Backyard Astronomy Glossary: 24 Terms Explained
The words that show up in backyard astronomy specs, reviews and how-to guides, explained without jargon. Open any term for the full explanation and what to look for when buying.
- Altazimuth mount
- A simple telescope mount that moves up-down and left-right like a camera tripod, easier to use casually than an equatorial mount.
- Read more about altazimuth mount
- Aperture
- The diameter of a telescope's main lens or mirror, the single biggest factor in how much light it gathers and how much detail it can resolve.
- Read more about apertureRelated: Best Dobsonian Telescopes for Deep-Sky Views (2026)
- Apochromatic refractor
- A high-end refractor using special glass elements to bring multiple colors of light into focus together, virtually eliminating color fringing.
- Read more about apochromatic refractor
- Averted vision
- A viewing technique of looking slightly to the side of a faint object so its light falls on more light-sensitive parts of the eye's retina.
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- Barlow lens
- An accessory placed before an eyepiece that multiplies magnification, commonly by two or three times, without buying more eyepieces.
- Read more about barlow lensRelated: Best Barlow Lenses to Boost Your Magnification (2026)
- Collimation
- The process of aligning a reflector's mirrors so light meets at a single sharp focus point instead of a blurred smear.
- Read more about collimationRelated: Best Collimators for Razor-Sharp Optics (2026)
- Coma corrector
- An accessory lens that removes the star-shaped distortion, called coma, that fast reflector telescopes produce toward the edge of the field.
- Read more about coma corrector
- Dobsonian
- A Newtonian reflector on a simple, low-friction rocker box mount, the classic value choice for visual deep-sky observing.
- Read more about dobsonianRelated: Best Dobsonian Telescopes for Deep-Sky Views (2026)
- Equatorial mount
- A telescope mount with one axis aligned to Earth's rotational axis, so a single slow-motion control tracks the sky's apparent motion.
- Read more about equatorial mountRelated: Best Equatorial Mounts for Tracking the Sky (2026)
- Exit pupil
- The width of the light beam leaving an eyepiece, found by dividing aperture by magnification, which determines how bright an image looks.
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- Finder scope
- A small, low-power scope mounted on the main telescope tube that helps locate objects before centering them in the main eyepiece.
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- Focal length
- The distance light travels inside a telescope before coming to focus, used with eyepiece focal length to work out magnification.
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- Focal ratio
- Focal length divided by aperture, written like f/5 or f/10, showing whether a telescope favors wide fields or high magnification.
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- GoTo mount
- A computerized mount that automatically points to and tracks celestial objects once aligned with a few reference stars.
- Read more about goto mountRelated: Best GoTo Mounts for Automated Object Finding (2026)
- Intervalometer
- A remote timer that triggers a camera shutter automatically at set intervals, essential for long-exposure astrophotography and star trails.
- Read more about intervalometerRelated: Best Intervalometers for Automated Astro Shots (2026)
- Light pollution filter
- A filter that blocks specific wavelengths from artificial lighting to boost contrast on certain nebulae from bright suburban or city skies.
- Read more about light pollution filterRelated: Best Light Pollution Filters for City Skies (2026)
- Maksutov-Cassegrain
- A compact catadioptric telescope combining a mirror and a corrector lens, giving a long focal length in a short, sealed tube.
- Read more about maksutov-cassegrainRelated: Best Maksutov Telescopes for Planets and Moon (2026)
- Red dot finder
- A finder that projects an illuminated red dot onto a viewing window to show where a telescope is aimed, with no magnification at all.
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- Reflector
- A telescope that uses curved mirrors instead of lenses to gather light, delivering more aperture per dollar than a refractor.
- Read more about reflectorRelated: Best Reflector Telescopes for Deep-Sky (2026)
- Refractor
- A telescope that uses a glass lens at the front to bend and focus light, prized for sharp, low-maintenance views.
- Read more about refractorRelated: Best Refractor Telescopes for Sharp Views (2026)
- Seeing
- The steadiness of the atmosphere on a given night, which limits how much fine detail any telescope can resolve regardless of aperture.
- Read more about seeing
- Solar filter
- A certified filter fitted securely over a telescope's front aperture that reduces sunlight to safe levels for direct solar viewing.
- Read more about solar filterRelated: Best Solar Filters for Safe Sun Observing (2026)
- Star diagonal
- A mirror or prism attachment that bends the light path 90 or 45 degrees at the eyepiece, making high-angle viewing far more comfortable.
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- True field of view
- The actual width of sky visible through an eyepiece, calculated from the eyepiece's apparent field and the telescope's magnification.
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