August 23, 2016


JWST (James Webb Space Telescope) is composed of 4 main parts:

  • the platform to maintain orbit and attitude, to manage power, temperature, data and to ensure communication inside JWST and outside with Earth,
  • the sunshield to protect JWST against Sun light and Sun heat,
  • the telescope to receive and distribute the light to the different instruments,
  • the "payload" called ISIM (for Integrated Science Instrument Module) where the 4 instruments are settled.


The JWST satellite

Its total weight is close to 6200 kg.

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 6 K.
The JWST pointing is very challenging, but no direct requirement has been emitted on it. Long exposure time would require about 0.01µrad accuracy in attitude.

An important part of the temperature management is done through the orientation of the sunshield. It is a large surface of elongated hexagon shape, slightly less than a tennis court. It will face the Sun, Earth and Moon, i.e. the 3 light sources that can heat up JWST and disturb the infra-red light collection of the telescope.

The telescope comprises 2 mirrors: the primary unfoldable mirror collects light from the sky with its 6.5 m large diameter. It is launched folded and then is divided in 18 hexagonal elements that have to fit with a very high accuracy. The secondary mirror concentrates the light coming from the primary mirror and distributes it toward the ISIM.

Cryostat in rocketThe ISIM has 3 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.

In addition, the Fine Guidance System (FGS) will include a near-IR tunable filter imaging capability covering 0.6 - 5 µm wavelengths.