Practical co-phasing metrology sensor for E-ELT telescope (Conference Presentation)

Break through large astronomical telescopes of the near future (ELT, TMT, MSE, LOT, etc.) will be comprised of extremely large optical elements, requiring segmented primary mirrors. Optical performance of the primary mirror requires an optical shape with an accuracy of a fraction of a wavelength over the whole radius. This can be practically achieved by composing the largest primary mirrors of scores to several hundreds of individually actuated hexagonal mirrors. Multi-layered control of these actuators is performed over sensors observing the position of non-surface parts of these mirror elements. Alignment calibration or co-phasing to the front-surface is required. This procedure needs to be performed at the initial assembly and updated after each (scheduled) daily mirror replacement and to compensate for residual drift. Capital investment costs of these telescopes create the necessity to minimize the duration of maintenance during the valuable night time hours. TNO has developed an affordable instrument methodology, capable of delivering coordinates of each mirror element with respect to its neighbors during day time. This instrument combines nanometer accuracy/precision with low latency and meets the strict requirements of co-phasing the large multi-segmented telescopes of the future. This article describes the requirements, design and specifications of this instrument. TNO benefits from a rich heritage of designing world class optical instruments for science and industry. This metrology system takes advantage of the development of fast, full-field, time-domain, white-light interferometry, which has been demonstrated previously for in-line quality inspection at harsh industrial machining workshops.