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Inside the JWST: How the Telescope Actually Works - Leejohnston - 11-16-2025
✨ Inside the James Webb Space Telescope:
How It Actually Works
A deep, beginner-friendly guide to the engineering behind humanity’s most
advanced space observatory.
? 1. Why JWST Sees the “Invisible” Universe
JWST observes mainly in:
• Near-Infrared
• Mid-Infrared
These wavelengths reveal:
• Early galaxies
• Dust-shrouded star nurseries
• Exoplanet atmospheres
• Cool brown dwarfs
• Molecular chemistry in nebulae
? 2. The Giant Mirror — 18 Hexagonal Segments
Specifications:
• Diameter: 6.5 m
• Material: Beryllium
• Coated in: Gold (~100 nm thick)
Why hexagons?
They tile perfectly, fold compactly, and can be adjusted individually.
Each segment has actuators for:
• tip
• tilt
• piston
• curvature
• alignment
?️ 3. The Sunshield — A Five-Layer Space Refrigerator
To see faint IR light, JWST must stay colder than:
50 K (−223 °C)
Its sunshield:
• Blocks sunlight
• Blocks Earthlight
• Blocks Moonlight
• Lowers temperature by ~300°C
• Works like 5 giant reflective umbrellas
Size: a tennis court.
?️ 4. L2: JWST’s Home in Space
JWST orbits the Sun–Earth L2 point:
• Constant alignment
• Minimal fuel usage
• Stable thermal environment
• No Earth eclipses
• Perfect for deep-field imaging
? 5. JWST’s Instruments
• NIRCam — early galaxies, deep fields
• NIRSpec — spectroscopy of thousands of objects
• MIRI — mid-infrared imaging & spectroscopy
• FGS/NIRISS — exoplanet transit science, precision pointing
? 6. What JWST Has Already Changed
• Galaxies earlier than expected
• New exoplanet atmospheric chemistry
• Carbon-rich worlds
• Star formation deeper in dust
• Gravitational lensing maps with high resolution
• Planet-forming disks in unprecedented detail
It is already rewriting textbooks.
Written by Leejohnston