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30 January 2003 Analysis of curvature sensing for VISTA
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VISTA is the Visible and Infrared Survey Telescope for Astronomy that has been designed by the UK ATC. The design incorporates two cameras covering the visible and near-IR wavelengths. The 4-m primary mirror has an active support system with 84 axial actuators that is used, in conjunction with the 5-axis support system for the 1.2-m secondary mirror, to maintain the image quality at seeing-limited conditions (~0.6 arcsec at Cerro Paranal, Chile). A system of curvature sensing is proposed to measure the low-order aberrations of the telescope and camera, which uses a pair of CCD arrays located at the edge of the camera focal-plane array. The analysis presented here uses simulated extra-focal images of point sources generated using the optical design program ZEMAX and demonstrates that this off-axis curvature-sensing technique will provide adequate measurements of low-order aberrations under the expected SNR, seeing conditions and field crowding. Even with the VISTA f-ratio of 3.26, an extra-focal distance of only 1 mm is shown to be sufficient to obtain rms wavefront errors accurate to a few tens of nanometres (under ideal and fully time-averaged seeing conditions). The demonstrated insensitivity to crowding means a field of 35 arcmins2, such as obtained with VISTA on a standard CCD, is sufficient to guarantee finding a suitably bright guide star.
© (2003) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Brett A. Patterson and Will J. Sutherland "Analysis of curvature sensing for VISTA", Proc. SPIE 4842, Specialized Optical Developments in Astronomy, (30 January 2003);


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