What is a telescope? How it works and the main types explained
A concise guide to what a telescope is, how it works, its main parts, and the differences between refractors, reflectors and catadioptric designs.
Atacama Stargazing
5/1/20263 min read
A telescope is not, first and foremost, an instrument to magnify images. It is a machine to collect light. That distinction changes everything about how you understand, choose, and use one.
Imagine it starts to rain and you try to catch water with your hands: you capture a few drops. Use a bucket and you collect liters from the same shower. The telescope is the bucket; your naked eye, the hands. The goal is not to see bigger — it is to gather enough light to detect objects that would otherwise be invisible.
Refraction and reflection: two ways to bend light
All telescopes exploit one of two optical phenomena to direct light toward a focus:
Refraction (lenses): when light passes from air to glass it changes direction according to Snell's Law. A convex lens converges parallel rays to a focal point. The problem: different colors focus at slightly different points, producing halos known as chromatic aberration.
Reflection (mirrors): a concave parabolic mirror reflects all rays parallel to the optical axis to exactly the same focal point, regardless of wavelength. No chromatic aberration. Newton demonstrated this in 1668 by building the first functional reflector specifically to eliminate the color halos plaguing the refractors of his era.
The telescope trinity: aperture, resolution, magnification
1. Aperture (D) — the most important
The diameter of the objective in millimeters. Light-collecting power scales with the square of the diameter: a 150 mm telescope gathers four times more light than a 75 mm one, not twice. Aperture also sets the limiting magnitude — the faintest detectable object:
Limiting magnitude ≈ 2 + 5 × log₁₀ (D in mm)
A 200 mm telescope can reach magnitude ~13.5 under a dark sky. The naked eye reaches ~6.5.
2. Angular resolution
The finest detail the telescope can distinguish, set by the physics of diffraction (Rayleigh criterion):
θ (arcsec) ≈ 138 / D (mm)
A 100 mm telescope can theoretically separate details of ~1.4 arcseconds. Atmospheric turbulence — seeing — limits practical resolution to 0.7–2 arcsec even under excellent skies.
3. Magnification — the least important
M = F_objective / F_eyepiece
The useful magnification limit is approximately 2 × D (mm). A 200 mm telescope gains nothing beyond ~400×: past that threshold you get "empty magnification" — bigger but not sharper, and noticeably darker and harder to track. The "600×" myth: a 60 mm telescope at 600× produces a shaky, unrecognizable image. Aperture is what counts.
Types of telescopes
Refractors
Lens-based objective. Sealed tube, no collimation, low maintenance. Best for planets, Moon, and double stars. Practical aperture limit: beyond ~100 mm, lens weight and price grow cubically with diameter.
- Achromatic: two lenses (crown + flint glass). Corrects chromatic aberration for two wavelengths. Accessible price.
- Apochromatic (APO): triplet with ED glass or fluorite. Virtually free of chromatic aberration. The best accessible refractor for amateurs, at a significantly higher price.
Newtonian Reflector
Parabolic primary mirror + flat secondary that diverts the beam 90° to a lateral eyepiece. No chromatic aberration. Greater aperture per dollar than any other design. Requires periodic collimation (aligning the mirrors with a laser collimator: 5–10 minutes).
Dobsonian — the best starting point for deep sky
A Newtonian on a low-cost altitude-azimuth friction mount, popularized by John Dobson in the 1960s. Not a different optical design: a philosophy — maximize aperture per dollar spent.
The 8-inch Dobsonian (200 mm) is the most recommended entry-level telescope for anyone serious about deep-sky observing. It combines enough aperture to resolve galaxies, nebulae, and globular clusters with a tube compact enough to transport in a car. Under Bortle 3 skies — like those of the San Pedro de Atacama area — you can see the spiral structure of M51, resolve the Hercules Cluster into individual stars, and detect M31's dust lane. The aperture-to-price ratio in this category is unmatched. Limitation: no motorized tracking, so it is not suited for long-exposure astrophotography.
Schmidt-Cassegrain (SCT)
Schmidt corrector plate + spherical primary + convex secondary. The optical path "folds" into a short tube: an 8-inch SCT with 2000 mm of focal length fits in a 46 cm tube. Versatile for visual and photographic use. Typical f-ratio ~f/10. Central obstruction 35–40%.
