sidereal Time (LST) is measured by the right ascension that is currently at the local observer’s meridian. Astronomers use LST to aim telescopes at celestial objects.
Magnification A measure of the enlargement of an object seen through an optical instrument. More specifically, it is a measure of the angle subtended by an image viewed with optical aid divided by the angle subtended by the same object without optical aid. Magnification is determined by dividing the focal length of the telescope (or binocular) by that of the eyepiece. Hence a telescope with a 1200mm focal length and an eyepiece of 40mm yields a magnification of 30x. The human eye is considered 1x magnification.
Magnitude, Absolute – A measure of a star’s true or intrinsic brightness. Essentially, astronomers decide this by gauging how bright the star would appear to the eye if brought to a standard distance of 10 parsecs, or 32.6 light-years. Alnitak, the easternmost star in Orion’s belt, has an apparent magnitude of 2.05 but an absolute magnitude of -5.9, because that’s how bright it would appear if it lay 10 parsecs away. The Sun, with an apparent magnitude of -26.7 has an absolute magnitude of 4.8.
Magnitude, or Apparent Magnitude – the means of measuring the brightness of a star or other deep sky object. Lower numbers to the brighter stars. Our Sun’s magnitude is -26.7, the full Moon is -12, Venus at brightest is -4.7 and the star Sirius is -1.5. The faintest star visible through any visual telescope is around magnitude 20. The naked eye, from a dark location, can perceive stars of 6, or slightly higher.
A catadioptric reflecting telescope similar to a Schmidt, except that it employs a deeply curved full-aperture lens called a meniscus to correct for spherical aberration. Maksutovs utilize spherical mirrors and can be designed with a Cassegrain configuration, in which they are called Maksutov-Cassegrains, or as Newtonians, in which they are called Makstutov-Newtonians (or MAK-Newts, for short).
Maksutov Cassegrain A catadioptric telescope using a spherical primary mirror and secondary mirror, and a spherical corrector plate. Common Maksutov Cassegrain designs include the Questar and the popular Meade ETX series.
Maksutov-Cassegrain telescope (MCT)
A type of compound telescope that uses both lenses and mirrors. Also called a Mak-Cass telescope.
Meniscus lens – The front lens in Maksutov telescope.
The list of 110 deep-sky objects compiled by eighteenth century astronomer and comet researcher Charles Messier. He cataloged bright objects– “nebulas” (star clusters, nebulae, and galaxies) — for the benefit of other comet hunters, so they could avoid mistaking them for comets. These brightest od deep space objects are often the first targets of amateur astronomers.
A frame which holds the primary mirror of a reflecting telescope. The frame must be designed to hold the mirror securely, but not tightly to avoid “pinching” the glass (and thus introducing image distortion), well ventilated to allow the glass to cool down to ambient air temperature, and adjustable so the mirror can be properly collimated.
The imaginary north-south line in the sky that passes through the observer’s zenith overhead.
Messier objects –
Mount – The mechanical device that connects the telescope to its tripod and allows the telescope to move freely. The commonest types of mount are the ‘altazimuth’ and the ‘equatorial’.
Multi-coated – All air-to-glass surfaces are coated with at least one layer of magnesium fluoride, with some surfaces receiving multiple anti-reflection coatings. A step up from “fully coated”, but not quite as good as “fully multi-coated”.All air-to-glass surfaces are coated with at least one layer of magnesium fluoride, with some surfaces receiving multiple anti-reflection coatings. A step up from “fully coated”, but not quite as good as “fully multi-coated”.
A discrete cloud of dust and gas associated with either star formation (see EMISSION NEBULA), reflecting the light of adjacent stars (see REFLECTION NEBULA), or as a shell surrounding a dying star (see PLANETARY NEBULA). A nebula can also appear as a dark cloud seen silhouetted by stars behind it or as a “hole” in space. These are referred to as Dark Nebulas.A large, deep-space object comprised of gas and dust. A nebula that glows with its own light is called an emission nebula while one lit from nearby stars is called a reflection nebula. A nebula that blocks light is called a dark nebula.
New General Catalog (NGC) – A classification system that lists over 13,000 deep-sky celestial objects. Abbreviated as NGC.
