You’ve probably seen or heard many unfamiliar terms while researching telescopes. We’ve included the definitions of some of the most commonly used words in astronomy as a quick and easy reference.
A type of telescope mount that moves up and down (altitude), and side to side (azimuth) similar to a camera tripod.
Altitude and Azimuth
The two positions in the horizontal coordinate system. Altitude refers to the angle between the observer’s horizon and the object. Azimuth refers to the angle of the object along the horizon measured from north to east.
Aperture is the diameter of the telescope’s light-gathering lens or mirror. The larger the aperture, the more light will be collected resulting in brighter and sharper images. Aperture is typically measured in millimetres or inches.
Apparent Field of View
The angular diameter (in degrees) of the image viewed through the eyepiece. It is a property of the eyepiece.
A number that is the measure of the brightness of a celestial object to the naked eye. The lower the value, the brighter the object appears.
A minor planet orbiting the Sun with a diameter greater than one metre and mainly composed of minerals and rock.
A type of optical aberration where light does not come to focus on a single plane. Most telescope designs do not suffer from noticeable astigmatism.
Astronomical Unit (AU)
The average distance between the Earth and the Sun used to measure distances within our Solar System. One AU is about 93 million miles or 150 million kilometres.
The study of celestial objects and phenomena.
Taking photographs of celestial objects and phenomena.
A natural light display in the sky produced by the interaction of charged particles when solar wind disturbs the magnetosphere. Known as Aurora Borealis in the Northern Hemisphere and Aurora Australis in the Southern Hemisphere.
Viewing an object by looking slightly off to its side and not directly at it. This allows fainter details to appear.
A lens used along with an eyepiece that increases the telescope’s focal length and the magnification of the eyepiece. A 2x Barlow would double the eyepiece’s magnification.
A system of two stars that orbit around their common center of mass. Also known as double stars.
An optical device with a lens for each eye used to view distant objects.
Also known as a compound telescope. Uses a hybrid design with a combination of lenses and mirrors in a sealed tube which fold the optical path to form an image. This provides a focal length much longer than the length of the compact optical tube. They are more commonly known by the two popular designs: Schmidt-Cassegrain and Maksutov-Cassegrain.
A natural object that exists in the Universe and is located outside of Earth’s atmosphere.
An imaginary sphere with a large radius and the Earth located at its center. An observer can view celestial objects as though they are projected on to the inner surface of the celestial sphere.
A type of optical aberration present in some refractor telescopes. When light passes through the objective lens, each wavelength of light is refracted by a different amount and the colors fail to converge at the same focal point. Think of the rainbow effect of light passing through a glass prism. Chromatic aberration is typically seen as color fringing around bright objects, like a purple halo around planets and stars.
The process of manually adjusting the mirrors of a telescope when they are out of alignment.
A type of optical aberration (comatic) present in Newtonian reflector telescopes. With coma, stars appear distorted near the edge of the field of view while stars in the center of the field of view are unaffected.
A large ball of ice and rock that orbits the Sun. When a comet passes close to the Sun, it releases gas forming a coma (gas cloud) and a tail. Comets can range in size from hundreds of metres to tens of kilometres in diameter.
See ‘Catadioptric Telescope’
A telescope on a GoTo mount with a small motor drive, built-in computer and hand-held controller which allows you to look up celestial objects in a database. The GoTo will automatically locate the specific object and track its movement across the sky.
When two planets or celestial objects are aligned so that they have the same right ascension and appear to be close together as observed from Earth. It is caused by the perspective of the observer as the two objects involved are not actually close to each other in space.
A recognizable grouping of stars that form a pattern. There are 88 constellations which are typically named after animals, mythological characters or objects.
Cooldown Time (Thermal Stabilization)
The time required for a telescope’s optics to be brought to ambient temperature for optimal viewing.
The adjustment of the eyes to darkness or low light conditions. Dark adaptation is necessary to see faint objects.
One of two angles used to locate a point on the celestial sphere. It is comparable to geographic latitude.