Maksutov-Cassegrain (MCT)
Thick meniscus corrector + spherical primary. The secondary is a silvered zone of the meniscus itself — no spider vanes, no diffraction spikes on bright stars. Smaller central obstruction (25–35%), higher contrast. Slower cooling due to the thick glass. f-ratio f/13–f/15: excellent for planets and double stars, slow for deep-sky photography.
| Property | SCT | MCT |
|---|---|---|
| Corrector | Aspheric Schmidt plate | Thick spherical meniscus |
| Central obstruction | 35–40% | 25–35% |
| Cool-down time | Moderate | Slow |
| Available aperture | Up to 22 inches | Rarely >180 mm |
| Best use | Visual + photography | Planets and double stars |
Components: from star to eye
Objective: collects and focuses light. Sets the system's aperture, focal length, and f-ratio.
Focuser: focusing mechanism. The Crayford type (friction roller) provides smooth, backlash-free movement, preferred for photography. Rack-and-pinion is more robust for heavy eyepieces.
Eyepiece: magnifies the image at the focal plane. The Plössl design (4 elements) is the accessible standard — ~50° apparent field, good correction. Nagler and Ethos designs (7–8 elements) offer 82–100° fields and an immersive experience, at premium prices.
Finder scope: points the telescope at the target. A red dot finder (LED with no magnification) is quick under light-polluted skies. A 9×50 RACI finder (correct-image, 9×) enables precise star-hopping under Bortle 1–4 skies.
Mount — three types:
- Altitude-azimuth: moves in altitude and azimuth. Simple and intuitive. Not suitable for long-exposure astrophotography without field rotation correction.
- Equatorial: one axis aligned with the celestial pole. A single motor on the right ascension axis tracks any object indefinitely. Standard for astrophotography.
- GoTo: computerized. Automatically locates objects from a database after a brief star alignment. Speeds up observing; does not replace learning to navigate the sky.
What you can see by aperture
| Aperture | Accessible objects | Limiting magnitude (Bortle 3) |
|---|---|---|
| 60–80 mm | Lunar craters from 8 km, Saturn's rings, Galilean moons, M42 (Orion Nebula) | ~10.5–11.5 |
| 100–130 mm | Cassini Division, Jupiter's cloud bands, M13 (diffuse), basic structure of M31 | ~12.0–13.0 |
| 150–200 mm | M51 spiral structure, M81/M82, planetary nebulae with shape, Hercules Cluster resolved | ~13.0–14.0 |
| 200 mm+ | Detailed galactic structure, Encke Division in Saturn, thousands of galaxies to mag. 14.5+ | ~14.5+ |
Magnitudes given for Bortle 3 skies, such as those in the San Pedro de Atacama area and its ayllus.
The sky matters as much as the telescope
A 200 mm telescope under Bortle 7 (average Latin American city) cannot reach the same depth as a 130 mm under Bortle 3. Artificial sky background brightness competes directly with faint objects: when the background exceeds the surface brightness of a nebula or galaxy, the object vanishes into it — regardless of aperture.
The Bortle Dark-Sky Scale, published by John E. Bortle in Sky & Telescope in February 2001, rates sky darkness on a 9-level scale:
| Bortle Class | NELM (mag.) | Visible reference |
|---|---|---|
| 1 — Exceptional darkness | 7.6–8.0 | Milky Way casts shadows; M33 naked-eye |
| 2 — Truly dark | 7.1–7.5 | Gegenschein visible; zodiacal light casts shadows |
| 3 — Rural sky | 6.6–7.0 | Complex Milky Way with detail; M33 with averted vision |
| 4 — Rural-suburban transition | 6.3–6.5 | Light domes on horizon; Milky Way less detailed |
| 5–6 — Suburban | 5.1–6.0 | Washed-out Milky Way; light pollution omnipresent |
| 7–9 — Urban | ≤5.0 | Milky Way nearly or completely invisible |
The surroundings of San Pedro de Atacama and its ayllus — including our observatory at Ayllu de Cucuter — reach Bortle 3: a genuinely rural sky where the Milky Way shows complex structure and a 200 mm Dobsonian can push to magnitude 14. Bortle 1 skies exist only in the high Andes, at temperatures between –10 and –30 °C — conditions impractical for the vast majority of observers.
A 130 mm under Bortle 3 outperforms a 300 mm under Bortle 7 for deep-sky observing. It is not the size of the mirror that defines the experience — it is the sky you point it at.
Use a real telescope under San Pedro de Atacama's dark skies
Now that you know how a telescope works, imagine using a professional one under Chile's darkest skies. Our stargazing tour in San Pedro de Atacama lets you observe lunar craters, Saturn's rings, and southern hemisphere nebulae with expert guidance.
