19 July 2010 Metrology of complex astigmatic surfaces for astronomical optics
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This paper will focus on the metrology of multiple complex surfaces that are to be integrated into the KBand Multi- Object Spectrograph (KMOS). KMOS is a multi-field astronomical spectrograph designed for integration with the 8.2m diameter European Southern Observatory Very Large Telescope (VLT). There are 1080 separate optical surfaces in the design, many of them complex freeform surfaces. Optical surfaces were manufactured in aluminium by precision freeform diamond machining. This flexible technique allows the fabrication of extremely complex surfaces with an accuracy of better than 15 nm RMS over a 20 mm aperture, giving the designer great freedom in generating powerful and unorthodox designs. However, the complexity of these freeform surfaces poses a challenge to their accurate characterisation. This paper will discuss in detail the metrology of a specific freeform component in the instrument. The form of these complex astigmatic surfaces was measured using spherical wavefronts by adapting a tilted Twyman-Green Interferometer arrangement. There are eight separate designs for this type of component, each with a different orientation and magnitude of astigmatism. Careful mechanical fixturing is essential to align the astigmatic axis to the test set up. The impact of mechanical tolerances on measurement uncertainty will be discussed in detail.
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Stephen Rolt, Andrew K. Kirby, David J. Robertson, "Metrology of complex astigmatic surfaces for astronomical optics", Proc. SPIE 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 77390R (19 July 2010); doi: 10.1117/12.856428; https://doi.org/10.1117/12.856428


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