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We are happy to provide this glossary of astronomical terms which includes definitions for some of the most common words used in the amateur astronomy hobby. Click on the letters and symbols below to filter your selection.

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1/4"-20 Mounting
A 1/4"-wide threaded socket or adapter for attaching binoculars, small telescopes, and cameras to the pan head of a standard photo tripod. Some binoculars and telescopes come equipped with a 1/4"-20 adapter plate or L-bracket.

Aberration, Chromatic
An aberration of a lens in which light of different colors is not brought to the same focus point. This causes false colors and fringing in the image. See also ACHROMAT.

Aberration, Spherical
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.

Absolute Magnitude
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. See also MAGNITUDE

Accessory Tray
A shelf or bin attached to a telescope's mount for holding eyepieces, diagonals, barlow lenses, etc. It may also serve as a center-brace for a tripod.

A refractor objective lens comprised of two elements of different types of glass, which serves to greatly reduce chromatic aberration. Can also be called an achromatic objective lens.

Active Region
A small area on the solar surface that has a strong and complex magnetic field. Sunspots are a good indicator of an active region, though not all active regions include sunspots. Active regions also can be the source of solar flares. Similar to sunspots, the number of active regions present on the sun at any given time is dependent on the solar cycle, an approximate 11-year cycle of activity, most obvious in the number of sunspots visible at any particular time.

Altazimuth Mount
A mount that allows a telescope to move freely in altitude (up and down) and azimuth (around the horizon). A standard photo tripod is an example of an altazimuth mount.

Altitude and Azimuth
Coordinate terms used in the horizontal coordinate system. Altitude is the angular distance of a celestial object above (numbered positive) or below (numbered negative) the horizon. Azimuth is the angular distance of an object measured eastward along the horizon from cardinal north (or 0°).

AMCap is a small yet fully functional video capture and preview application compatible with Microsoft DirectShow (formerly ActiveMovie, hence the name). Used in planetary imaging applications.

Angstroms ()
A unit of length commonly used to measure the wavelength of light. It is equal to 0.1 nanometers, or 10 billionths of a meter. The wavelength of Hydrogen-alpha light is 656.28 nanometers, or 6562.8 angstroms.

The diameter of a refractor's light-gathering objective lens or a reflector's primary mirror.

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.

Apparent Magnitude
Refers to how bright the star appears to the naked eye. The difference between the apparent brightness of two stars follows a logarithmic ratio of 2.512. Hence, a star that is three magnitudes less than another is (2.512) 3, or about 16 times brighter. Using this system, stars can also have negative magnitude values, and these are the brightest we see in the sky. See also MAGNITUDE.

Arc Minute
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 Second
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.

An acronym for Astronomy Common Object Model, which is an open source initiative to create standard interfaces for a range of astronomical equipment that operate in a standard Microsoft Windows environment. ASCOM Standard Drivers are typically provided with cameras designed for astrophotography, including many sold by Orion.

An aberration of a lens or mirror in which light does not come to focus at the same plane. Most often encountered with inferiorly ground optics.

Astronomical Seeing

Atmospheric Transparency

An electronic accessory used to take long exposure photography. The Autoguider locks onto the image of a star and controls the tracking of the telescope to a far finer degree than a telescope drive system can do by itself.

AVI is an acronym for Audio Video Interleave, a electronic multimedia file format introduced by Microsoft in 1992. Planetary images are often captured and manipulated with this format.

A series of rings placed along the light path within a refractor's tube with apertures that gradually "step down". Baffles prevent internal light scatter and hence increase contrast.

BAK-4 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.

Balance System
A system of weights most often used with equatorial mounts to counterbalance the optical tube assembly of a telescope.

The bandpass specification of a hydrogen-alpha (H-α) solar telescope refers to the range of wavelengths around the (H-α) line that is transmitted through the telescope. The bandpass range is usually between 1 angstrom () for the PST, down to a very narrow <0.5 for a double stack system. The narrower the bandpass, the higher the contrast will be for viewing surface features such as filaments, flares and surface granulation.

Coronado PST's with a single etalon have a bandpass of < 1
Coronado SolarMax telescopes with a single etalon have a bandpass of < 0.7.