NGC – An acronym for New General Catalog of Nebulae and Clusters of Stars, first compiled in 1888 by Danish astronomer J.L.E. Dreyer at Armagh Observatory, Ireland. Today the catalog contains 7840 deep-sky objects. A fair number are visible in small telescopes, but many require large-aperture instruments and dark skies.
Near focus – The minimal distance at which a binocular and/or spotting scopes can attain focus. Near focus is an important quality for bird watchers and wildlife devotees who want to get as close as possible to their quarry.
Newtonian reflector – A telescope designed with a spherical or parabolic primary mirror at one end of the tube that reflects and focuses light back along the optical axis to a secondary mirror, which, in turn, redirects the light at a right angle to the optical axis and into an eyepiece. First designed by Isaac Newton about 1670.
Objective, Objective lens The main light-gathering lens or lens system of a refracting telescope or binocular.This is the large lens at the front of the telescope. It is invariably either achromatic or apochromatic. Simple lenses are never used nowadays as the objective lenses of telescopes. It is often called ‘the OG’ (objective glass).
Ocular – Another name for ‘eyepiece’.
Off-axis-guider – A device employing a lateral prism to capture a small outlying area of the telescope’s field of view allowing an astrophotographer to view it through a guiding eyepiece during a long exposure.
Open cluster – A loose array (though some can be remarkably compact) of tens to thousands of stars located near each other in space. Open clusters are weakly bound gravitationally and tend to gradually disperse over periods of millions of years. Examples include the Pleiades, Hyades, and the Double Cluster.
Opposition – When the Earth is exactly between a planet and the Sun, that planet is said to be in opposition.
Invented by Ernst Abbe (1840-1905). Often marked ‘O’ or ‘Or’. This is a four-element eyepiece, which consists of a triplet field lens and a singlet eye-lens. It performs excellently in telescopes.
Optical tube assembly (OTA) –
The main tube of a telescope including the primary mirror or objective lens, focuser, and finder scope. The optical tube assembly does not include a mount or tripod.The housing and optical system of a telescope. It does not include the mount, eyepieces or other accessories.
Parfocal – Commonly refers to a group of eyepieces that can be swapped without refocusing. Some makers produces sets of eyepieces that are designed to be parfocal.Refers to eyepieces of varying focal lengths but designed so that little or no focusing is required when switching from one to another. Usually manufactured as a series.
Parsec – A unit of distance equal to 3.26 light-years, and the the distance at which a star would have a parallax of 1 second of arc.
Parabolic mirror – A mirror ground to a shape of a paraboloid, which brings all incoming light rays to a focus.
Polar axis – The telescope mount axis that is parallel to the Earth’s axis. With a motorized drive, celestial objects can be tracked and remain in the eyepiece field of view throughout the night.
Polar alignment – Aligning the right ascension axis of an equatorial mount to the celestial pole so that it is parallel with Earth’s axis. Rough polar alignment is required for a clock drive or guiding system to track east-to-west without appreciable north-south (declination) drift. For astrophotography, polar alignment must be precise.
Equatorial mounts must have one axis- the Right Ascension, or polar, axis aligned with the Earth’s axis of rotation in order to be able to track the rotation of the earth. This is called Polar Alignment.
Polar alignment scope – A small finder scope built into the right ascension axis of an equatorial mount that greatly facilitates polar alignment. Also called a polar axis finder.
Polar axis – The right ascension axis of an equatorial mount aligned such that it is parallel to Earth’s rotational axis. See also RIGHT ASCENSION and SETTING CIRCLES.
Primary mirror –
The main light-gathering mirror of any reflecting telescope.
Pole Star – A star located almost due north or due south and used for navigation or telescope alignment. Polaris is currently the pole star of the Northern Hemisphere.
A device into which an eyepiece is inserted and adjusted to bring a telescopic image to focus. A focuser can be as simple as a manual drawtube, but the more efficient type is the “rack-and-pinion” design, whereby a threaded axle affixed with knurled knobs at each end meshes with a threaded drawtube, enabling it to be moved up or down through the focal plane.A device that is used to move the drawtube of a telescope so that the eyepiece is correctly focused. A toothed strip (the rack) is attached to the drawtube and a toothed wheel (the pinion) is meshed with the rack and is rotated by a handwheel on the side of the telescope. The rotation of the pinion causes the rack, and thus the drawtube, to move forwards or backwards.
