Webb Telescope

Table of Contents

  1. Telescopes
    1. Basic Idea
    2. Early Telescopes
    3. Modern Telescopes
      1. Radio Wave
      2. Microwave
      3. Infrared
      4. Optical (Visible Light)
      5. Ultraviolet
      6. X-ray
      7. Gamma Ray
  2. Webb Telescope
    1. Basics
    2. Infrared Radiation
    3. Why the Webb Telescope was designed for infrared
    4. The Challenge of Infrared Astronomy
    5. How Webb Works
    6. Orbit
    7. Webb’s 30-Day Journey
      1. Before Launch
      2. Unfurling on its Voyage
      3. In Orbit
    8. Links


Basic Idea

Early Telescopes


Modern Telescopes

Radio Wave

Very Large Array (VLA)

  • Located in New Mexico
  • Began operating in 1980
  • Consists of 27 parabolic dishes, each 82 feet in diameter.
  • The radio signals recorded by the component dishes are integrated by computer to give one consolidated image.


  • 2009-2013
  • Studied the cosmic microwave background (CMB).
  • Had same orbit around the Sun as Webb
Image Credit esa.int/Planck

 Herschel Space Observatory

  • Predecessor of the Webb
  • 2009-2013. Ran out of liquid Helium.
  • Mirror 11.5 feet diameter (Webb 21.3 feet)
  • Same orbit around the sun as Webb
  • Designed for wavelength of 60 to 500 microns (far-infrared)
Optical (Visible Light)

Hubble Space Telescope

  • 1990 – present
  • Orbits Earth
  • Mirror 7.8-foot
  • Observes ultraviolet, visible, and near-infrared
  • Maintained in space by astronauts.

Galaxy Evolution Explorer (GALEX)

  • 2003-2013
  • Orbited Earth

Chandra X-ray Observatory

  • 1999 – present
  • Elliptical orbit around Earth
Gamma Ray

Fermi Gamma-ray Space Telescope

  • 2008 – present
  • Orbits Earth

Webb Telescope

Image Credit Nasa


  • Took $10 billion and 30 years to design, build, test, and put in orbit.
  • Launched Dec 25, 2021 and took 30 days to reach its destination
  • Orbits the Sun, just beyond Earth’s orbit
  • Has a huge mirror: 21.3 feet in diameter (versus Hubble 7.8 and Hershel 11.5)
    • The larger the mirror the more photons it collects from an object.
  • Unlike Hubble, it’s too far to be maintained by astronauts.
  • Sees near-infrared radiation

Infrared Radiation

  • Infrared radiation can be detected as a sensation of warmth on the skin
  • Sub-regions:
    • Near-infrared: wavelengths from 0.78 to about 2.5 microns
    • Middle-infrared: wavelengths from 2.5 to about 50 microns
    • Far-infrared, wavelengths 50 to 1,000 microns
  • Most of the radiation emitted by a moderately heated surface is infrared
  • Humans at normal body temperature radiate chiefly at wavelengths around 10 microns, in the middle-infrared range.
  • Slightly more than half of the energy the Earth receives from the Sun is Infrared radiation.
  • Infrared devices: remote controls, fiber optic cables, night vision goggles, thermal imaging cameras, electric heaters, infrared lamps
  • Musician and astronomer William Herschel discovered infrared rays in 1800. He also discovered Uranus.

Why the Webb Telescope was designed for infrared

It would have been nicer to see ordinary photographs of galaxies

  • Primary Reason
    • Infrared telescopes can see further back in time.
      • Light from faraway galaxies is redshifted as it travels because of the expansion of space. The further the galaxy the greater the redshift. So a galaxy whose light we see as infrared is further away from a galaxy whose light we see as visible red.
      • The hope is that Webb observes the first galaxies formed after the Big Bang.
    • View Expansion of Space
    • In the diagram below JWST sees further back in time that HST and others.
  • Secondary Reasons
    • Unlike visible light, infrared passes easily through dust
    • It’s good at observing exoplanets.

The Challenge of Infrared Astronomy

  • The challenge facing any infrared telescope is that the instrument itself emits infrared radiation that can then become part of what the telescope observes.
  • Standard operation is to prevent the telescope from emitting infrared rays by making it very cold.
  • The problem is how.
  • The Herschel Space Observatory, Webb’s infrared predecessor, cooled itself using liquid helium, which ran out after three years.
  • Webb, on the other hand, uses a sunshield that, because of Webb’s orbit, always faces the Sun, Earth, and Moon. The sunshield consists of five layers of a material named Kapton.

How Webb Works

  • Infrared rays from the cosmos strike the golden Primary Mirror, which reflects them back to the Secondary Mirror, which in turn reflects them through the aperture in the middle of the primary mirror, entering one of Webb’s four detectors, labeled Scientific Instruments.
  • On the cold size of the Sunshade is the Spacecraft Bus, which houses six subsystems for
    • Electrical Power
    • Attitude Control
    • Communication
    • Command and Data Handling
    • Propulsion
    • Thermal Control
Image Credit Nasa


  • Webb’s orbit comprises two component orbits.
  • The first component orbit is that of a point in space called L2 that orbits the Sun, a million miles beyond Earth’s orbit so that Earth is always between the Sun and the point.
  • The second component orbit is that of Webb orbiting L2 so that its sunshade always faces the Sun, Earth and Moon. This “halo” orbit keeps Webb out of Earth’s shadow, thus maintaining a constant environment for Webb’s mirrors and detectors.
Image Credit webbtelescope.org

Video of Webb’s Orbit

Nasa: About Webb Orbit

Webb’s 30-Day Journey

Before Launch
Unfurling on its Voyage

Video of Unfurling

In Orbit