Prominences at the edge of the sun are easy to see even with a wider single etalon bandpass system (single stacked) since they stand out in profile against the black background of space and thus have higher contrast to start with. But to help pull out surface disk details even further, a narrower bandpass double stacked system is advantageous.

You can further narrow the bandpass of your primary single stack etalon filter by adding an additional etalon filter to the telescope. The term "double stacking" is used to describe this process.

Coronado PST's and SolarMax telescopes with two etalons (double stacked) have a bandpass of <0.5.

Barlow Lens
An achromatic lens that increases the effective focal length of the objective lens or primary mirror and doubles or triples eyepiece magnification. Hence, a 30mm eyepiece becomes a 15mm eyepiece when used with a 2x barlow. Given a telescope with a focal length of 1200mm, a 30mm eyepiece yields 40x alone and 80x with a barlow lens.

A procedure used to enhance the sensitivity of astronomical cameras. Binning 1x1 simply means each pixel on the CCD imaging chip counts as one pixel. Binning 2x2 or more is a method of increasing the sensitivity of the CCD imaging chip at the expense of resolution; adjacent pixels are summed together into super-pixels. For example binning 2x2 would cause 4 pixels to act as one larger pixel — at a quarter of the resolution but about four times more sensitive.

A portable optical instrument that consists of two small telescopes mounted together with a central focusing device and viewed through using both eyes.

Binocular Tripod Adapter
An L-shaped adapter that couples a binocular to the pan head of a standard tripod.

BK-7 Glass
Also known as "borosilicate" glass. Most optical prisms are made of BK-7 glass.

Blocking Filter (BF5, BF10, BF15, BF30)
The 2nd critical part of the SolarMax system (the first being the etalon). The Blocking Filter (BF) is not a single filter, but instead, multiple filters that make Coronado telescopes safe for viewing the Sun. The Coronado etalon passes the hydrogen-alpha (H-α) wavelength of light through the telescope to the Blocking Filter, but also allows its harmonics through as well. The Blocking Filter blocks all of the light transmitted by the etalon, except H-α. When combined with the etalon, only the very specific H-α wavelength centered at 6562.8 angstroms () is transmitted to the eyepiece, and overall image brightness is reduced. Coronado telescopes MUST be used with both an etalon and rear Blocking Filter together in order to provide a safe and enjoyable view of the sun.

There are different sizes and shapes of BF filters available, using a number designation to differentiate between models. The number designation describes the clear aperture in millimeters of the H-a cutoff filter used in the blocking filter. For example, the BF10 uses a 10mm clear aperture H-α cutoff filter, while the BF15 and BF30 use a 15mm and 30mm clear aperture filter respectively. The larger the clear aperture, the larger the unvignetted field of view provided through to the eyepiece or camera sensor. When using wide angle eyepieces, higher magnification, or a larger camera sensor, it is always recommended to use the largest BF available in your budget.

The blocking filter selected also determines whether or not a full disk image of the Sun will be visible when using the etalon and BF set together on a different telescope:

BF5: provides full disk image when used with telescopes having a focal length of 500mm or less.
BF10: provides full disk image when used with telescopes having a focal length of 1,000mm or less.
BF15: provides full disk image when used with telescopes having a focal length of 1,500mm or less.
BF30: provides full disk image when used with telescopes having a focal length of 3,000mm or less.

The BF5, BF10 and BF15 filters are built into the right-angle star diagonal located directly below the eyepiece. These diagonals accept 1.25" eyepieces and accessories only but also have a t-thread connection ideal for connecting imaging equipment.

The BF30 uses a convertible design allowing use in a straight through or 90-degree configuration. When used in the straight through configuration, a 2" receiver with removable 2" to 1.25" zero-length adapter is used. The straight through design is advantageous when using larger format camera sensors, or when using 2" accessories. When the BF30 is used in the 90-degree diagonal configuration, the BF30 attaches to the included diagonal body which accepts 1.25" eyepieces or accessories, but also has a t-thread connection available.

All Coronado Blocking Filters require use of a Coronado SolarMax etalon to make the telescope safe for solar viewing.

Blower Bulb
A pliable hand-held bulb that, when squeezed, produces brief gusts of air. Used to blow dust off of optical surfaces.

Camera Adapter
An adapter, either a T-ring, T-adapter, or tele-extender used to attach an interchangeable lens camera to a telescope for astrophotography.

