Telescope Glossary

1. Telescope Types

Refractor – Uses lenses to bend light. Sharp images, low maintenance, great for planets.
Reflector – Uses mirrors to gather light. Excellent for deep-sky objects, affordable for large sizes.
Catadioptric – Uses both lenses and mirrors (e.g., Schmidt-Cassegrain). Compact and versatile.
Dobsonian – A Newtonian reflector on a simple alt-az base; popular for visual observing.
Schmidt-Cassegrain (SCT) – Compact catadioptric design with long focal length.
Maksutov-Cassegrain (Mak) – Similar to SCT but often sharper; slower cooldown.
Newtonian – Reflector design with a flat secondary mirror.
Rich-Field Telescope – Optimized for wide views of the sky.

2. Optical Terms

Aperture – Diameter of the main lens or mirror; determines light-gathering power. Bigger = brighter.
Focal Length – Distance light travels to focus (mm). Affects magnification and field of view.
Focal Ratio (f/number) – Focal length ÷ aperture (e.g., f/5). Lower = faster/brighter image.
Light Gathering Power – How much light the telescope collects (depends on aperture).
Resolving Power – Ability to separate close objects (e.g., double stars).
Field of View (FOV) – How much sky you can see at once.
True Field of View (TFOV) – Actual sky area seen.
Apparent Field of View (AFOV) – Eyepiece’s perceived width (°).
Exit Pupil – Beam of light exiting eyepiece; affects brightness.
Magnification – Telescope focal length ÷ eyepiece focal length.
Limiting Magnitude – Faintest object visible.
Collimation – Alignment of mirrors/lenses for sharp images.

3. Eyepieces & Accessories

Eyepiece (EP) – Lens you look through; determines magnification.
Plössl – Common eyepiece design, good quality and affordable.
Wide-Angle Eyepiece – Large AFOV (68°–100°).
Barlow Lens – Multiplies magnification (2×, 3×).
Diagonal – Bends light path for comfortable viewing.
Finder Scope – Small scope to help aim the telescope.
Red Dot Finder – Projects a red dot for easy targeting.
Filters – Enhance contrast or block unwanted light.
UHC Filter – Boosts nebula contrast.
O-III Filter – Enhances oxygen emission nebulae.
Moon Filter – Reduces brightness for lunar viewing.

4. Mounts & Tripods

Mount – Structure that holds and moves the telescope.
Alt-Az Mount – Moves up/down and left/right; simple and intuitive.
Equatorial Mount (EQ) – Aligned with Earth’s axis; tracks stars smoothly.
German Equatorial Mount (GEM) – Common EQ mount design.
Fork Mount – Holds telescope between arms; often used on SCTs.
GoTo Mount – Computerized; automatically finds objects.
Tracking – Mount movement compensating for Earth’s rotation.
Polar Alignment – Aligning EQ mount with celestial pole.
Counterweight – Balances telescope on EQ mount.

5. Observing Terms

Seeing – Atmospheric stability; affects image sharpness.
Transparency – Clarity of the sky; affects faint objects.
Light Pollution – Artificial sky brightness reducing visibility.
Dark Adaptation – Eyes adjusting to darkness (~20–30 minutes).
Averted Vision – Looking slightly away to detect faint objects.
Star Hopping – Navigating sky using stars as guides.
Zenith – Point directly overhead.
Horizon – Edge of visible sky.
Meridian – Imaginary line from north to south through zenith.

6. Celestial Objects

Planet – Body orbiting a star, reflects light.
Moon – Natural satellite.
Asteroid – Rocky object, mostly between Mars & Jupiter.
Comet – Icy body that forms a tail near the Sun.
Nebula – Cloud of gas and dust.
Emission Nebula – Glows from ionized gas.
Reflection Nebula – Reflects starlight.
Planetary Nebula – Shell of gas from dying star.
Galaxy – Massive system of stars, gas, and dust.
Globular Cluster – Dense spherical star cluster.
Open Cluster – Loose star group.
Double Star – Two stars appearing close together.

7. Astrophotography Terms

Imaging Camera – DSLR, mirrorless, or astro-camera.
CCD / CMOS – Camera sensor types.
Exposure – Length of time sensor collects light.
Stacking – Combining multiple images to reduce noise.
Bias Frames – Sensor noise calibration frames.
Dark Frames – Correct thermal noise.
Flat Frames – Correct vignetting and dust shadows.
Guiding – Using a second camera to improve tracking accuracy.
Autoguider – System that corrects mount tracking errors.
Plate Solving – Software that identifies where telescope is pointing.

8. Sky Coordinates & Time

Right Ascension (RA) – Celestial longitude.
Declination (Dec) – Celestial latitude.
Celestial Equator – Sky equivalent of Earth’s equator.
Ecliptic – Sun’s apparent path across the sky.
Sidereal Time – Time based on Earth’s rotation relative to stars.
Epoch – Reference date for star positions (e.g., J2000).

9. Practical & Technical Terms

Cooldown Time – Time for optics to reach ambient temperature.
Dew Shield – Prevents condensation on optics.
Dew Heater – Actively prevents dew.
Vibration Damping – Reducing shake for clearer views.
Back Focus – Distance required for accessories to focus properly.
Vignetting – Darkening at image edges.
Chromatic Aberration – Color fringing in refractors.
Coma – Off-axis distortion in reflectors.

10. Popular Abbreviations

DSO – Deep-Sky Object
FOV – Field of View
AFOV – Apparent Field of View
TFOV – True Field of View
LP – Light Pollution
OTA – Optical Tube Assembly
SCT – Schmidt-Cassegrain Telescope
EQ – Equatorial
AZ – Alt-Azimuth