Astronomy Glossary of Terms
April 11, 2020
Aberration – defects in images produced by a telescope (or optical system). They are caused by limitations in the design and manufacture of the optics. Find out more about aberrations here.
Astigmatism – an off-axis effect that causes rays from a star to come to different focal points in the vertical and horizontal plane.
Coma – a spherical aberration caused by photons striking a plane off-axis. When an image suffers from coma, stars appear as little, off-axis, comet-shaped blobs that point inwards towards the center of the field and get bigger as you look towards the edge of the field of view.
Chromatic Aberration – the failure of a lens or lens system to bring all light to a common focus.
Field curvature – a visual defect that occurs when objects or stars scattered in a field of view of a telescope do not come into focus on the same flat plane. An image that suffers from field curvature will have pinpoint stars at the center of the frame, but out-of-focus stars at the edges.
Distortion – a variable magnification across the field of view.
Spherical Aberration – the failure of photons at different distances from the center of a lens or mirror to come into focus on the same plane.
Achromat or Achromatic Lens – a system of two or more lenses that is substantially free from chromatic aberration. An achromat’s lenses are made of different substances so that the focal length of the system is the same for two or three wavelengths of light.
Airy disk – a spot of light that a circular aperture can make, limited by the diffraction of light. An airy disk becomes smaller as the aperture of the telescope gets larger.
Alignment – the process of setting up your equipment to better locate celestial objects.
GoTo Alignment – the process of synching up your computerized telescope with the sky in order to use the GoTo feature to locate celestial targets
Polar alignment – the process of accurately aligning your equatorial mount’s polar axis with the celestial pole.
Altazimuth or Alt-azimuth Mount – a simple two-axis mount for supporting and pointing a telescope in altitude (the vertical axis) and azimuth (the horizontal axis).
Altitude – the vertical position of a star or other astronomical object in the sky measured from the horizon
Azimuth – the horizontal position of a star or other astronomical object in the sky measured from compass north
Aperture – the diameter of a telescope’s objective lens. For more information, click here.
Astrograph – a telescope designed for astrophotography
Astrophotography – photography of astronomical objects
Autoguider or Autoguiding – a digital guidance tool used to keep a telescope pointed precisely at an object to prevent it from drifting across the field of view during long-exposure photography.
Back-focus – the distance from the last surface of the telescope - either focuser drawtube, rear cell thread (SCT), or camera adapter flange (RASA) - to the focal plane of the camera.
Backlash – the play in your telescope’s drive gears. Backlash causes a delay in the scope movement when using the direction arrows on the hand control. For more information, click here.
Bandpass or Bandpass filter – a type of filter or filtering process that passes frequencies within a certain range and rejects frequencies outside that range.
Barlow Lens – a diverging lens, which, when used with an eyepiece, can double or triple a telescope’s focal length.
Binoviewer – an optical device that splits the light coming into the telescope for binocular-style viewing of the object.
Interpupillary distance – the distance measured in millimeters between the centers of the pupils of the eyes, an important factor in properly setting up your binoviewers.
Clock-drive – a motor used with an equatorial mount to track objects. When observing the sky with a properly aligned clock-drive mount, it compensates for the Earth's rotation, keeping celestial objects centered in the field of view.
Collimation – the proper alignment of the optical elements in a telescope, which is critical for achieving the best possible views.
Computerized telescope – see GoTo
Coma – see Aberration – coma
Contrast – the amount of difference between dark and light in a telescope’s eyepiece view or an astroimage.
Cooldown/Thermal equilibrium – the time a telescope’s optical tube needs to reach ambient temperatures. Using the tube for visual astronomy or astroimaging before the cooldown period has been completed can result in degraded image quality.
Cord wrap – an issue that can occur when your mount slews more than 360° in azimuth and tangles your power and accessory cables around the base. Cord wrap can cause damage to the scope’s casing and electronics. For more information, click here.
Corrector plate/Corrector lens – a thin lens that corrects the spherical aberration and coma that the primary mirrors produce.
Counterweight – a weight placed on the counterweight shaft to balance out the weight of the optical tube and accessories on the other side of the right ascension axis. For more information, click here.
Counterweight shaft – a metal rod used to attach counterweights to your equatorial mount.