The optical configuration of a reflecting telescope in which a convex hyperboloid secondary mirror mounted inside the primary mirror's focal plane reflects light back down the tube through a hole in the primary mirror to a focal point just behind it. This, in effect "folds" the focal length of the light path.

A compound telescope 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.

CCD Camera
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.

CCD Imaging Chip
The solid state electronic detector used in a CCD Camera.

Coronado's line of eyepieces and Barlow designed specifically for use with hydrogen-alpha (H-α) telescopes. The CEMAX contrast enhanced series have premium coatings optimized for solar viewing and to minimize unwanted internal reflections.

A small dot centered on the primary mirror of a Newtonian reflector that greatly aids in collimating the optics. The dot does not interfere with the performance of the mirror because it lies within the obstruction of the secondary mirror.

The chromosphere is a layer in the sun's atmosphere, just above the photosphere (which can be considered the visible surface of the sun). While the photosphere is around 6000C on average, the temperature rises in the chromosphere to around 20,000C. The temperature of this layer causes the hydrogen gas to emit strongly in the hydrogen-alpha (H-α) portion of the electromagnetic spectrum, giving it the characteristic red glow seen through an H-α solar telescope. The activity in the chromosphere changes daily and is where many of the interesting features to view through a Coronado telescope are located, including prominences, filaments, and plages.

An acronym for Complementary Metal-Oxide Semiconductor, this is a type of imaging chip (which is an integrated circuit) named after the technology used to create this type of integrated circuit that has high noise immunity and low power consumption. Each pixel is sensitive to photons and converts incoming light to electrical current and finally to digital data used to construct the final image.

Denotes that a coating of magnesium fluoride has been applied to at least one optical surface to enhance light transmission and contrast.

Coated, Fully
All air-to-glass surfaces are vacuum-coated with a single layer of magnesium fluoride to increase light transmission and contrast. A definite step up in quality from "coated."

Coated, Fully Multi
All air-to-glass surfaces receive multiple anti-reflection coatings.

Coated, Multi
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".

Coating, Anti-Reflection
A coating of magnesium fluoride or other chemicals that reduces internal reflections of light bouncing between one element and another within the optical path.

Coating, Phase
Special coatings applied to roof prism binoculars that enhance the contrast for the roof prism optical design.

A process of adjusting the optical components of a telescope so they are aligned with each other.

Collimation Cap
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.

CorrecTension (XT)
A tension-friction system engineered into the Orion XT series of dobsonian telescopes that creates friction on the altitude bearings to ensure continual smooth motion and tube balance.

A weight that is placed on an equatorial, dobsonian, or other mount, to counterbalance the weight of the telescope tube assembly.

Also known as reticle, it is a system of cross wires, dots, or rings in the focus of a finder scope or eyepiece for centering purposes.

Crown Glass
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 Adaptation
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.

Dark Signal
Every CCD produces a certain amount of "false signal" which accumulates in the pixels during imaging. This electronic noise is produced by heat and can be reduced with a thermoelectric cooler.

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.

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
A covering of ABS plastic or metal 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.

Dew Zapper
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.

Diagonal, Correct-Image
A 45° or 90° diagonal used primarily for terrestrial viewing with refractors and cassegrains 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.

Diagonal, Mirror
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. Mirrors are used to redirect the light within the diagonal.

Diagonal, Prism
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.

Diffraction Limited
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.

Digital Setting Circles
Intelliscopes from Orion and Most GOTO or robotic telescope systems have digital electronic read-outs of the precise position of where a telescope is pointed in the sky (the correct astronomical coordinates in R.A. and Dec., if the telescope has been set up correctly).

The breaking of white light into its component colors when it passes through one medium, like air, into another medium, such as glass. Dispersion is what causes chromatic aberration in lenses.

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. Orion produces 8 different families of Dobsonian telescopes, such as the Orion SkyQuest XT8 Classic Dobsonian Telescope.

Double Stack
Stacking two etalons in line with each other creates a double stack system, and narrows the bandpass of the entire system in order to boost contrast on surface features such as filaments, granulation, and plages. This greatly increases contrast by narrowing the bandpass to <0.5 angstroms () and gives a spectacular 3D effect showing increased surface detail. The SolarMax III filters can be double stacked by adding an additional SolarMax III Etalon with T-Max of the same aperture. Contact your Coronado dealer or Meade Instruments directly for pricing.

