December 29, 2023


Tipping the scales at 6,200 kilograms, the James Webb Space Telescope (JWST) features a 6.5-metre primary mirror—compared with 2.4 metres on Hubble—able to collect images nine times faster than Hubble.

JWST’s sunshield is the size of a tennis court—22 metres long and 11 metres wide—to protected it from our star’s light and heat. This sunshield is composed of five very thin layers of a highly reflective material called Kapton, each coated with aluminium. This marvel of engineering keeps the telescope’s mirror in the shade at a temperature of around –233°C (50 K). Some of JWST’s instruments, like MIRI, even require active cooling down to –266°C (7 K).

JWST has also accomplished a number of technological feats, principally:

    • The first mirror to be deployed in space and the largest space telescope ever
    • The first multi-object spectrograph (MOS) in space (NIRSpec)
    • The first phase-mask coronagraph in space (MIRI)


    JWST is composed of four main parts:

      • the platform to maintain orbit and attitude, to manage power, temperature and data, and to support communications inside JWST and with Earth,
      • the sunshield to protect JWST against sunlight and heat,
      • the telescope to receive and distribute light to the instruments,
      • the "payload", called ISIM (for Integrated Science Instrument Module), where the four instruments are integrated behind the primary mirror.



      Webb satellite

      The platform is the central element of JWST, comprising the main structure, the solar panels to generate power, the thrusters and fuel tanks to control orbit and pointing, the antennas to receive telecommands and send back technical and scientific data, electronics and flight software to execute all planned operations. Moreover, the platform includes a cryo-cooler to operate MIRI at 7 K.

      JWST pointing is very challenging, but no direct requirement has been formulated in this respect. Long exposure times would require about 0.01µrad accuracy in attitude.

      Temperature management is largely achieved through the orientation of the sunshield. It is a large surface with an elongated hexagon shape, slightly smaller than a tennis court. It faces the Sun, Earth and Moon, i.e. the three light sources that can heat JWST and interfere with infrared light collection.

      The telescope comprises two mirrors: the primary unfoldable mirror collects light from the sky with its 6.5-m diameter. It is launched folded and is divided into 18 hexagonal elements that have to fit with great accuracy. The secondary mirror concentrates the light coming from the primary mirror and distributes it to the ISIM’s instruments.

      The ISIM is composed of four main instruments:

      • NIRCam (Near-InfraRed Camera): a wide field camera operating in the 0.6 - 5 µm near-infrared wavelength range
      • NIRSpec (Near-InfraRed Sprectrometer): a multi-object spectrometer operating in the 1 - 5 µm near-infrared wavelength range
      • MIRI (Mid InfraRed Instrument): a camera/spectrometer assembly operating in the 5 - 28 µm near-infrared wavelength range
      • NIRISS (Near-InfraRed Imager and Slitless Spectrograph): a secondary instrument associated with the fine guidance system (FGS) but independent from it. It is an imaging spectrometer operating in the 0.6-5 µm wavelength range


      In addition, the FGS includes a near-IR tunable imaging filter covering 0.6-5 µm wavelengths.