Dew shield – an accessory that protects your optical tube from excess moisture forming on the corrector lens and helps block out stray light for better viewing and astroimaging.
Erect image diagonal – a right angle or 45° star diagonal that replaces a regular prism with an Amici or pentaprism.
Star diagonal – a prism or mirror set to direct light at a 90° angle to the telescope’s optical axis, providing more convenient and comfortable viewing of objects nearly overhead so observers can avoid neck-craning angles.
Diffraction spikes – spikes of light that appear to extend from a star in some astroimages. Diffraction spikes are caused by a non-circular obstruction in front of the primary mirror, such as the vanes holding a secondary mirror or an irregular-shaped imaging camera. The shape and number of spikes depend on the shape of the obstruction. For more information, click here.
Dobsonian – a type of Newtonian reflector alt-azimuth telescope with a more simplistic “rocker box” mount made from wood or metal. For more information, click here.
Dovetail – a rail that attaches your OTA and/or accessories to the amount or another dovetail bar on the top of the optical tube. Celestron offers three styles of dovetails: the smaller CG-5, the larger CGEM, and a dual saddle that accepts both. For more information, click here.
EdgeHD – Celestron’s proprietary, flat-field, aplanatic optical design. EdgeHD provides sharp, diffraction-limited images all the way across the field of view of today’s largest eyepieces and imaging sensors. For more information, click here.
Equatorial mount (EQ) – a type of telescope base that compensates for Earth's rotation by having its right ascension axis rotate parallel to the Earth's axis.
German equatorial mount (GEM) – a specific type of equatorial mount that forms a T-shape when the OTA is mounted directly to the declination axis and balanced by a counterweight shaft perpendicular to the right ascension axis.
Declination or Dec – celestial coordinates that correspond to latitude (north to south) on the celestial sphere. Declination is measured in degrees, minutes, and seconds of degrees, minutes, and seconds of arc. For more information, click here.
Right ascension (RA) – celestial coordinates that correspond to longitude (east to west) on the celestial sphere. Right ascension is measured in hours, minutes, and seconds of time. For more information, click here.
Eyepiece – an ocular lens that magnifies the view through your telescope—an essential part of visual observing. For more information, click here.
Erect image eyepiece – a series of prisms combined with eyepiece lenses designed to rotate the image from the telescope’s main optics by 180°. Erect image eyepieces are most commonly used with Newtonian reflectors.
Exit pupil – the width of the light beam leaving the eyepiece. You can calculate an eyepiece’s exit pupil using this equation: exit pupil = aperture/eyepiece magnification.
Eye relief – the distance between the observer’s eye and the eyepiece at which you can see the entire field of view. The best distance for optimal comfort is at least 15mm.
Kellner eyepiece – an achromatic eyepiece that uses a 3-lens design, a plano-convex front element, and a plano-concave eye lens. Kellner's are known for their good image quality from low to medium power. Their apparent field of view is 40-50◦.
Plössl eyepiece – a symmetrical eyepiece that uses a 4-lens design: a two-group, two-element design that uses back-to-back plano-convex front element and eye lenses. Their apparent field of view is 50◦ or more.
Zoom eyepiece – an eyepiece with a rotating barrel that increases or decreases the magnification.
Fastar – a Celestron technology that gives imagers that capability to transform their EdgeHD or Schmidt-Cassegrain optical tube into an f/2 focal ratio astroimaging system by removing the secondary mirror. For more information, click here.
Field flattener – a type of lens placed at the optic’s focal plane that counteracts a concave or convex curvature, providing an image closer to being in focus over the full field. Field flatteners are found built-in our EdgeHD and RASA optical tubes.
Field of view – the amount of sky that is visible through the objective lens with each eyepiece.
Apparent field of view – a measurement dependent on the design of the eyepiece. The larger the apparent field of view, the wider the field the eyepiece will give.
True field of view – the actual angle of sky seen through the eyepiece when attached to the telescope. To calculate an eyepiece’s true field of view, use this equation: TFOV = Eyepiece AFOV / magnification.
Field rotation – the apparent rotation of a celestial object in the field of view of an altazimuth telescope during the night. For more information, click here.
Filter – a circular disk of film or colored glass that attaches to a telescope to enhance details or improve contrast.