Double Star/Binary Star
A star that looks singular to the naked eye but, upon closer scrutiny with a telescope, appears to have two or more members. There are two types of double stars: optical doubles are mere chance alignments of stars with different distances. Physical doubles are gravitationally bound to each other. Such a system is often called a binary star.

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. Larger versions can be hade to attach entire telescope tubes to their mounts and come in two size standards, Vixen and Losmondy.

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.

Electronic Drive
A motorized system incorporated into the telescope mount that enables a telescope to track celestial objects electronically. Drive systems are available for both right ascension and declination axes.

Emission Nebula
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.

Emission-Type Objects
Galactic deep sky objects consisting of interstellar dust that is lit by the presence of hot nearby stars. The dust absorbs, and then re-emits the photons, causing the dust to give off a glow. See also EMISSION NEBULA.

Enhanced Reflectivity Coatings
It's the aluminum deposited on a first surface telescope mirror that makes it reflective. Normal aluminum coatings reflect ~90% of the light, by overcoating the aluminum with ultra thin coatings (called dielectric coatings), the reflectivity can be increased several percent.

Equatorial Mount
A telescope mount designed to facilitate easy tracking of celestial objects as they move across the night sky. One axis (the polar axis) is aligned to the celestial pole (parallel with Earth's axis) and the other (the declination axis) is oriented at right angles to it. Also called German equatorial mount.

Erecting Prism
A prism that produces an image that is oriented the same as seen with the naked eye.

The heart of the SolarMax telescopes. An etalon is a type of ultra narrow bandpass interference filter, consisting of two parallel and partially reflective optical elements with a very specific separation distance between those glass plates. Light entering the etalon is transmitted only when at the correct incident angle and when in resonance between the optical elements. All other wavelengths are not transmitted. This is a result of a phenomenon of light known as interference, where the wavelengths of light that resonate between the etalon elements interfere with each other constructively and pass through the etalon (reinforcing the transmitted light at that wavelength), while other wavelengths will destructively interfere and won't pass through the system. In the case of the SolarMax etalon, hydrogen-alpha (H-α) light at 6562.8 angstroms () is the central wavelength to pass through the system.

Exit Pupil
The diameter of the light cone as it exits the telescope eyepiece. It appears as a distinct disk when the telescope is pointed at a bright source like the Moon. Also known as the Ramsden disk. The exit pupil varies with both aperture and magnification.

Eye Relief
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.

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. 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. 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 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.

Eyepiece, 2"
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 exactly positioned relative to the crosshairs or in eyepieces with an illuminated micrometric scale for making small angular measurements.

Eyepiece, Kellner
A basic three-element eyepiece that produces acceptably bright images, though with somewhat narrow fields of view. Kellners works best on long-focal-length telescopes and exhibit slight chromatic aberration.

Eyepiece, Lanthanum
An eyepiece design in which one of the eyepiece elements is made of Lanthanum, a rare-earth element that eliminates visual aberrations.

Eyepiece, Orthoscopic
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.

Eyepiece, Plössl
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, 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 internal reflections, unless they've been properly coated and their internal barrels blackened or glare-threaded.

Field 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.

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°.

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.

The same structure as a solar prominence, but seen directly on the surface of the sun instead of hanging off the edge of the solar disk. The plasma that a filament is composed of is relatively cool compared with the surface that it rises from, making filaments appear darker than the surrounding surface. Since the contrast is lower when viewing a filament on the solar disk compared with a prominence protruding away from the edge of the disk over the black background of space, it helps to view filaments with a narrower bandpass solar telescope. While filaments can be visible with all Coronado telescopes, the narrower <0.7 angstrom (), or even better a double stacked system at <0.5 will really make these features of the solar disk stand out.

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.

Filter, Variable-Polarizing
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%.

Finder Scope
A low-power, wide-field telescope mounted on the main optical tube assembly used to locate celestial objects for viewing through the main telescope.

Finder Scope, Right-Angle
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.

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.

