Glossario attrezzatura

Telescopes

$Refractor telescope on a tripod$

A classic refractor telescope

A telescope gathers light through its aperture (the diameter of the primary lens or mirror) and focuses it at a point determined by its focal length. Larger aperture means more light and finer detail; longer focal length means higher magnification for a given eyepiece.

Refractor

Uses a glass lens at the front to bend (refract) light to a focus. Produces sharp, high-contrast images with no central obstruction. Simple refractors (achromatic) show some chromatic aberration — color fringing around bright objects. Apochromatic (APO) refractors use extra-low dispersion (ED) glass to nearly eliminate this, but at higher cost.

Best for: Planets, Moon, double stars, wide-field views. Low maintenance.

Newtonian Reflector

Uses a concave primary mirror at the bottom of the tube and a small flat secondary mirror angled at 45° to direct light to the focuser on the side. No chromatic aberration. Offers the most aperture per dollar. Requires periodic collimation (mirror alignment).

Best for: Deep-sky objects, general observing. Often paired with Dobsonian mounts.

Dobsonian

A Newtonian reflector on a simple alt-azimuth rocker box mount. The design maximizes aperture while keeping cost and complexity low. Extremely popular for visual deep-sky observing. Available from 6″ to 24″+ aperture, including collapsible truss-tube designs for portability.

Best for: Deep-sky visual observing on a budget. The "light bucket" of choice.

Schmidt-Cassegrain (SCT)

A compound (catadioptric) design using a spherical primary mirror and a thin corrector plate at the front. Light bounces off the primary, reflects from a convex secondary mirror mounted on the corrector, and passes through a hole in the primary to the focuser at the back. The folded light path makes the tube very compact — an 8″ SCT is typically only 40 cm long. Focal ratio around f/10.

Best for: All-round use — planets, deep-sky, and astrophotography. Very portable for the aperture.

Maksutov-Cassegrain (Mak)

Similar to the SCT but uses a thick meniscus corrector lens instead of a thin plate. The secondary mirror is often an aluminized spot on the inside of the corrector. Produces very sharp, high-contrast images with a long focal length (typically f/12–f/15). Slower cool-down time due to the thick corrector.

Best for: Planets, Moon, double stars. Excellent optics in a compact tube.

Ritchey-Chrétien (RC)

A specialized Cassegrain using two hyperbolic mirrors to eliminate coma and produce a flat, sharp field across the entire focal plane. The design used by the Hubble Space Telescope and most professional observatories. Requires careful collimation but delivers outstanding imaging performance.

Best for: Astrophotography, especially with large sensors. Not typically used for visual.

Try our interactive Optics Simulator to compare how different telescope types perform.

Eyepieces

A collection of telescope eyepieces

The eyepiece magnifies the image formed by the telescope. Magnification = telescope focal length ÷ eyepiece focal length. Eyepieces come in 1.25″ (31.7mm) and 2″ (50.8mm) barrel diameters.

Key Specifications

Focal length Given in mm. Lower = higher magnification. Common range: 4mm (high power) to 40mm (low power).

Apparent FOV The angular width of the view through the eyepiece alone. Ranges from ~40° (Plössl) to 100°+ (ultra-wide). Wider = more immersive.

Eye relief Distance from the eye lens to where the full field is visible. At least 15mm recommended for eyeglass wearers.

True FOV The actual sky area you see: apparent FOV ÷ magnification. This determines how much sky fits in the view.

Common Designs

Plössl

4-element symmetric design. ~50° AFOV. Sharp, affordable, good all-rounder. Short eye relief in focal lengths under 10mm.

Kellner / Modified Achromat

3-element budget design. ~40–45° AFOV. Adequate for long focal lengths, some edge softness.

Wide-field (68°–72°)

Multi-element designs (e.g., Explore Scientific 68°, BST Explorer). Good balance of field, sharpness, and price.

Ultra-wide (82°–100°+)

Premium multi-element designs (e.g., Nagler, Ethos, Nikon NAV-HW). Immersive "spacewalk" views. Heavy and expensive.

Zoom

Variable focal length (e.g., 8–24mm). Convenient for quick power changes, but narrower AFOV than fixed eyepieces.

Orthoscopic

4-element classic design. ~45° AFOV. Superb sharpness and contrast, excellent for planetary observation.

Mounts

$Astro-Physics German equatorial mount with refractor$

A German equatorial mount with refractor

The mount supports the telescope and allows you to point and track objects across the sky. The mount is at least as important as the optics — a shaky mount ruins any view.