Solar filter – a circular disk of film that covers the main aperture of a telescope, allowing safe viewing of the Sun by blocking nearly all its light. For more information, click here.
Color Filters – dyed glass filters that screw into the bottom of an eyepiece that increases contrast on certain features on certain planets.
Narrowband Filters – glass filters with special coatings that allow you to see specific wavelengths of light while rejecting others, such as O-III and Light Pollution Reduction (LPR) filters
Finderscope – a small auxiliary telescope attached to the main telescope to assist in locating objects to observe through the main telescope.
Polar axis finderscope – an aid placed at the right ascension axis of an equatorial mount for quickly polar-aligning your telescope accurately enough for visual work. For more information, click here.
Red Dot Finderscope – a 1x finder that reflects the image of a small red dot in a glass window, making it easier to use the finderscope from any distance rather than having your eye right up against it
Flat field – a telescope with no field curvature, providing images that are sharp across the center of the field to the edge of the eyepiece or camera sensor.
Focal length – the distance from the objective lens or mirror to where the image forms.
Focal ratio or (f/number) – the photographic speed of a telescope’s optics; the ratio of the optical tube’s focal length to the diameter of the camera or eyepiece. Equation: Focal ratio = focal length / aperture.
Focal Reducer – an optical element lens design used to reduce focal length and increase lens speed, which increases field of view.
Focuser – the part of the telescope mechanics that allows a user to turn a knob on the telescope and bring the view into focus.
Rack and pinion focuser – a device that uses gears to move the telescope eyepiece precisely to the telescope's focal point.
Crayford focuser – a device that uses a smooth, spring-loaded shaft against a bearing surface to move the telescope eyepiece to the telescope's focal point.
Fork arm mount – an alt-azimuth telescope mounting system that holds an optical tube either with one or two arms and moves the telescope in altitude and azimuth.
Single fork arm – a fork arm mount that holds an optical tube on only one side, such as the NexStar SE and NexStar Evolution.
Dual fork arm – a fork arm mount that holds the optical tube on both sides, such as the CPC and CPC Deluxe HD.
GoTo telescope – a telescope that can automatically point a telescope at celestial objects the user chooses and track them as they appear to move across the sky.
Guidescope – a small refractor telescope mounted at the top of your telescope and attached to an autoguider, precisely guiding it for long exposure astroimaging.
Light gathering ability – a telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture.
Light pollution – unwanted light that is projected up into the night sky (city lights, flashlights) that significantly impacts the visibility of faint objects in telescopes. For more information, click here.
Limiting Magnitude – the apparent magnitude of the faintest object that is visible with the naked-eye or a telescope. For more information, click here.
Magnification or Power – the amount by which a telescope appears to enlarge the view of its subject. To calculate magnification or power, use this equation: Magnification=telescope focal length / eyepiece focal length. For more information, click here.
Magnitude – a measure of the brightness of a star or other celestial body. For more information, click here.
Magnitude limit – the magnitude of the dimmest star or other celestial body that can be detected by a given instrument.
Maksutov-Cassegrain – a catadioptric telescope that uses a spherically curved corrector plate to reduce off-axis aberrations found in reflecting telescopes, such as coma, while avoiding chromatic aberration. The Maksutov-Cassegrain design features a mirrored spot in the center of the corrector instead of a secondary mirror, so the system never needs to be collimated. For more information, click here.
Manual telescope – a telescope that has no electronics and must be moved by hand.
Meridian flip – the process of flipping the tube’s positions 180° on the RA and Dec axis of your GE mount to avoid the optical tube from hitting the mount when it tracks an object across the meridian. For more information, click here.
Newtonian reflector – a telescope has a concave parabolic or spherical primary mirror at the back of the telescope that collects and focuses incoming light onto a flat secondary (diagonal) mirror. In turn, the secondary mirror reflects the image to an eyepiece on the side of the optical tube. For more information, click here.
Objective – the primary lens or mirror that gathers light.
Off-axis guider – an accessory that allows you to guide through the same telescope that you are imaging with, eliminating the need for a separate guide scope.
Optical coating –There are two types of coatings: Mirror and Anti-Reflection (AR) coatings. Mirror coatings enhance reflectivity and AR coatings minimize reflection and allow more light to pass through a glass lens. For more information, click here.
