MICADO’s filter wheel

MICADO is planned as one of the first instruments to operate on ESO’s Extremely Large Telescope in Chile’s Atacama Desert, observing the Universe at infrared wavelengths. Its rotating wheel carries a different filter in each slot, with the filters letting light through at different colours or wavelengths. The practical point is wavelength selection: the filters determine which colours of light pass through, giving the wheel an optical role beyond simply holding components.

The Netherlands Research School of Astronomy, NOVA, built this Centre Wheel Mechanism as part of the MICADO consortium led by the Max Planck Institute for Extraterrestrial Physics. ESO puts its size at about 1.5 metres, compared with filter wheels typically a few tens of centimetres across in today’s telescopes. That comparison establishes the scale of the mechanism; understanding the engineering also requires looking at the conditions in which its optical elements must work.

The mechanism must operate at cryogenic temperatures to prevent its own thermal glow from contaminating the infrared observations, so cooling serves a specific optical purpose. Yet materials behave differently at very low temperatures, which ESO says complicates the accurate positioning of the optical elements housed in the wheel. Some elements, including special masks, need positioning accuracy of just a few microns; this is a requirement for those elements, not a stated tolerance for every filter.

The engineering takeaway is that suppressing thermal contamination and achieving accurate positioning are linked challenges, because the cold required for one complicates the other. The supplied account describes these requirements but provides neither an operating temperature nor positioning test results, so it does not establish demonstrated performance under those conditions.