Deep-Sky Objects (DSO)
Celestial objects that are not individual stars and are outside of our Solar System such as star clusters, nebulae and galaxies.
A simplified altazimuth mount for Newtonian reflectors. The mount sits on the ground, swivels 360 degrees and moves up and down.
See ‘Binary Star’
When a celestial object like a moon or planet temporarily moves into the shadow of another celestial object. A Lunar Eclipse occurs when the Earth blocks the sun and the Earth casts a shadow on the moon. A Solar Eclipse occurs when the moon blocks the sun and the moon casts a shadow on the Earth.
The circular path on the celestial sphere that the Sun appears to follow over the course of a year.
Equatorial Mount (EQ)
A type of telescope mount that is more precise than an altazimuth mount and better for astrophotography. Aligning one axis with Polaris allows you to adjust only the polar axis to track an object as the Earth rotates.
One of two times per year when the Sun crosses the plane of the Earth’s equator causing day and night to be of equal length.
The width of the cone of light that exits the eyepiece at the exact eye relief distance.
The part of the telescope that you look into. They are typically 1.25″ or 2″ in diameter. To change the telescope’s magnification, you can switch eyepieces with different focal lengths.
The distance between the eyepiece lens and the observer’s eye while still being able to see the entire field of view. Outside of this distance, the observer will see a reduced field of view.
Field of View (FOV)
See ‘Apparent Field of View’ and ‘True Field of View’
An accessory that attaches to the bottom of an eyepiece to enhance details of celestial objects. Neutral density or polarizing filters can reduce glare from bright objects like the moon. Color filters can enhance specific planetary details by blocking certain wavelengths of light.
An optical device attached to a telescope used to locate a celestial object and aim the telescope in its direction.
The distance from the objective lens or mirror to the focal point.
The point where parallel rays of light meet after passing through a lens or reflecting off of a mirror.
Focal Ratio (f/ratio)
Equal to the focal length of a telescope divided by its aperture size. A telescope with a focal length of 1200mm and an 8″ (203mm) aperture would have a focal ratio of f/5.9.
A device attached to the telescope into which an eyepiece is inserted. It can be adjusted to bring the image viewed through the telescope into focus. Types of focusers include helical, rack and pinion, and Crayford.
A system of stars, gas, dust, and dark matter that is gravitationally bound.
A spherical collection of stars that is tightly bound by gravity and has a high stellar density towards its center. They are found orbiting in the outer regions of a galaxy. Globular Clusters are much older and contain more stars than Open Clusters.
See ‘Computerized Telescope’
The angle between a reference plane and the orbital plane of an orbiting object.
Brightening of the sky caused by artificial light which can significantly reduce the ability to observe celestial objects.
The distance that light travels in one year, used to express astronomical distances. Equal to about 5.9 trillion miles or 9.5 trillion kilometres.
Equal to the focal length of a telescope divided by the focal length of an eyepiece. For example, a telescope with a focal length of 1200mm and an eyepiece with a focal length of 10mm would provide a magnification of 120x. Also known as power.
A type of catadioptric telescope design that uses a combination of a spherical mirror and a full aperture meniscus lens to correct for spherical aberration. Also known as a Mak.
A catalog of 110 deep-sky objects including nebulae, open clusters, globular clusters and galaxies. The initial list was created in 1771 by French astronomer Charles Messier.
A streak of light caused by a meteoroid, comet or astroid entering the Earth’s atmosphere at rapid speed causing it to heat up. Also known as a shooting star.
A small rocky or metallic body in outer space ranging in size from a small grain to a width of 1 metre.
A number of meteors appearing to originate from the same point in the sky. They occur at particular dates each year when the Earth passes through streams of cosmic debris.
The galaxy that contains our Solar System.
A mechanical structure that supports a telescope which allows it to point in the direction of celestial objects. The two types of telescope mounts are Altazimuth and Equatorial.
A cloud of gas and dust in outer space. Nebulae (plural) are visible as bright patches in the night sky or as opaque clouds that block light from luminous objects behind them (dark nebula).
