Active Optics II. Results of an Experiment with a Thin 1 m Test Mirror

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Abstract
Part I of this paper [1] presented the theory, and practical basis, of a complete system for active control of optical quality, with particular application to large telescopes. Reference was made to an experimental verification using a thin model mirror of 1 m diameter. Part II now gives a complete account of these experiments which consisted essentially of two parts: the verification of the theoretical modal calibrations and their orthogonality in practice, and the application of the calibrations to achieve the active modal correction of the mirror. Both aspects of the experiment were successful, with a precision higher than expected. The residual errors left after final correction are, by definition, of a higher order than the five corrected low spatial frequency terms and are small. However, one residual term is dominant (fifth-order astigmatism) but this was omitted in the correction process as being probably too high a mode to appear in practice. A calibration for this term will be performed which will enable it also to be removed. It is not expected that this term will be detectable in the full-scale New Technology Telescope.

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