Glossary of Common Astronomy Terms

Plain-language definitions for sky coordinates, imaging time, calibration, tracking, light, narrowband channels, and other terms used throughout Learn.

Use the search field below to find a term. Each entry links back to the Learn article where the concept is used in context.

Term	Definition	Learn more
Aperture	The diameter of a telescope's main lens or mirror. A larger aperture collects more light and can potentially reveal more detail.	In AstroGuide: Learn Aperture 6 matching references in this guide section. Myth: Bigger Aperture Always Wins 4 matching references in this guide section. External references Wikipedia: Aperture Optics background for the opening that admits light into an instrument.
Bortle Scale	A nine-level scale used to describe the brightness of the night sky caused by light pollution. Lower numbers represent darker skies.	In AstroGuide: Learn Light Pollution 6 matching references in this guide section. Myth: Lower Bortle Always Produces Better Images 5 matching references in this guide section. External references Wikipedia: Bortle scale Summary of the nine-level dark-sky brightness scale.
Celestial Object	Any natural object found in space, such as stars, planets, moons, asteroids, nebulae, and galaxies.	In AstroGuide: Learn Visual Astronomy vs Astrophotography 1 matching reference in this guide section. Understanding the Night Sky 1 matching reference in this guide section. External references Wikipedia: Astronomical object General reference for natural objects and structures in space.
Deep-Sky Object (DSO)	Objects outside our solar system, such as nebulae, star clusters, and galaxies.	In AstroGuide: Learn Myth: Fast Telescopes Are Always Superior 4 matching references in this guide section. Moonlight 2 matching references in this guide section. External references Wikipedia: Deep-sky object Overview of non-Solar-System subjects such as nebulae, clusters, and galaxies.
Exposure	The amount of time a camera collects light. Longer exposures allow more faint details to be recorded.	In AstroGuide: Learn Why Great Images Take Time 4 matching references in this guide section. Myth: Stacking Fixes Everything 3 matching references in this guide section. External references Wikipedia: Exposure Photography background on exposure time and captured light.
Field of View (FOV)	The amount of sky visible through a telescope or camera system. A wider field of view captures larger areas of the sky.	In AstroGuide: Learn Focal Length 6 matching references in this guide section. Optimal Focal Length: 400–1,500 mm Related guide topic for this term. External references Wikipedia: Field of view Reference for angular extent visible through an optical system.
Focal Length	The distance between a telescope's optical system and the point where light is focused. It determines image scale and field of view.	In AstroGuide: Learn Focal Length 9 matching references in this guide section. Myth: Long Focal Length Produces Better Images 6 matching references in this guide section. External references Wikipedia: Focal length Optics reference for focal distance and image scale.
Focal Ratio	The relationship between a telescope's aperture and focal length, expressed as an f-number (such as f/4 or f/10). It affects light-gathering speed and imaging characteristics.	In AstroGuide: Learn Myth: Fast Telescopes Are Always Superior 19 matching references in this guide section. Focal Ratio 13 matching references in this guide section. External references Wikipedia: F-number Reference for focal ratio and how it relates aperture to focal length.
Galaxy	A massive collection of stars, gas, dust, and dark matter held together by gravity. Our home galaxy is the Milky Way.	In AstroGuide: Learn Galaxies 22 matching references in this guide section. Star Clusters 5 matching references in this guide section. External references NASA: Galaxies NASA overview of galaxy types, structure, and evolution. Wikipedia: Galaxy Broad reference on galaxies and their components.
Light Pollution	Artificial light that brightens the night sky and reduces the visibility of faint astronomical objects.	In AstroGuide: Learn Light Pollution 6 matching references in this guide section. Myth: Lower Bortle Always Produces Better Images 5 matching references in this guide section. External references Wikipedia: Light pollution Reference for causes and effects of artificial nighttime light.
Light-Year	The distance light travels in one year. It is used to describe distances between stars and galaxies.	In AstroGuide: Learn Galaxies 1 matching reference in this guide section. Understanding Light Related guide topic for this term. External references NASA Space Place: Light-year Beginner-friendly explanation of distance measured by light travel time. Wikipedia: Light-year Reference for the astronomical distance unit.
Magnification	How much larger an object appears through a telescope compared to viewing it with the naked eye.	In AstroGuide: Learn Seeing 2 matching references in this guide section. Myth: Long Focal Length Produces Better Images 1 matching reference in this guide section. External references Wikipedia: Magnification Optics reference for apparent size increase.