A telescope’s main light-gathering lens or mirror.
A loose grouping of stars found in the disk of a galaxy. They can contain up to a few thousand stars. Open Clusters are younger and less dense than Globular Clusters.
Optical Tube Assembly (OTA)
The main tube of a telescope which contains the optics (objective lens or primary mirror). It does not include the mount.
The Universe beyond Earth’s atmosphere.
A reflective surface with a concave shape designed to bring light to focus at a single point.
A circular star map that can be adjusted to show the stars and constellations as they would appear at a specific date and time.
The brightest star in the constellation Ursa Minor. It is very close to the celestial north pole which is why it is commonly referred to as the North Star. It is used for navigation as it appears to remain stationary as the Earth rotates.
The main light-gathering mirror of a reflector telescope.
A form of computerized telescope with a hand-held controller which allows you to look up a celestial object in a database. After selecting your desired object, the controller will provide directional arrows for you to move the telescope by hand until it is in the correct position. Unlike a GoTo, a PushTo requires manual tracking.
Red Dot Finder
A reflex sight finder that doesn’t provide any magnification. Look through the red dot finder’s viewing window and align the red LED dot with the celestial object you wish to observe by moving the telescope.
A type of telescope that is open at one end and uses a concave primary mirror to collect and focus incoming light onto a flat diagonal secondary mirror which reflects the image to an eyepiece on the side of the optical tube.
A type of telescope that has a convex objective lens at the front end of a sealed tube and an eyepiece at the rear end. Refractors were the first telescopes and the image most people have when thinking of the word “telescope”.
The angular distance measured eastward along the celestial equator. It is the celestial equivalent of geographic longitude.
A type of catadioptric telescope design that uses a combination of a spherical primary mirror, a convex secondary mirror and a corrector plate (aspheric lens). Also known as an SCT.
A filter placed at the front of a telescope when viewing the Sun to block most its light. They are typically made from glass or plastic film. Failure to use a solar filter when your telescope is pointed at the Sun can cause severe eye damage and blindness.
The system of eight planets and their moons as well as dwarf planets, comets, asteroids and meteoroids that orbit the Sun.
The two times of the year when the Sun is at its most northerly or southerly point in the sky.
A type of optical aberration where a spherical mirror is unable to focus light to a single point. It can be eliminated by using a parabolic mirror.
A type of galaxy structure that has spiral arms extending from the galaxy’s center.
A luminous sphere of gas held together by its own gravity. Thermonuclear fusion of helium and hydrogen in its core produces energy. The Sun is the closest star to Earth.
A grouping of stars which are gravitationally bound. See ‘Globular Cluster’ and ‘Open Cluster’.
An accessory consisting of an angled mirror or prism that fits in the telescope’s focuser and accepts an eyepiece. It allows a more comfortable viewing angle especially when the telescope is pointed at or near the zenith.
A technique for locating faint celestial objects by using bright stars as a reference and a star chart.
A temporary dark spot that appears on the surface of the Sun. A sunspot has a reduced surface temperature compared to its surroundings. Sunspots can be viewed with a telescope using a proper solar filter.
Occurs when a full moon is also at its closest distance to Earth along its orbit.
A dying star that experiences a massive explosion which expels most of its mass. It appears as a very bright star before it fades away.
A reflex sight finder that doesn’t provide any magnification. It projects three red LED concentric circles onto the viewing window. This helps when star hopping as the circles aid in judging the angular distances between objects.
Moving the telescope in small increments as the earth rotates to keep the celestial object within the field of view.
When a celestial object appears to move across the surface of another celestial object, blocking a small part of it. Typically seen as Mercury or Venus transiting the Sun.
True Field of View
The angular size (in degrees) of the amount of sky that can be seen through an eyepiece when used with a telescope.
The imaginary point in the sky directly above the observer.
The area of the sky centered on the ecliptic along which the Sun, moon and most of the planets move over the course of a year. It is divided into 12 parts with each part named for a nearby constellation.