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“The Essential Guide to the James Webb Space Telescope”Dr Jenifer Millard, 18th May 2022


“And this is just an Engineering shot!” Dr Jenifer Millard exclaimed excitedly, presenting the first image of the star 2MASS J17554042+6551277 produced by the James Webb Space Telescope (JWST) having had each of its primary mirror segments adjusted to produce one unified image. Her enthusiasm was infectious – everyone who attended our first live/hybrid post-pandemic lecture at Coleshill Village Hall, whether in the Hall or at home, was absorbed by her fact-filled lecture about this amazing new telescope.

To illustrate what a technological advancement this instrument represents, Dr Millard made comparisons with what went before – the Spitzer and Hubble Space Telescopes. For example, JWST’s 6.5m primary mirror compared with Hubble’s 2.4m, JWST’s 74 cm secondary mirror is almost as big as Spitzer’s primary, Hubble is primarily an optical telescope whilst JWST will look at the near- and mid-infrared parts of the electromagnetic spectrum. Interestingly, Hubble and JWST have pretty similar resolutions (.05 and 0.1 arc secs, respectively) – this drops out from the physics, because JWST will be operating at much longer wavelengths – but both knock spots off Spitzer’s 2 arc sec resolution which was considered pretty revolutionary at the time.

The facts kept coming: the launch using the ultra-reliable Ariane 5 rocket was so perfect that it is estimated that there remains maybe 20 years’ worth of fuel on board; the actuators for the mirror elements can be moved slower than a blade of grass grows; the mirrors can be aligned to within one ten thousandth the width of human hair; the unfolding of the mirror and the 5-layer heat shield (24m x 14m) went flawlessly, though not without risk since there was no redundancy built in – if any element of the shield failed, the whole telescope failed!

Positioned at Lagrange Point 2 and by looking in the infrared, JWST will be able to deal with cosmological red-shift and observe stars and galaxies from much earlier in the history of the universe. It will be able to peer through cosmic dust (which optical telescopes cannot) to reveal up-till-now invisible parts of the universe, and it will be able to analyse the characteristics of exoplanets’ atmospheres (the TRAPPIST-1 seven planets system being a special target).

JWST carries four scientific instruments - the Mid-Infrared Instrument (MIRI), the Near-Infrared Camera (NIRCam), the Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec). More than one instrument will be usable at any one time and will make JWST particularly productive in its imaging and spectroscopic tasks.

Deciding who gets to use the telescope, and on what projects, required a highly selective process; carefully designed to reduce the effect of unconscious biases or prejudices, it forced decision-makers to focus on the scientific merit of a proposal rather than who submitted it. However, the scientific community must wait another couple of months before they can actually start to use the telescope, but Dr Millard was in no doubt that what we shall learn from it will touch every single element of our understanding of astronomy and cosmology.

Sandy Giles

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