Solar flares are powerful bursts of radiation, localized in a small active region of the sun in and around sunspots. The magnetic field lines around sunspots can get twisted and tangled, and suddenly untangle or reorganize, causing a massive release of electromagnetic radiation. This will heat the surrounding plasma to a much higher temperature, causing a brightening of the area in hydrogen-alpha (H-α) light, which will be visible in the Coronado telescope as a bright patch in the area of the sunspot undergoing a solar flare. Flares are a relatively rare occurrence, compared with the daily visibility of sunspots, prominence and filaments, so keep watching to hopefully catch one of these events!

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.

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.

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 Plane
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 Ratio (F-Ratio)
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 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, Dual Speed
A mechanical focuser design with both course and fine adjustment speeds.

Focuser, Crayford
A mechanical focuser design developed in Crayford, England that uses precision bearings and finely machined surface tolerances to deliver far more precise and blacklash free focuser movement.

Full Well Capacity
A measure of the amount of signal electrons collected by each individual pixel in a CCD imaging chip. The higher the full well capacity, the more light the CCD pixels can collect without oversaturating the signal.

A vast system of several billion to hundreds of billions of stars. Galaxies come in multifarious shapes but are generally classed as spirals (pinwheel), ellipticals (spherical), and irregulars (amorphous). Examples, the Andromeda Galaxy (M31) and the Milky Way, our galaxy.

German Equatorial Mount
A telescope mount designed to more facilitate easy tracking of celestial objects as they move across the night sky. One axis (the polar axis) is aligned to the celestial pole (parallel with Earth's axis) and the other (the declination axis) is oriented at right angles to it. Also called equatorial mount.

Globular Cluster
A compact spherical aggregate of several tens of thousands to millions of stars. Globulars are highly concentrated in the center, but thin out around the periphery. They are not galaxies themselves, but they are gravitationally bound to their "parent" galaxy. Many astrophysicists believe globular clusters are remnants of galaxy formation. Examples, the Great Hercules Cluster (M13) and Omega Centauri.

GoTo (also GOTO)
A telescope equipped with a computer, position encoders and high speed motors that will automatically find and track astronomical objects. Think of it as a robotic telescope that can give you a tour of the night sky.

GoTo Telescope

The solar "surface" that we see through a hydrogen-alpha (H-α) telescope is the sun's photosphere. Within the photosphere are convection cells of plasma that rise and fall to and from the surface, called granules. The tops of these granules are what we see as granulation on the disk of the sun. Granulation is a fairly low contrast feature of the sun, and thus a double stacked narrower bandpass filter system is recommended. A <0.5 angstrom () Coronado telescope is best for bringing out the contrast of the granulation. It can also be seen in <0.7 single stack systems, but imaging the solar disk and increasing the contrast in post processing of the photograph will provide the best "pop" for this feature when using wider bandpass filtering.

Guide Scope
A telescope affixed to another telescope on the same mounting and used in astrophotography to ensure that the object being imaged remains fixed in the camera's field. The observer monitors a "guide star" through the guide scope and makes small adjustments in right ascension and declination to prevent drift that would otherwise blur the target and hence ruin the exposure. Alternatively, an autoguider can be used with the guide scope instead of manually guiding the telescope.

Guide Star

Hydrogen-alpha (H-α)
Hydrogen-alpha (H-α) light sits on the electromagnetic spectrum at 6562.8 angstroms (), near the middle of the red part of the visible spectrum of light. It is the first spectral line in the Hydrogen Balmer series and is emitted when an electron transitions from the third to second energy level in Hydrogen. It is also an exciting wavelength to view when solar viewing as the sun's chromosphere is a strong emitter at this wavelength.

This wavelength is where many of the beautiful features of the solar disk and atmosphere radiate best. Coronado filters are designed to isolate this wavelength from the rest of the light of the sun, revealing prominences spilling off the edge of the solar disk, along with surface features such as solar flares, filaments, plages, and surface granulation.

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.

An Orion, electronic telescope technology that guides you where (which way) to move a telescope to find an object in the night sky — it contains the coordinates of thousands of nebula and galaxies.

Interpupilary Distance
The distance between the two pupils of an observer's eyes. Binoculars can be adjusted to accommodate variations in a person's interpupilary distance.

IR Cut-Off Filter (IR Filter)
CCD and CMOS sensors in astronomical cameras are sensitive to longer infrared wavelengths of light, if this light is not blocked, image sharpness or contrast may be affected.

