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Max Planck Institute for Astronomy - Annual Report 2005

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(TFT) of the Canadian Space Agency is much more sensitive<br />

than NirCaM and thus will be able to faster turn<br />

the attention to very young galaxies with their highly<br />

redshifted Lyman-alpha-lines.<br />

Common to the three focal-plane instruments <strong>for</strong><br />

JWST presented above is that they have to be operated<br />

in a cryo-vacuum. For NirCaM and NirSpeC a temperature<br />

of – 240 °C suffices. Miri has to be cooled to below<br />

– 260 °C so that its own thermal emission will not outshine<br />

the cosmic infrared radiation. Miri’s infrared detectors<br />

have to be operated at – 268 °C, only 5 °C above absolute<br />

zero, in order to keep the »dark current« of the camera<br />

sufficiently low. Another common property of the instruments<br />

is that all of them have large »optical exchange<br />

additional<br />

pin<br />

electrical subassembly<br />

including the motor<br />

ratchet<br />

assembly<br />

support<br />

structure<br />

ring with 6 flexural<br />

rods<br />

index bearings<br />

ratchet<br />

subassembly<br />

filter wheel subassembly<br />

(with optical elements)<br />

IV.1 Instruments <strong>for</strong> the James Webb Space Telescope 91<br />

wheels« with numerous gratings, filters, beam splitters,<br />

mirrors, prisms and coronagraphic masks mounted on<br />

their peripheries (Fig. IV.1.5). By exchanging these<br />

elements in the light path (i.e. by turning the wheels)<br />

it is possible to select the different operation modes of<br />

the instruments (spectroscopy, imaging, coronagraphy,<br />

calibration) as well as the particular wavelength ranges.<br />

Although every space technician is anxious to avoid<br />

mechanisms like these wheels (»… may fail …«), powerful<br />

scientific instruments without moving parts are not<br />

feasible. Because of the successful developments <strong>for</strong><br />

the European space telescopes iSo and HerSCHel, our<br />

institute was well prepared <strong>for</strong> these high-risk challenges<br />

with Miri and NirSpeC.<br />

Fig. IV.1.5: Filter wheel <strong>for</strong> the Mid-Infrared Instrument<br />

(Miri). The 18 positions are occupied by filters <strong>for</strong> the<br />

wavelength range from 5 to 28 µm and a prism <strong>for</strong> sensitive<br />

low-resolution spectroscopy. Several coronagraphic<br />

masks allow searches <strong>for</strong> extrasolar planets near a very<br />

bright star, which is masked out. The wheel is moved by a<br />

central torque motor and positioned with a detent latching<br />

into a ball-bearing outer race. (CEA, TTL, ADR, ZeiSS,<br />

MPIA)<br />

bearing / axis<br />

subassembly with<br />

the KIP<br />

magnet ring<br />

support structure<br />

including a parallel<br />

shift assembly

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