How Telescopes Work

How Telescopes Work

From mirror designs to orbital mechanics, discover the engineering behind observing the invisible universe.

Reflecting vs Refracting Telescopes

Reflecting Telescope

Reflecting telescope mirror system diagram

Light Path

StarlightPrimary MirrorSecondary MirrorFocal Point / Detector

Uses curved mirrors to collect and focus light. The primary mirror reflects light to a secondary mirror, directing it to instruments. All major space telescopes (Hubble, JWST, Chandra) are reflectors.

  • No chromatic aberration
  • Can be made very large
  • Lighter than equivalent lenses

Refracting Telescope

StarlightLensFocusEyepiece

Light Path

StarlightObjective LensFocal PointEyepiece

Uses glass lenses to bend (refract) light to a focal point. Effective but limited in size — large lenses are heavy, costly, and suffer from chromatic aberration.

  • Suffers from chromatic aberration
  • Heavy at large sizes
  • Not used for modern space telescopes

Segmented Mirror Design

JWST golden hexagonal segmented mirror close-up

JWST’s primary mirror comprises 18 hexagonal segments made of beryllium coated in gold, working together as a single 6.5-meter mirror.

Why Hexagonal Segments?

18 segments = 6.5m mirror
1
Lightweight
Far lighter than a single solid 6.5m mirror, reducing launch costs
2
Foldable
Segments fold to fit inside the Ariane 5 rocket fairing, then unfold in space
3
Active Optics
Each segment has actuators for nanometer-precision alignment adjustments

JWST Sunshield & Detector Path

5-Layer Sunshield

The sunshield is tennis-court-sized and blocks sunlight, keeping instruments at -233°C (-387°F).

Layer 1 (Sun-facing): ~85°C
Layer 2: ~55°C
Layer 3: ~15°C
Layer 4: ~-45°C
Layer 5 (Cold side): ~-233°C

Each layer is made of Kapton®, as thin as a human hair, with vacuum gaps as insulation.

Light → Discovery Path

Infrared Photons
Travel millions of light-years from distant objects
Primary Mirror
18 gold-coated beryllium segments collect light
Secondary Mirror
Redirects light back through center of primary
Instruments
NIRCam, NIRSpec, MIRI, FGS/NIRISS analyze photons
Detectors
Convert photons to electrical signals at -233°C

Orbit Locations: LEO vs L2

SunEarthHubble (LEO ~540km)Orbits every 95 minL2 Point1.5 million km from EarthJWST (L2)Unblocked cosmic view~1.5 million km

Low Earth Orbit (LEO)

Hubble orbits at ~540 km altitude. Close enough for servicing missions, but Earth blocks the view for half of each orbit, and radiation belts can interfere with instruments.

L2 Lagrange Point

JWST orbits the Sun at L2, 1.5 million km from Earth. Gravitational forces balance here, providing an unobstructed view and a stable, cold thermal environment — ideal for infrared observations.