Porro Prisms – A binocular prism system that contains two right-angle prisms in each barrel offset from one another, requiring that the objective lenses be spaced further apart than the eyepieces. Optically, porro prisms often perform better than their roof prism counterparts (see PRISMS, ROOF).
Parfocal Photography –
A technique in which a camera is coupled to a telescope’s tube assembly via a bracket (or mounted separately on a tripod) and positioned where the eye would normally observe the image. The image formed by the eyepiece is then photographed through the camera lens.
Photography, eyepiece projection –
An astrographic technique similar to afocal photography except that the camera lens is not used. Instead, the eyepiece image projects directly onto the film. Projection systems can use barlow lenses or any type of eyepiece.
Photometry – The measurement of apparent magnitudes of stars.
Planoconvex – This describes the shape of a lens that is flat on one face and convex on the other.
A type of nebular where a cloud of gas that has been discharged by a central star.
Planetary alignment – When many of the planets in the solar system are in a line.The shell of gas expelled by a dying star. Many appear circular or doughnut shaped. Examples include the Ring and the Dumbbell nebulas.
Planisphere – A hand-held circular star map or “star wheel” that shows the positions of the constellations and prominent deep-sky objects in the sky from a given range of latitude at any desired date and time.
Prime Focus Photography – An astrographic technique in which a 35mm camera is positioned at a telescope’s prime focus — the focal plane of the primary mirror or objective lens. No eyepiece is used. Instead, the telescope and camera work together as a unit. This method provides the highest light transmission with lowest possible power, the widest field of view, and the best definition. Sometimes the image plane cannot be accessed because it falls within the focusing tube.
Piggyback Photography – A method whereby a 35mm camera is “piggybacked” via a bracket or some other means to the tube assembly of telescope on an equatorial mount equipped with an electronic drive. As the telescope tracks in right ascension, exposures of wide-angle regions of the sky can be made. If a telephoto is attached, magnified images of the Moon, comets, or deep-sky objects can be captured.
RACI-Finder scope – right-angle correct image finder
Same as a finder scope, but with a right-angle diagonal to divert incoming light 90° to the light path rather than sighting along the tube. Permits more comfortable viewing for observers using reflectors.
Right Ascension – The axis of rotation of an Equatorial telescope mount aligned with the earth’s axis of rotation. It’s calibrated in in 24 hour segments, since the Earth takes 24 hours to complete one rotation.
RACI-Finder scope – right-angle correct image finder– Same as a finder scope, but with a right-angle diagonal to divert incoming light 90° to the light path rather than sighting along the tube. Permits more comfortable viewing for observers using reflectors.
Reflection nebula – An interstellar dust cloud that reflects light from adjacent stars as opposed to having its constituent atoms ionized by embedded hot stars, which allows the nebula to shine by its own light (see EMISSION NEBULA). In long-exposure images, reflection nebulas are blue in color. Examples include the blue half of the Trifid nebula and the nebulosity surrounding the Pleiades star cluster.
Refracting or refractor telescope
A telescope that uses lenses, instead of mirrors, to magnify and focus images.
roof Prisms – A compact binocular prism system that allows the objective lenses to line up directly with the eyepieces and hence yields a more portable binocular. Roof prism binoculars lose slightly more light to reflections than porro prism binoculars. High-quality roof prism binoculars compensate for this with special optical coatings.Invented by Giovanni Battista Amici (1786-1863). This type of reflecting prism is used to divert the light through 90 degrees and at the same time it completely inverts the image. It is used in a telescope to produce a view that is the right way up and the right way round.
Reflector – A type of telescope in which the light from a distant object is gathered by a large, concave mirror at the rear of the telescope.
Resolving Power – The ability of a telescope to separate, or split, objects that are close together.
The ability of a telescope to resolve, or reveal, details.The resolving power of a telescope can be calculated with the following formula: Resolving power (in arc seconds) = 4.56 ÷ aperture of telescope (in inches). In metric units, this is: Resolving power (in arc seconds) = 116 ÷ aperture of telescope (in millimeters). Note that the formula is independent of telescope type or model, and is based only upon the aperture of the telescope. So the larger the telescope’s aperture, the more it is capable of resolving. This important to keep in mind when observing astronomical objects which require high resolution for best viewing, such as planets and double stars.