Short for "light-emitting diode," an LED emits light at low voltage levels. They are used in electronic displays and, as an emitter of pure red light, in astronomy flashlights.

Lens 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.

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.

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.

Magnesium Fluoride
A coating applied to the optical surfaces of refractors, binoculars, and eyepieces that greatly enhances light transmission.

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.

A measure of the brightness of a star or deep-sky object. The magnitude system is not exactly intuitive since the brighter the object the lower the value assigned. A magnitude 1 star, for example, is brighter than one of magnitude 2.

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.

Maksutov (MAK)
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 case they are called Maksutov-Cassegrains, or as Newtonians, in which case they are called Makstutov-Newtonians (or MAK-Newts, for short).


Maxim DL
A commonly used image processing and camera control software produced by Diffraction Limited. Trial versions are included with many Orion astrophotography cameras.

Meniscus Lens
The front lens in Maksutov telescope.

Messier Objects
Bright "nebulas" (subsequently identified as star clusters, nebulas, and galaxies) cataloged in the late 18th century by French astronomer Charles Messier (pronounced MEZ-ee-ay). Messier ostensibly compiled his catalog in order to avoid confusing the cloudlike objects with comets, which was his specialty. Today, the list contains 110 objects, though the identities of some of the entries are still debated.

Mirror Cell
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.

A single "pocket-sized" telescope used as a handy spotting scope.

Motor Drive
Automatically moves the telescope in right ascension at the same rate as the east-west drift of the stars (See SIDEREAL RATE) so stars can be continuously tracked in the eyepiece without manual adjustment. Motor drives are usually equipped with a hand control that allows the telescope's tracking speed to be slightly increased or decreased, which is particularly critical when taking long-exposure astrophotographs.

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".

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.

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.

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.

NGC Catalog
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.

NTSC, named for the National Television System Committee is the analog television system standard used in most of North America and many other areas around the world.

Objective lens
The main light-gathering lens or lens system of a refracting telescope or binocular.

An alternate term for eyepiece.

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 (or use an autoguider).

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.

Open Star Cluster

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.

Parabolic Mirror
A mirror with a concave, polished, and aluminized surface designed to bring light to a focus at a certain prescribed distance or focal length.

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.

Peltier Cooler

Photography, Afocal
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.

Photography, Piggyback
A method whereby a 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.

Photography, Prime Focus
An astrophotography technique in which an interchangeable lens camera is positioned at the focal plane of a telescope?s primary mirror or objective lens. No eyepiece is used. Essentially the telescope acts as a very large telephoto lens. This method provides the highest light transmission with lowest possible power, the widest field of view, and the best definition. However, in some cases the telescope?s image plane cannot be accessed because it falls within the focusing tube.

Pixel (or Picture Element)
In digital imaging, this is smallest single element in a digitally produced image and corresponds to a physical element on the chip. The smaller the size of the pixel, the higher the image resolution the imaging chip can produce. Pixels are usually measured in microns.

Bright regions of the solar disk, usually found surrounding sunspots. They can appear as small patches or tendrils of a lighter shade compared with the background surface. Also known as bright flocculi.

Planetary Nebula
The shell of gas expelled by a dying star. Many appear circular or doughnut shaped. Examples include the Ring and the Dumbbell nebulas.

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.

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.

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.

Power refers to a telescope's magnification (i.e., 40x can be referred to as 40 "power").

Primary Mirror
The main light-gathering mirror of any reflecting telescope.

A solid glass figure cut with flat surfaces. Optical prisms are used in binoculars and star diagonals and are made from either BK-7 (borosilicate) glass or BAK-4 (barium crown) glass.

Prisms, Porro
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).

Prisms, Roof
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.

A solar prominence is a large and bright structure of plasma rising up from the solar surface into the outer atmosphere of the sun. They are visible around the edge of the sun, in profile against the black background of space when viewed through a solar telescope. These same structures when seen directly on the solar disk are called filaments. Prominences are the most easily visible structure seen through a hydrogen-alpha (H-α) telescope, and can be seen with all Coronado telescopes, even with wider bandpass filtering, such as the single stack <0.7 angstrom () SolarMax telescopes and the <1 PST telescope.