Optical tube assembly (OTA) – the name of the tube at the top of your telescope setup where the optics are housed. To find out which optical tube best suits you, click here.
Pan handle – a rod with a clutch to move your telescope around
Parfocal – eyepieces that can be swapped out without the need to refocus the telescope.
Payload – the weight that can be supported by the mount and dovetail clamp. When determining your setup’s payload, you should include the weight of the optical tube and all accessories you wish to use. For more information, click here.
Periodic error (PE) – the name for the inevitable small mechanical errors that occur in the drive gearing as a GoTo mount is tracking during long-exposure imaging. For more information, clickhere.
Precession – the movement of the north celestial pole (NCP) among the constellations over long periods of time. For more information, click here.
Polar alignment – the process of accurately aligning your equatorial mount’s polar axis with the celestial pole.
All-Star Polar Alignment or ASPA – Celestron’s exclusive, innovative polar alignment procedure that allows users to use any bright star for polar alignment while the software calculates and assists. For more information, click here.
Rough polar alignment – the process of orientating your mount so the polar axis is roughly north (facing Polaris) or south (facing Sigma Octantis).
Polar axis – the axis of rotation of an equatorial mount that is parallel to the earth's axis of rotation.
Primary mirror – a large mirror in a reflector or Cassegrain telescope that captures light from celestial objects.
Rack and Pinion – a type of linear gear system that comprises a circular gear (pinion) engaging a linear gear (rack) to change circular motion into a linear motion.
Reflector – see Newtonian reflector
Refractor – a long, thin optical through which light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. For more information, click here.
Resolution/resolving power – the ability of an optical tube to distinguish small or closely adjacent objects. A telescope’s aperture determines its maximum resolution.
Angular resolution – The measurement of a telescope’s ability to resolve small details, usually measure in arc-seconds. If the angular distance separating a close double-star is smaller than the angular resolution of a telescope, this scope would only see a single star or slightly elongated star. If the angular separation is larger, the scope (under stable seeing conditions) would see the individual stars.
Rayleigh limit – the limit at which two objects in a field of view can be clearly identified as separate components.
Dawes limit – the separation required to see a black space between two objects.
Rowe-Ackermann Schmidt Astrograph (RASA) – a wide-field astrograph designed to be a perfect companion to today’s color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras. For more information, click here.
Schmidt-Cassegrain Telescope (SCT) – an optical design that uses both mirrors and lenses (catadioptric optics) to fold the optical path and form an image. The light enters through a thin aspheric Schmidt correcting lens, strikes the spherical primary mirror, reflects the image on a small convex spherical secondary mirror, and then reflects the light through a hole in the primary mirror to the eyepiece or camera. For more information, click here.
Secondary mirror – a small, flat or curved mirror that diffracts incoming light in reflecting and catadioptric optical systems.
Secondary mirror obstruction – the obstruction caused by the secondary mirror and its support.
Setting circles – a tool used to measure or set the pointing angles in the right ascension (RA) and declination (Dec) coordinates. For more information, click here.
Sidereal rate – the speed at which a scope must track to counteract the rotation of the Earth and keep a celestial object in the eyepiece (about 15 arc seconds per second of time.)
Slew – the movement of a telescope around an axis. A telescope can slew manually or electronically.
Slow motion controls/knobs – Two knobs that move a telescope mount in small increments in either altitude & azimuth or RA & Dec, depending on the style of mount.
Solar filter – see Filter – Solar Filter
T-Adapter – an accessory used with a T-Ring that turns a telescope into a camera lens for your SLR or DLSR camera. T-Adapters can be universal or optical tube-specific.
T-Ring – an accessory used with a T-Adapter that attaches your SLR or DSLR camera to a telescope for astrophotography. T-Rings are camera brand-specific.
T-Mount – a system that uses T-Threads of metric dimensions (42mm and 48mm diameter and a pitch of 0.75mm) to mate cameras to lenses or telescopes.
Tracking – the task of keeping a celestial object centered in the telescope’s eyepiece as the Earth rotates.
Vignetting – a situation in an optical system in which the field of view is not fully illuminated at the edges.
Vibration Suppression Pads (VSP) – anti-vibration pads that go under your tripod legs to stabilize the telescope and prevent unwanted shaking.