Alt-Azimuth (Alt-Az)

Moves up/down (altitude) and left/right (azimuth). Intuitive to use. Does not track the sky's rotation without a computerized dual-axis drive. Includes Dobsonian rocker boxes and single-arm fork mounts.

Equatorial (EQ)

One axis (the polar axis, or RA axis) is aligned with Earth's rotation axis. Once polar-aligned, a single motor can track objects by rotating around this axis alone. Essential for long-exposure astrophotography. Common types: German Equatorial Mount (GEM) and fork equatorial.

GoTo

A computerized mount (alt-az or equatorial) with a built-in object database. After an initial alignment procedure (usually 2–3 stars), the mount can automatically slew to any cataloged object. Some use GPS and accelerometers for faster setup.

Star Tracker

A lightweight, portable equatorial tracking platform for camera lenses and small telescopes. Polar-aligned to Polaris, it rotates at sidereal rate to compensate for Earth's rotation. Popular for wide-field astrophotography from dark-sky sites.

Mount Terminology

Payload capacity — Maximum recommended weight of telescope + accessories. A common rule: load the mount to no more than 60–70% of rated capacity for imaging.

Polar alignment — Aligning the mount's RA axis with the celestial pole. Needed for accurate equatorial tracking.

Periodic error (PE) — Small tracking inaccuracies caused by imperfect gears. Measured in arc-seconds. Lower is better; can be corrected with autoguiding or PEC.

Slew speed — How fast the mount can move to a new target, measured in degrees per second.

Counterweight — Weight on the opposite side of a GEM to balance the telescope. Proper balance reduces motor strain and improves tracking.

Finders & Guides

A 50mm finder scope on a telescope

Auxiliary optics attached to the telescope to help aim at targets.

Red-dot finder Projects a red dot or bullseye onto a small window. No magnification. Intuitive for aiming at naked-eye objects. The Telrad projects three concentric circles (0.5°, 2°, 4°) and is a favorite for star hopping.

Finder scope A small, low-power telescope (e.g., 6×30, 8×50, 9×50) mounted on the main tube. Shows fainter stars than the naked eye, with crosshairs for centering. Larger aperture = fainter stars visible.

Right-angle finder A finder scope with a 90° prism or mirror, so you look down instead of along the tube. More comfortable, especially for objects near the zenith.

Guide scope A small refractor (50–80mm) used with a guide camera for autoguiding during long-exposure astrophotography. Sends corrections to the mount to keep tracking precise.

Filters

Astronomik O-III narrowband nebula filter

An O-III narrowband nebula filter

Filters thread into the eyepiece barrel or camera adapter to selectively pass or block wavelengths of light.

Visual Filters

Light pollution (broadband) — Blocks common artificial light wavelengths (sodium, mercury) while passing nebula emission lines. Subtle improvement; most useful from moderately light-polluted skies.

UHC (Ultra High Contrast) — Passes both O-III and H-beta emission lines. Excellent general-purpose nebula filter. Darkens stars and sky background, making emission and planetary nebulae pop.

O-III — Passes only the doubly-ionized oxygen lines (496nm, 501nm). The strongest visual nebula filter for planetary nebulae and supernova remnants (Veil Nebula, Dumbbell). Darkens the field significantly.

H-beta — Passes only the hydrogen-beta line (486nm). Very specialized. Essential for the Horsehead Nebula and California Nebula; not useful on most other objects.

Moon / ND filter — Neutral density filter that reduces overall brightness. Makes lunar observing comfortable and can improve planetary contrast.

Color filters — Colored glass (red, orange, yellow, green, blue, violet) for enhancing planetary detail. Red for Mars surface features, blue for Jupiter cloud bands, etc.

Imaging Filters

H-alpha (Ha) — Narrowband filter passing the hydrogen-alpha line (656nm). Captures emission nebulae in stunning detail. Works well even under severe light pollution.

S-II — Passes ionized sulfur emission (672nm). Often combined with Ha and O-III for the "Hubble Palette" (SHO) false-color images.

L-R-G-B — Set of Luminance, Red, Green, Blue broadband filters for color imaging with monochrome cameras.

Duo-/tri-narrowband — Multi-bandpass filters (e.g., L-eXtreme, L-eNhance) that pass Ha + O-III in a single filter. Designed for one-shot color cameras.

Solar filter — Full-aperture filter (Baader film or glass) placed over the front of the telescope for safe white-light solar viewing. Never observe the Sun without a proper solar filter.

