Andrew Peter Doel

NAOMI adaptive optics system for the 4.2-m William Herschel telescope

NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) is a recently completed and commissioned astronomical facility on the 4.2m William Herschel Telescope. The system is designed to work initially with Natural Guide Stars and also to be upgradeable for use with a single laser guide star. It has been designed to work with both near infrared and optical instrumentation (both imagers and spectrographs). The system uses a linearised segmented adaptive mirror and dual-CCD Shack-Hartmann wavefront sensor together with a multiple-DSP real-time processing system. Control system parameters can be updated on-the-fly by monitoring processes and the system can self-optimize its base optical figure to compensate for the optical characteristics of attached scientific instrumentation. The scientific motivation, consequent specification and implementation of NAOMI are described, together with example performance data and information on future upgrades and instrumentation.

The MARTINI adaptive optics instrument

Abstract The Multi-Aperture Real Time Image Normalisation Instrument (MARTINI) is an adaptive optics system designed to operate at one of the Nasmyth focal stations of the 4.2 m William Herschel Telescope. Correction of atmospherically distorted images is achieved by the action of a six element segmented mirror in a real-time servo loop controlled by the signal from a CCD based Shack–Hartmann wavefront sensor. A unique aspect of the MARTINI instrument is that, from its conception, it was designed to be capable of image sharpening at visible wavelengths. An outline of the mechanical and electronic arrangement of MARTINI is given, together with a description of its modes of operation, facilities and performance. Also described are some of the first scientific results obtained with the instrument.

STELLAR IMAGE STABILIZATION USING PIEZO-DRIVEN ACTIVE MIRRORS

As applied to the 4.2-m William Herschel Telescope, the multiaperture real-time image-normalization system presented implies a wavefront whose size requires a mask of six optimally-scaled subapertures. These subaperture images are separated and examined on a single image photon detector which yields x, y, and t coordinates for each recorded photon. The motions of these images feed back to six independent piezoactuated active mirrors which act to null the image motions at a CCD focus. Data are presented from two image normalization runs, with and without active mirrors, which illustrate the size and variation behavior of the coherent seeing length, characteristic seeing times, and power spectra.

Comparison of Shack-Hartmann and curvature sensing for large telescopes

This paper reports the results of a simulation comparing the performance of the following wavefront sensors when used in a closed loop astronomical adaptive optics (AO) system: a 48- element Shack-Hartmann, a 20-element Shack-Hartmann, and a 20-element curvature sensor. The method chosen for wavefront reconstruction in each case is based on a modal interaction matrix technique with zernike polynomials chosen as the basis modes. No attempt is made to include a real mirror model in the simulation, thus the evaluation of the sensing technique is decoupled from specific mirror technologies. Two different seeing conditions are simulated with various guide star magnitudes. The wavefront distortions are sensed in the visible and the point spread function of the corrected wavefront is recorded in the near infra-red. The effects of photon statistics and various levels of sensor pixel readout noise are also included in the simulations.

Toward a large lightweight mirror for AO: development of a 1m Ni coated CFRP mirror

We present our recent developments towards the construction of a large, thin, single-piece mirror for adaptive optics (AO). Our current research program aims to have completed fabrication and testing of a 1m diameter, nickel coated carbon-fibre reinforced cyanate ester resin mirror by the last quarter of 2009. This composite mirror material is being developed to provide a lightweight and robust alternative to thin glass shell mirrors, with the challenge of future large deformable mirrors such as the 2.5m M4 on the E-ELT in mind. A detailed analysis of the material properties of test mirror samples is being performed at the University of Birmingham (UK), the first results of which are discussed and presented here. We discuss the project progress achieved so far, including fabrication of the 1m flat moulds for the replication process, manufacturing and testing methods for 20 cm diameter sample mirrors and system simulations.

OSCA - an optimized stellar coronagraph for adaptive optics: description and first light

We describe a coronagraph facility built for use with the 4.2 metre William Herschel Telescope (WHT) and its adaptive optics system (NAOMI). The use of the NAOMI adaptive optics system gives an improved image resolution of ~0.15 arcsec at a wavelength of 2.2 microns. This enables our Optimised Stellar Coronagraph for Adaptive optics (OSCA) to null stellar light with smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA is a fully deployable instrument and when in use leaves the focus of the NAOMI beam unchanged. This enables OSCA to be used in conjunction with a number of instruments already commissioned at the WHT. The main imaging camera to be used with OSCA will be INGRID; a 1024×1024 HgCdTe cooled SWIR detector at the NAOMI focus. OSCA can also be used in conjunction with an integral field spectrograph for imaging at visible wavelengths. OSCA provides a selection of 10 different occulting mask sizes from 0.25 - 2.0 arcsec and some with a novel gaussian profile. There is also a choice of 2 different Lyot stops (pupil plane masks). A dichroic placed before the AO system can give us improved nulling when occulting masks larger than the seeing disk are used. We also present results from initial testing and commissioning at the William Herschel Telescope.

TEIFU: a thousand element integral field unit for the WHT fed by the ELECTRA AO system

A description is given of the fiber based Thousand Element Integral Field Unit (TEIFU) that is being built by Durham University for use with the WYFFOS fiber spectrograph and the ELECTRA Adaptive Optics system at the William Herschel Telescope. With TEIFU there will be at least one thousand spatial elements with selectable sampling scales of 0.125, 0.25 and 0.5 arcsec corresponding to field areas of 14, 56 and 222 square arcsec, the field can be divided into 2 to facilitate background subtraction. The first two scales of 0.125 and 0.25 arcsec are designed to take advantage of the improved image quality provided by the ELECTRA AO system. The 0.5 arcsec sampling is designed for use without ELECTRA and samples the uncorrected image quality provided by the WHT. In this mode there is a larger field that is not constrained by the guide star requirement of the ELECTRA AO system. The operating wavelength of the system will be approximately 500 - 1000 nm.

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 metre William Herschel Telescope (WHT) and its Nasmyth Adaptive Optics system for Multi-purpose Instrumentation (NAOMI). The use of the NAOMI system gives an improved image resolution of � 0.15 arcsecs at a wavelength of 2.2� m. 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 arcsecs in diameter, including two with full greyscale 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.

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, reflective 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 briefly 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.

Adaptive secondary mirror demonstrator: design and simulation

Atmospheric turbulence distorts the wavefront of the incom- ing light from an astronomical object and so limits the ability of a tele- scope to form perfect images. Adaptive optics is a combination of tech- nologies that enable the correction of the wavefront distortion in real time. Conventional adaptive optics operate like auxiliary instruments and use additional relay optics, which reduce total throughput and introduce extra IR emissivity and polarization. Adaptive secondary mirrors avoid additional optical surfaces by providing the optical correction at an exist- ing telescope surface (the secondary mirror). Previous studies have demonstrated the optical efficacy and mechanical feasibility of perform- ing the adaptive correction in this way. A technique demonstrator is be- ing developed to explore features and capabilities applicable to a large adaptive secondary mirror and to explore manufacturing, assembly/ disassembly, calibration, and measurement techniques. The paper de- scribes the design of the demonstrator and its predicted performance.