Prominences are composed mostly of ionized hydrogen and helium, and rise up from the surface following twisted magnetic field lines. They can form shapes including hedgerows, loops, and in more powerful cases such as an erupting prominence, a loop that breaks away, sending plasma outward away from the sun. A prominence can form and disappear over the timescale of a single day, so while you don't see "live" motion in real time, if you study the prominence over an hour or two, you can often notice changes to its shape.

A high-quality glass known for its low thermal-expansion properties, meaning that it is less affected by changing temperatures, and hence preferred for making primary mirrors of reflecting telescopes.

Rack-and-Pinion Focuser
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.

RCA Cable (or RCA Connector)
Sometimes called a phono connector this is a type of electrical connector used to carry audio or video signals.

Read Noise
Electronic noise present in all CCD chips.

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.

A telescope that uses a mirror to collect and focus light. Reflectors come in a variety of designs, including Newtonians and Maksutov-Cassegrains.

Reflex Sight
A lenseless finder that superimposes a pinpoint red LED-powered dot focused at infinity on the sky. When aligned with the main optical tube of the telescope, the dot indicates where the main field of the telescope is aimed.

Refractive Index
The ratio of the velocity of light in a vacuum to the velocity of light through a material medium. Glass with different refractive indices bend light at different angles, which affects optical design. Different types of glass have difference indices of refraction.

A freeware or free software program that is used in the "processing" of an astronomical image (manifpulation and enhancement of n image after you actually capture the data). This allows the stacking of multiple frames of a digital image to increase contrast and resolution in the final "processed" image.

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.

A system of cross wires, dots, or rings in the focus of a finder scope or eyepiece for centering purposes.

RGB Color
A digital color image of an astronomical object constructed from the superposition of three separate Red, Green and Blue images. This type of imaging can give very high contrast, dramatic results, even in light polluted skies. Special filters and a "monochrome" camera are required for best results.

RichView Tuning
Coronado's patented RichView tuning mechanism uses a proprietary method of adjusting the primary etalon directly to achieve optimal hydrogen-alpha (H-α) performance. To see the ever-changing solar features with the highest contrast, etalons need to be fine-tuned in real-time to bring it "on-band" for H-alpha. This is done by either tilting the etalon slightly, or changing the gap size within the etalon cavity. Coronado can do both!

With RichView tuning, the air-spaced etalon gap size can be mechanically adjusted quickly to see surface details such as filaments and granulation, or again to see prominence details on the solar limb. This optimizes the exact wavelength of light that passes through the etalon and provides the largest range of adjustment, all while maintaining precise optical alignment.

Solar features often move quickly which requires further adjustments to the etalon due to Doppler shifting. Even the sun's rotation itself can affect the wavelength you observer as one side of the sun moves toward the observer and other side moves away. RichView tuning allows for adjusting the etalon to compensate for this Doppler shift.

Barometric pressure at your observing site also changes the etalon's performance which can move the H-alpha line too far for normal tip-tilt tuning. With Coronado's patented RichView tuning, the additional tuning range it provides makes using solar scopes at high altitudes still possible.

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.

A type of cassegrain reflector telescope design. Both primary and secondary mirrors are polished to a hyperbolic shape, which delivers a very flat image plane; used primarily by advanced astrophotographers.

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.

Secondary Mirror
A small flat or curved mirror that intercepts light coming from the primary mirror of a reflecting telescope and directs it into an eyepiece. In Newtonian reflectors, the secondary mirror is often referred to as the diagonal.

The general term used by professional and amateur astronomers to describe the "stability" of the air above you and therefore the steadiness of an image viewed through a telescope. "Poor seeing" comes from turbulent air and "good seeing" is the result of viewing through stable, steady atmospheric conditions. Seeing will vary nightly and even moment to moment; it is generally better in early morning (after midnight) when the ground has had time to cool from the heat of the day since heat rising off the ground is a major source of "poor seeing."

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.

Setting Circles — Digital
Intelliscopes from Orion and Most GOTO or robotic telescope systems have digital electronic read-outs of the precise position of where a telescope is pointed in the sky (the correct astronomical coordinates in R.A. and Dec., if the telescope has been set up correctly). Such telescopes may or may not have mechanical setting circles as well.

Sidereal Rate
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.

Silicon Dioxide
Similar to silicon monoxide (see below) but provides a more durable layer to better protect aluminum coatings.