Reticule – A glass disc that is inserted at the focal plane of an eyepiece. The disc is engraved with cross wires, dots, rings, or a scale, so that the eyepiece can be used to measure an object. Also used in the in finder scopes or eyepieces for centering purposes.
Relay lenses – A set of lenses inside a telescope, between the objective lens and the eyepiece. The purpose of this set of lenses is to turn the telescope image the right way up and the right way round.
Right Ascension (R.A.) – A coordinate used in the equatorial coordinate system, somewhat analogous to longitude on Earth, but scaled in hours, minutes, and seconds eastward along the celestial equator. One hour of right ascension is equal to 15 degrees, which is the angle through which the celestial sphere rotates in an hour’s time. See also POLAR AXIS and SETTING CIRCLES.
Part of a coordinate system similar to latitude on the Earth’s surface. Right-ascension (RA) is measured in hours, minutes and seconds. Zero degrees of right ascension is the position of the Sun during the Vernal (Spring) equinox. RA is used along with Declination to find objects in the night sky.
Schmidt-Cassegrain telescope A type of compound telescope, abbreviated SCT, that uses both lenses and mirrors to focus light.
A catadioptric configuration consisting of a spherical primary mirror and a convex secondary mirror affixed to an aperture-wide correcting plate. The secondary mirror directs the focused light back down the tube through a hole in the center of the primary mirror and into an eyepiece or camera. The thin correcting plate is curved along its periphery so that it counteracts aberrations inherent in the spherical mirror. This adaptation produces a flat field and high-quality images in a compact telescope tube.
Schmidt camera – A type of telescope used for astrophotography that has an extremely fast focal ratio and provides very high-quality images.
Secondary mirror – In reflecting telescopes, the smaller mirror that reflects light from the primary mirror to the eyepiece.
Setting circles – Two graduated circular scales provided on the right ascension and declination axes of a telescope that aid in locating an object by its celestial coordinates. The declination scale runs from -90° to 90°. The R.A. scale goes from 0 hours to 24 hours, subdivided into minutes and, if the scale is fine enough, seconds. Setting circles are set up by first sighting on star of known right ascension and declination and adjusting the R.A. and Dec. circles to those coordinates. Then the observer need only maneuver the telescope to the coordinates of the desired target, which should be in the field of view or very near it.
Circular scales attached to the telescope marked off in degrees of Declination and hours of
Sidereal drive – A motorized drive used to make a telescope track stars across the sky as the Earth rotates.
Sidereal Time – The rate at which an equatorial mount has to be driven around its right ascension or polar axis to track stars continuously across the sky. It is equivalent to one rotation per 23 hours 56 minutes and 4 seconds.
Slew – To move, manually or electronically, a telescope about its mount’s axes of rotation at a relatively fast rate. Typically, a telescope is slewed to an object’s general location, then fine adjustments are made to position the object within the telescope’s field of view.
Slow-motion controls – Knobs, or cables affixed with knobs, attached to a telescope mount that allow the observer to manually move a telescope’s tube incrementally. Slow-motion controls can be used to center an object within the field of view.
Spherical aberration – A blurring of an image caused by the inability of a mirror to focus properly.
Spider, Spider Vanes – The arms or struts used to support a secondary mirror in Newtonian reflecting telescopes.A 3- or 4-vaned frame at the front of a reflecting telescope tube that supports the secondary mirror.
Schmidt-Cassegrain A catadioptric telescope design, commpnly referred to as an “SCT” that uses two mirrors and an aspheric “corrector plate” on the front of the tube to focus incoming light.
Spiral galaxy – A disk galaxy with a whirlpool or pinwheel shape. The pattern may appear tightly wound or extremely loose. Seen “edge-on” a spiral galaxy looks saucer shaped and exhibits a central bulge and dust lane.
Spotting scope – A small, portable telescope used primarily for terrestrial observing, such as nature study and bird watching. Most spotting scopes use prisms to provide an image that matches the naked eye.
