The James Webb Space Telescope (Fig. 8.20), initially known as the NGST (New Generation Space Telescope) is the planned successor to the Hubble Space Telescope, which has been in Earth orbit since 25 April 1990. JWST is being developed as a collaboration between NASA, ESA, and the Canadian space agency.

Unlike the HST, which was optimized for the visible and ultraviolet, the JWST is optimized for the visible, near and medium infrared regions. It consists of a telescope, 6.5 m in diameter, with a segmented mirror, passively cooled, and protected from sunlight by a thermal sunshade. Its launch is expected to be in 2013. JWST will be fitted with 4 instruments:

• MIRI (Mid InfraRed Instrument): This instrument, the product of a European consortium and JPL, is an imaging spectrometer that covers the range 5-27 |m. It consists of two modules, one for wide-band imagery, and the other for mediumresolution spectroscopy. The imaging instrument is also fitted with a corona-graph, which will allow direct imaging of giant planets and sub-stellar objects. The operating temperature of the instrument is 7K. This temperature cannot be reached by passive cooling of the telescope. The instrument is therefore fitted with additional cooling systems such as a Joule-Thomson expansion system.

• NIRcam (Near InfraRed Camera). This is a wide-field imager with a high angular resolution that exploits the size of the JWST in the wavelength range 0.6-5 |m (visible and near infrared). The instrument may also be used to check the quality of the wavefront for the whole telescope.

Fig. 8.20 Artist's impression of the JWST (image credit: courtesy NASA, ESA)

• NIRspec (Near InfraRed Spectrograph). This is a multi-object spectrograph (100 objects simultaneously) covering a field 9 arcminutes square on the sky. NIRspec will obtain medium-resolution spectra in the 1-5 |m region, and low-resolution spectra in the 0.6-5 | m region.

• FGS (Fine Guidance Sensor). This is a wide-spectral-band camera (1-5 |m) with a wide field giving accurate guiding over 95 per cent of the sky, using a reference star.

The JWST's scientific programme is clearly geared to the study of 'origins' -of the Universe and of planetary systems. It is a multipurpose observatory which will cater, simultaneously, for extremely varied programmes in cosmology, galactic physics, the physics of the interstellar medium, and in planetology. The MRI instrument, in particular, which has coronagraphic capability, is the favourite for the study of giant planets, where it should be able to obtain some images and spectra in the middle infrared (5-30 | m). In addition, it should be able to image protoplane-tary disks with great accuracy, thanks to the telescope's excellent angular resolution (which is diffraction-limited).

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