Silicon Monoxide
A hard, abrasive material used as thin surface films to protect aluminum coatings on optical mirrors.

Single Stack
A solar telescope with only one etalon filter is commonly referred to as being "single stacked."

The Coronado Solarmax III single stacked telescopes have a bandpass of <0.7 Angstroms and the PST has <1 Angstrom.

These single etalon telescopes have a wider bandpass than their double stacked counterparts and are ideal for observing solar features on the limb of the sun. Edge features such as solar prominences stand out extremely well in single stacked telescopes as they are viewed against the black background of space. These telescopes can also view surface features such as granulation and filaments, although with a lower contrast than what is observed with a double stacked system.

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.

Sol Ranger Solar Finder
A solar viewfinder designed by Coronado to help pointing the solar telescope at the Sun in a safe and effortless manner. The Sol Ranger projects an image of the solar disk onto the rear window of the finder, and is visible at any angle when looking at the rear port. There is no need to look through the finder, or line your eye up behind it — simply look at the rear port and center the tiny image of the sun on the screen.

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 3- or 4-vaned frame at the front of a reflecting telescope tube that supports the secondary mirror.

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.

Star Cluster

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.

Small dark spots or blotches that appear on the disk of the sun. They appear dark because they have a cooler temperature compared with the surrounding area (though still very hot at around 4000-4500 Celsius!). They form where the magnetic fields are especially strong, and can be fun to watch slowly migrate across the surface of the sun over the course of a few weeks as the sun rotates. The number of sunspots visible at any given time is known to be variable with an approximate 11-year cycle known as the solar cycle. Solar cycle 25 began in late 2019, and is expected to reach peak (known as solar maximum) sometime around 2024 or 2025.

A camera adapter that connects to a T-ring attached to a camera body to connect it to a telescope focuser for prime-focus astrophotography.

T-Max Tuning
T-Max tuning is a method of slightly tilting the etalon in order to move the etalon transmission "on-band" with hydrogen-alpha (H-α), providing the highest contrast and brightest image possible. It is also used to minimize any ghost reflections caused by internal light bouncing off various solar filters within the telescope.

Converts the lens mount on a camera body to a standard "T-thread" that can accept a T-adapter or universal camera adapter for either prime focus or eyepiece projection photography.


A frictionless synthetic material used as a bearing surface.


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, permitting shorter, or faster, exposures when used with a camera. ?Fast? telescopes also provide a wider field of view compared to higher focal ratio scopes.

Telescope, "Rich-Field"
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 requires a slower exposure to produce an image at prime focus. For visual observation, ?fast? and ?slow? telescopes with the same aperture will produce images with the same apparent brightness at the same magnification.

Refers to bird watching, landscape or seascape daytime observing with a telescope, binoculars, or spotting scopes.

Terrestrial Scope
A scope used during the day or in low light to observe terrestrial fields of view. Applicable to birding, sightseeing, and nature study.

Thermal Electric Cooler (or Peltier Cooler)
A solid state electronic device that can be used for cooling; the process of using such a device is called Thermoelectric Cooling or TEC.

Using a motor drive to remain continuously fixed on a celestial object as the earth rotates.

Transparency (also Atmospheric Transparency)
The term astronomers use to rate how "clear" the night sky is; a very transparent night is very clear with no clouds or haze and objects seen through the telescope are bright and full of contrast. Transparency can very from night to night and hourly during a single night.

Tripod, Camera
A three-legged stand with a swivel or pan head upon which a camera, spotting scope, or binocular can be attached.

USB (or USB Connector or USB Port)
Acronym for Universal Serial Bus is a standard for cables and connector ports used to establish communications between devices and a host controller (usually a PC).

Variable Star
A star that fluctuates in apparent brightness over a period of time. Examples include Delta Cephei, Betelgeuse, and Algol.

Capable of being splashed with water without internal leakage. Not to be confused with "water resistant", which does not guarantee that moisture cannot enter and damage the optics.

Wave Surface Accuracy
The standard tolerance for high-precision optics is one-quarter of the wavelength of yellow-green light, which is 22 millionths of an inch. In numeric terms, 1/4-wave means that the surface of a parabolic mirror must not depart more than about 6 millionths of an inch from a perfect theoretical shape.

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.

The point in the sky directly over the observer's head.