Integral Field Unit for the James Webb Space Telescope.

The James Webb is a very large space telescope – with a 6.5m diameter primary mirror and a huge sun-shield.  It will be launched in 2018 into a solar orbit 1.5 million km from Earth (at the second Lagrange point which keeps it aligned between the Sun and Earth).  The NASA programme is supported by European industry and the European Space Agency, who provide spectrometers for the telescope focal plane.  Surrey Satellite Technology Ltd have built and delivered a small (1kgm) but significant unit for NIRSpec, the Near-IR Spectrometer which covers wavelengths 0.7 to 5 microns.  The Integral Field Unit, designed by Dan Lobb, receives a 1.2mm square of the telescope image (a 3 arc-second square of sky), typically containing a galaxy.  Without the IFU, the spectra of the galaxy would be analysed by scanning the field across the spectrometer entrance slit.

The IFU speeds-up analysis by converting the square image into a single narrow slit image; this allows the spectra of all points in the square field area to be analysed simultaneously.  The IFU splits the focused star image-area into 30 parallel strips, at a 30-facet “slicer”.  The light from each slice then follows a path through two relay mirrors (“pupil” and “field” mirrors); the 1.2mm square input image is output towards the spectrometer as two 18mm x 0.04mm slit-areas with a 10mm separation.

Challenges for design included fitting the IFU into a very small volume (since it was a late addition to NIRSpec) and a requirement to work over a range of temperatures from normal lab levels for assembly down to around -230⁰C for testing and operation.  The optical design of course needs to be very complex, with a total of 95 mirror facets, most including complex shapes.  Manufacture of the optics was made feasible by very precisely-controlled diamond machining of the mirrors – performed by the Centre for Advanced Instrumentation in Sedgefield – in aluminium alloy.  Construction is mainly in the same alloy to avoid thermal expansion differences.

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