Spherical aberration – A common form of optical aberration caused when the light from the periphery of a mirror or lens doesn’t have the same focal point as light from the center. A mirror or lens can be otherwise perfect, but suffer from spherical abberation from being improperly figured. The original space telescope mirror suffered from spherical abberation that was corrected by later installing an additional set of corrective lenses.
Spherical, Spherical mirror – The type of mirror used in catadioptric telescopes (Schmidt-Cassegrains and Maksutovs) whose curve forms part of a sphere. Spherical mirrors are easier to manufacture than parabolic mirrors, however they produce positive spherical aberration.
A mirror on some refracting telescopes that bends light 90 degrees, allowing an observer to look down through the eyepiece instead of squatting to look directly through the telescope.
Star Diagonal – An eyepiece adapter containing either a flat mirror or prism that directs the light at a right angle to the optical axis. Used primarily for refractors, Cassegrains, or Schmidt-Cassegrains so that the observer doesn’t have to look “straight through” the telescope tube at an uncomfortable angle.
Star hopping – A deep-sky observing technique whereby the telescope is moved in successive steps from star to star closer toward the field of view of the object of interest
Star cluster – A group of stars that are bound together gravitationally.
T-Adapter – A camera adapter that attaches to the body of a 35mm camera (without the lens) and then connects to the focuser for prime-focus astrophotography.
A camera adapter required for eyepiece projection astrophotography. An eyepiece goes within the body of the tele-extender.
Telescope, “Fast” – A short focal-length (f/4 to f/6) telescope that renders bright prime-focus images on film, permitting shorter, or faster, exposures. Fast telescopes also provide a wider field of view compared to longer scopes.
A short focal-length telescope designed for sweeping very large regions of sky such as star fields (hence the name “rich”). Also known as wide-field telescopes.
Telescope, “Slow” – A long-focal-length telescope (f/8 or greater) that takes more time to produce a bright image on film at prime focus. Visually, images in both fast and slow telescopes, when viewed at the same magnification, have exactly the same apparent brightness.
A scope used during the day or in low light to observe terrestrial fields of view. Applicable to birding, sightseeing, and nature study.
Singlet – A lens that is made of only one piece of glass. It is a single-element lens.
Star diagonal – A device that contains a mirror or right-angle prism whose purpose is to turn the light from the objective lens of the telescope through a right-angle. This makes the telescope more comfortable to use when it is pointed upwards. The star diagonal is an accessory that fits in the rear of the drawtube.
Spherical Aberration – An aberration of a spherical lens or mirror whereby light rays falling near the edge of the optical element converge inside of focus while those near the center converge outside of focus, resulting in a sharp center and softer edges.
A popular brand of unit-magnification or single-power (1X) telescope finder that displays 3 red concentric circles in the viewfinder.
A telescope that produces an image that is the right way up and the right way round. This is achieved by the use of a set of relay lenses within the telescope or by a prism. To avoid light loss within the telescope, astronomical telescopes do not have these extra lenses or prisms.
Triplet – A lens that is made of three components. Small triplets are always cemented, but the large triplets that are used as the objective lenses for apochromatic refracting telescopes are usually air-spaced. If you are buying a telescope that has such an objective lens, check that the fluorite component is not the front element of the lens. Fluorite is delicate and if your lens has this component as its front element, you could be buying into a lot of trouble.
T-adapter – An adapter, either a T-ring, T-adapter, or tele-extender used to attach a 35mm SLR camera to a telescope for astrophotography.
Tracking – Using a motor drive to remain continuously fixed on a celestial object as the earth rotates.
Visual magnitude – A measure of the brightness of an object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness.
Visual limiting magnitude – The magnitude of the dimmest star that you can see overhead.
Wave surface accuracy The standard tolerance for high-precision optics is one-eighth of the wavelength of yellow-green light, which is 22 millionths of an inch. In numeric terms, 1/8-wave means that the surface of a parabolic mirror must not depart more than about 3 millionths of an inch from its shape. Zenith The point in the sky directly over the observer’s head.
Widefield eyepiece – An eyepiece with an apparent field of view (FOV) of more than about 50 degrees.
Worm gear – A toothed wheel that meshes with the thread of a “worm” or screw that, when rotated, uniformly moves an axis of a telescope mount.