Nebula	A large cloud of gas and dust in space. Some nebulae are regions where new stars are forming.	In AstroGuide: Learn Nebulae 21 matching references in this guide section. Other Types of Nebulae 14 matching references in this guide section. External references Wikipedia: Nebula Overview of gas and dust clouds in space. NASA: Hubble's Nebulae NASA overview of nebula types and star-forming regions.
Narrowband Filter	A filter that allows only specific wavelengths of light to pass through. It is commonly used to capture emission nebulae.	In AstroGuide: Learn Filters 10 matching references in this guide section. Moonlight 3 matching references in this guide section. External references Wikipedia: Narrowband Reference for narrow passbands that transmit a limited range of wavelengths. Wikipedia: Optical filter General optics reference for filters that selectively transmit light.
Pixel Scale	The amount of sky covered by each pixel of a camera image. It affects the level of detail that can be recorded.	In AstroGuide: Learn Focal Length 4 matching references in this guide section. Focal Ratio 2 matching references in this guide section. External references Wikipedia: Angular resolution Background on resolving fine angular detail in an image.
Seeing	The stability of Earth's atmosphere. Good seeing allows sharper views, especially for planets and high-resolution imaging.	In AstroGuide: Learn Seeing 7 matching references in this guide section. Choosing the Right Telescope: Optimal Aperture and Focal Length for General Sky Conditions 3 matching references in this guide section. External references Wikipedia: Astronomical seeing Reference for atmospheric turbulence and image sharpness.
Signal-to-Noise Ratio (SNR)	The amount of useful information compared to unwanted noise in an image. Higher SNR usually results in cleaner and more detailed images.	In AstroGuide: Learn Why Great Images Take Time 2 matching references in this guide section. Why Processing Matters 2 matching references in this guide section. External references Wikipedia: Signal-to-noise ratio Technical reference for useful signal compared with unwanted noise.
Stacking	Combining multiple images together to improve signal quality and reduce noise.	In AstroGuide: Learn Myth: Stacking Fixes Everything 9 matching references in this guide section. Why Great Images Take Time 4 matching references in this guide section. External references Wikipedia: Astrophotography processing Background on processing multiple astronomical images. Wikipedia: Lucky imaging Related technique that combines selected frames for sharper results.
Star Cluster	A group of stars held together by gravity. The two main types are open clusters and globular clusters.	In AstroGuide: Learn Star Clusters 18 matching references in this guide section. Light Pollution 4 matching references in this guide section. External references Wikipedia: Star cluster Overview of open clusters, globular clusters, and stellar groupings.
Telescope	An optical instrument that collects and focuses light to observe distant objects.	In AstroGuide: Learn Mounts 9 matching references in this guide section. There Is No Perfect Equipment 7 matching references in this guide section. External references Wikipedia: Telescope Reference for instruments that collect and focus electromagnetic radiation.
Transparency	The clarity of the atmosphere and how easily light from space can pass through it.	In AstroGuide: Learn Transparency 11 matching references in this guide section. Myth: Lower Bortle Always Produces Better Images 4 matching references in this guide section. External references Wikipedia: Atmospheric window Context for how Earth's atmosphere transmits light from space.
Tracking	The movement of a telescope mount to follow objects as Earth rotates, keeping them centred during observation or photography.	In AstroGuide: Learn Mounts 12 matching references in this guide section. Myth: More Expensive Telescope Equals Better Images 7 matching references in this guide section. External references Wikipedia: Equatorial mount Reference for mount geometry used to follow Earth's rotation. Wikipedia: Clock drive Background on mechanisms that move a telescope with the sky.