H-alpha solar — Specialized etalon system (e.g., Lunt, Coronado) for viewing solar prominences, filaments, and chromospheric detail in hydrogen-alpha light.

Cameras & Imaging

Nikon D810A — a dedicated astrophotography DSLR

Camera Types

Dedicated astronomy camera — Cooled CMOS or CCD sensor designed for long exposures. Thermoelectric cooling reduces thermal noise. Available as monochrome (used with LRGB/narrowband filters) or one-shot color (OSC).

DSLR / Mirrorless — Consumer cameras work well for astrophotography. Attach to the telescope via a T-ring adapter. Modified (IR filter removed) versions capture more H-alpha nebula emission.

Planetary camera — Small, fast-readout CMOS cameras for capturing thousands of frames per second. Used with lucky imaging / stacking software to freeze atmospheric turbulence on planets, Moon, and Sun.

Guide camera — Small, sensitive camera used with a guide scope or off-axis guider to provide real-time tracking corrections to the mount via software (PHD2, Lin_guider).

Sensor Specifications

Pixel size — Measured in microns (μm). Determines the angular resolution per pixel (arc-seconds/pixel) combined with focal length. Typical range: 2.4μm to 9μm.

Read noise — Electronic noise added each time the sensor is read. Lower is better. Modern CMOS sensors achieve <1e− read noise.

Full well capacity — Maximum electrons a pixel can hold before saturating. Higher means more dynamic range in a single exposure.

Quantum efficiency (QE) — Percentage of photons converted to electrons. Higher QE = more sensitive sensor. Modern sensors reach 80–95% peak QE.

Sensor size — Physical dimensions (or diagonal). Determines the field of view with a given focal length. Common formats: 1/1.2″, APS-C, full-frame (36×24mm).

Imaging Accessories

Focal reducer / Flattener — An optical element placed before the camera to shorten the focal length (reducing magnification and speeding up the focal ratio) and/or flatten the field for sharp stars to the edges.

Barlow lens — Diverging lens inserted before the eyepiece or camera to multiply the effective focal length (usually 2× or 3×). Increases magnification and image scale.

Off-axis guider (OAG) — Uses a small prism to pick off light from the edge of the main telescope's field and redirect it to a guide camera. Eliminates differential flexure issues of separate guide scopes.

Filter wheel — Motorized wheel holding multiple filters that can be switched remotely during an imaging session. Essential for monochrome camera workflows.

Coma corrector — Corrects the comet-shaped star distortion (coma) that fast Newtonian telescopes produce near the edges of the field.

Binoculars

8x42 binoculars — a popular choice for astronomy

Often the best first instrument for astronomy. Both eyes give a natural, immersive view of the sky. Binoculars are specified as magnification × aperture (e.g., 10×50 = 10x magnification, 50mm aperture).

7×50 — Classic astronomy choice. Wide 7.1mm exit pupil matches a dark-adapted eye. Lightweight, steady enough to hand-hold. Great for scanning the Milky Way and finding large objects.

10×50 — Slightly more magnification, narrower field. Still hand-holdable for most people. Good compromise for general astronomy.

15×70 and larger — More aperture and magnification but require a tripod or parallelogram mount. Impressive views of star clusters, nebulae, and the Milky Way.

Exit pupil — The bright disk visible in each eyepiece. Calculated as aperture ÷ magnification. Should not exceed your eye's dark-adapted pupil (5–7mm depending on age). Larger exit pupil = brighter image.

Porro vs. Roof prism — Porro prisms (offset barrels) generally give brighter images and better depth perception at a given price. Roof prisms (straight barrels) are more compact but need expensive coatings to match Porro performance.

BAK-4 vs. BK-7 — Prism glass types. BAK-4 has a higher refractive index, producing a round, fully illuminated exit pupil. Preferred for astronomy.

Accessories

A star diagonal — redirects light for comfortable viewing

Dew heater A heated strip wrapped around the corrector plate or objective lens to prevent dew formation. Essential in humid climates. Controlled by a dew controller that adjusts heat based on temperature and humidity.

Dew shield A tube extension in front of the corrector/objective that delays dew formation by reducing radiative cooling. Passive and always recommended as a first line of defense.

Collimation tools Cheshire eyepiece, laser collimator, or autocollimator used to align the mirrors in reflector and catadioptric telescopes. Regular collimation is essential for optimal performance.

