Segmented Mirrors

The James Webb Space Telescope (JWST) project required a method for verifying that the segments of the primary mirror, which are folded for launch, will align to within sub-wavelength tolerance upon deployment. The initial misalignment between segments is much greater than the measurement range of a conventional laser interferometer.

The multiple-wavelength PhaseCam MW Dynamic Laser Interferometer was developed for this application. The much longer “synthetic” wavelength generated by the instrument is capable of measuring the initial misalignment. As the segments are brought closer to alignment the wavelength is stepped down for increasing resolution.

The European Extremely Large Telescope (E-ELT), being developed by the European Southern Observatory (ESO), will also feature a segmented primary mirror, an amazing 39.3 meters in diameter. To creat it, more than 800 1.5 meter hexagonal segments will need to be ground, polished and assembled.

Zeeko Ltd. is a UK-based manufacturer of seven-axis robotic polishers that have been used in the production of some of the E-ELT prototype segments. Zeeko selected 4D Technology's laser interferometers for its polishing metrology. Their compact size and insensitivity to vibration enables 4D interferometers to be permanently installed above the polishing station, allowing full-aperture metrology of the surface.

Mounting the interferometer in situ means optics do not need to be moved between polishing and measurement stations, greatly reducing cycle time and protecting the optics from handling damage.

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• PhaseCam MW Dynamic Laser Interferometer

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