An adaptive optics system for astronomical image sharpening

Images of stars in the focal plane of large astronomical telescopes are many times larger than the images expected from diffraction theory. Much of this blurring of the image is due to variations in the optical properties of the atmosphere above the observatory which for small aperture telescopes causes motion of the image, and for large aperture telescopes causes the image to break up into a number of smaller sub-images. This thesis describes a prototype adaptive system which is designed to sharpen astronomical images in real-time. The sharpening is achieved by removing the atmospherically induced motions of the image with servo-looped plane mirrors driven by piezo-electric actuators. The results of real-time sharpening obtained with the adaptive system at the William Herschel Telescope (WET) are presented along with the results of an investigation into the characteristics of the atmospheric limitations at the La Palma observatory site

An Adaptive Secondary Mirror demonstrator: Construction and Preliminary Evaluation

Adaptive optics combines technologies that enable the correction of the wavefront distortion caused by the earth‘s atmospheric turbulence in real time. Adaptive secondary mirror (ASM) systems have been proposed and are now being developed. ASMs have advantages over conventional AO systems in terms of throughput, polarisation and IR emissivity. Previously, we reported the design of an ASM demonstrator along with its predicted performance. This paper reports the construction techniques and the results from the preliminary static and dynamic testing of such a demonstrator. In particular assembly methods that preserve the optical quality of the mirror are presented along with experimentally measured mirror influence functions and closed loop tip/tilt performance

Results from the adaptive optics coronagraph at the William Herschel Telescope

Described here is the design and commissioning of a coronagraph facility for the 4.2-m William Herschel Telescope (WHT) and its Nasmyth Adaptive Optics for Multi-purpose Instrumentation (NAOMI). The use of the NAOMI system gives an improved image resolution of 0.15 arcsec at a wavelength of 2.2 μm. This enables the Optimised Stellar Coronagraph for Adaptive optics (OSCA) to suppress stellar light using smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA provides a selection of 10 different occulting masks with sizes of 0.25–2.0 arcsec in diameter, including two with full grey-scale Gaussian profiles. There is also a choice of different sized and shaped Lyot stops (pupil plane masks). Computer simulations of the different coronagraphic options with the NAOMI segmented mirror have relevance for the next generation of highly segmented extremely large telescopes

Dynamic measurement of displacement with a phase-shifting technique

A simple spatial phase-shifting setup that uses a tiled mirror and a cat’s-eye retroreflector is presented. It can be used in displacement measurement when high speed and accuracy are required. An experiment is carried out to demonstrate the feasibility of the method. A root mean square (rms) accuracy of better than 5 deg is obtained in ordinary circumstance. The system errors are also studied briefly

Why adaptive secondaries?

Adaptive optics (AO) combines technologies that enable the correction in real time of the wavefront distortion caused by the terrestrial atmospheric turbulence. An adaptive secondary mirror (ASM), unlike conventional adaptive optics systems, does not add any polarization, reÑective losses, and emissivity. Following successful implementation of tip/tilt secondary mirrors, most recent large telescope projects have considered the possibility of incorporating ASMs. This paper brieÑy reviews the development of ASMs and examines the issues which have arisen and also presents the predicted performance of an ASM system. It is concluded that adaptive secondary approach is an equally satisfactory or preferred solution to conventional AO systems