Altitude and Azimuth	Observer-dependent sky coordinates. Altitude measures angle above the horizon; azimuth measures compass direction around the horizon.	In AstroGuide: Learn Understanding the Night Sky How local coordinates relate to the moving sky. External references
Right Ascension and Declination (RA/Dec)	Coordinates fixed to the celestial sphere and used to identify a sky position independently of one observer's current horizon.	In AstroGuide: Learn Choosing the Best Subjects Connects catalog coordinates to an observing window. External references
Airmass	The relative path length that a target's light travels through Earth's atmosphere; it is lowest near the zenith and increases toward the horizon.	In AstroGuide: Learn Target Altitude Why low targets are more affected by atmosphere and skyglow. External references
Culmination	The point in a target's daily transit when it reaches its greatest altitude for the observer.	In AstroGuide: Learn Choosing the Best Subjects Uses culmination to define a practical imaging window. External references
Subframe (Sub-exposure)	One individual camera exposure that may later be calibrated, registered, and combined with other frames.	In AstroGuide: Learn Exposure Length vs Integration Time Separates one exposure from the complete integration. External references
Integration Time	The total exposure time represented by the accepted subframes combined into an image.	In AstroGuide: Learn Exposure Length vs Integration Time Explains usable integration and diminishing returns. External references
Light Frame	An exposure of the astronomical subject, as distinct from a calibration frame.	In AstroGuide: Learn Calibrate, Register, and Stack Places lights in the complete preprocessing pipeline. External references
Flat Frame	An evenly illuminated calibration exposure used to model vignetting, dust shadows, and pixel-response differences in an unchanged optical path.	In AstroGuide: Learn Calibrate, Register, and Stack Explains flats and matching requirements. External references
Dark Frame	A covered calibration exposure matched to light-frame exposure, gain, and sensor temperature to model repeatable thermal and electronic signal.	In AstroGuide: Learn Calibrate, Register, and Stack Explains traditional calibration data. External references
Bias Frame	A shortest-practical covered exposure used in some workflows to model the sensor's readout offset.	In AstroGuide: Learn Calibrate, Register, and Stack Distinguishes bias from dark flats and hot-pixel maps. External references
Calibration	Correction of repeatable sensor and optical signatures using appropriate reference data and a matched workflow.	In AstroGuide: Learn Calibrate, Register, and Stack Canonical calibration and preprocessing lesson. External references
Registration	Geometric alignment of exposures so the same sky coordinates land on the same output positions.	In AstroGuide: Learn Calibrate, Register, and Stack Explains star alignment and shared coverage. External references
Normalization	Adjustment of compatible frames to reconcile scale or background differences before combination.	In AstroGuide: Learn Calibrate, Register, and Stack Separates normalization from registration and stacking. External references
Statistical Rejection	A comparison of aligned pixel samples that reduces outliers such as some trails, cosmic rays, and hot pixels during stacking.	In AstroGuide: Learn Myth: Stacking Fixes Everything Explains what rejection can and cannot repair. External references
Dithering	Small, deliberate changes in pointing between exposures that help separate fixed sensor patterns from the sky during registration and stacking.	In AstroGuide: Learn Diagnosing Common Artifacts Connects dithering to walking-noise prevention. External references
Drizzle	A reconstruction method that uses many sub-pixel-shifted frames to build a finer sampling grid; it does not create optical resolution.	In AstroGuide: Learn Why Processing Matters Explains prerequisites and tradeoffs. External references
Field Rotation	Rotation of the sky field relative to the camera during Alt-Az tracking even while the target remains centered.	In AstroGuide: Learn Mounts Shows why common edge coverage shrinks. External references
Alt-Az Mount	A mount that moves in altitude and azimuth; simple and convenient, but its tracked camera field rotates with respect to the sky.	In AstroGuide: Learn Mounts Compares Alt-Az and equatorial operation. External references
Equatorial (EQ) Mount	A mount whose right-ascension axis is aligned with Earth's rotation axis so it can track the sky without Alt-Az field rotation.	In AstroGuide: Learn Mounts Explains EQ benefits and setup costs. External references
Polar Alignment	Alignment of an equatorial mount's right-ascension axis with Earth's rotational axis.	In AstroGuide: Learn Mounts Places polar alignment in the tracking model. External references
Spectrum	The distribution of light across wavelength.	In AstroGuide: Learn Understanding Light Introduces continuum and emission-line light. External references
Continuum Light	Light distributed across a broad range of wavelengths, such as much of the starlight in galaxies and reflection nebulae.	In AstroGuide: Learn Filters Uses target light to choose a filter. External references
Emission Line	A narrow wavelength at which excited atoms or ions emit strongly, such as Hα, O III, or S II in a nebula.	In AstroGuide: Learn Narrowband Imaging Connects physical emissions to mapped color. External references
Hα, O III, and S II	Common narrowband measurements from hydrogen-alpha, doubly ionized oxygen, and singly ionized sulfur emission.	In AstroGuide: Learn Narrowband Imaging Explains the lines and their capture. External references
HOO and SHO	Display mappings that assign measured narrowband channels to RGB: HOO commonly uses Hα for red and O III for green and blue; SHO commonly maps S II, Hα, and O III to red, green, and blue.	In AstroGuide: Learn Narrowband Imaging Full channel-extraction and mapped-color workflow. External references
Image Scale	The angular portion of the sky represented by a pixel, usually expressed in arcseconds per pixel and set by pixel size and focal length.	In AstroGuide: Learn Focal Length Connects focal length, sensor, and subject scale. External references