Diagonal A 90° mirror or prism that redirects the light path for comfortable viewing. Star diagonals use mirrors (better for astronomy); prism diagonals are heavier but produce a correct image. Dielectric-coated mirrors reflect 99%+ of light.

Power supply Portable battery packs (12V or USB) to power GoTo mounts, dew heaters, cameras, and laptops in the field. Lithium iron phosphate (LiFePO4) packs are popular for their long life and stable voltage.

Red flashlight Preserves dark adaptation while reading charts or adjusting equipment. Red light does not trigger the eye's cone receptors, keeping the rods sensitive to faint objects.

Observing chair An adjustable-height chair or stool designed for comfortable extended observing. Proper seating dramatically improves how much detail you can see at the eyepiece.

Optical Concepts

Light path diagram of a Newtonian reflector telescope

Light path in a Newtonian reflector telescope

Aperture — The diameter of the primary light-gathering element (lens or mirror). The single most important telescope specification. More aperture = more light gathered = fainter objects visible and finer detail resolved.

Focal length — The distance from the primary optic to the focal point, in mm. Determines image scale: longer focal length = larger image = higher magnification with a given eyepiece.

Focal ratio (f/number) — Focal length ÷ aperture. A fast scope (f/4–f/5) has a wide field and short exposures for imaging; a slow scope (f/10–f/15) has a narrow field and higher image scale. Focal ratio does not affect visual brightness (the exit pupil does).

Magnification — Telescope focal length ÷ eyepiece focal length. Useful maximum magnification is roughly 2× the aperture in mm (e.g., 200mm scope → 400× max), limited by atmospheric seeing.

Exit pupil — The disk of light exiting the eyepiece. Equals aperture ÷ magnification, or eyepiece focal length ÷ focal ratio. Should be 0.5–7mm. Too large (>7mm) wastes light; too small (<0.5mm) is uncomfortably dim.

Resolving power (Dawes limit) — The finest angular detail a telescope can separate: 116 ÷ aperture in mm = arc-seconds. A 150mm scope resolves about 0.77″.

Limiting magnitude — The faintest star visible through the telescope under ideal conditions. Approximately: 2.7 + 5 × log₁₀(aperture in mm). A 200mm scope reaches about ~14th magnitude.

Collimation — The alignment of all optical elements along the optical axis. Reflectors and catadioptrics need periodic collimation for best performance. Refractors are factory-collimated and rarely need adjustment.

Chromatic aberration — Color fringing caused by a lens focusing different wavelengths at different points. Visible as purple or blue halos around bright stars in achromatic refractors. Minimized by apochromatic (ED/fluorite) lens elements.

Coma — An aberration in reflectors where stars near the edge of the field appear comet-shaped (flared). Worse at fast focal ratios. Corrected with a coma corrector (Paracorr, etc.).

Seeing — Atmospheric turbulence that causes stars to shimmer and blur. Measured on a 1–10 Antoniadi scale or in arc-seconds of blur. Good seeing (<2″) is essential for high-magnification planetary and double-star observing.

Transparency — How clear the atmosphere is, affecting how faint you can see. Can be excellent even with poor seeing, or vice versa. Best measured by the faintest naked-eye star visible (NELM).

Dark adaptation — The process of your eyes adjusting to darkness, taking 20–30 minutes for full rod sensitivity. Bright white light destroys adaptation instantly; use only red light at the eyepiece.

Averted vision — Looking slightly off-center to place a faint object on the more sensitive rod cells at the edge of your retina. Can reveal objects 1–2 magnitudes fainter than direct vision.

Useful Formulas

$Airy disk diffraction pattern from a circular aperture$

Airy disk — the diffraction pattern from a circular aperture

Property	Formula	Example
Magnification	f_telescope ÷ f_eyepiece	1200mm / 10mm = 120×
Focal ratio	f_telescope ÷ aperture	1200mm / 200mm = f/6
Exit pupil	aperture ÷ magnification	200mm / 120× = 1.67mm
True FOV	apparent FOV ÷ magnification	82° / 120× = 0.68°
Dawes limit	116 ÷ aperture (mm)	116 / 200 = 0.58″
Limiting magnitude	2.7 + 5 × log₁₀(aperture mm)	2.7 + 5 × 2.301 = 14.2 mag
Image scale	206.265 ÷ f_telescope (mm)	206.265 / 1200 = 0.172″/μm
Arc-sec per pixel	image scale × pixel size (μm)	0.172 × 3.76 = 0.65″/px

Equipment Glossary

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