John V. Major

Liquid-crystal prisms for tip-tilt adaptive optics

Results from an electrically addressed liquid-crystal cell producing continuous phase profiles are presented. The adaptive deflection of a beam of light for use in a tip-tilt adaptive optics system is demonstrated. We compare the optical performance of liquid-crystal prisms with experimental data on atmospheric seeing at the William Herschel Telescope.

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.

The application of Edser-Butler fringes to the measurement of the spatial optical structure of liquid crystal prisms and the determination of dispersion characteristics and thickness

An application of liquid crystals to astronomical adaptive optics which would produce sharpened images from optical telescopes is proposed. To measure the appropriate optical properties a method is described for determining the dispersion and thickness of a thin film based on a spectroscopic technique using Edser-Butler fringes. The application to birefringent media, in particular liquid crystals, and a method for assessing the fine structure is discussed. Results of measurements of the dispersion of liquid crystal BDH-E44 are given.

A High Resolution, Ground Based Observation of a Virgo Galaxy

We argue that a very simple inflationary model with Ωbaryon = 1 would be allowed, if Hubble’s constant lay below its presently accepted range. As part of a programme to check the value of Hubble’s constant we present new, high resolution observations of the Virgo galaxy, IC3583, using the technique of image sharpening from the William Herschel Telescope. Although our observed image of this galaxy has 0.″65 seeing the galaxy is not resolved into stars. We suggest that this preliminary result may imply a lower limit to the Virgo distance of 14Mpc and suggest that future observations will allow further improvements on this limit.

Astronomical adaptive optics system for use on a 4-m-class telescope at optical wavelengths

An astronomical AO system for use on a 4 m class telescope at visible wavelengths is described. The design of this instrument is based on the University of Durham semi common-user partial-AO system MARTINI. A brief technical resume and recent astronomical results are provided. The MARTINI system was designed on a partial, but non-modal, philosophy which is able to deliver modest image improvements normally associated with low order corrections at small D/ro. The new system will remove the limited aperture coverage of the MARTINI system and extend its operating philosophy to produce a corrected optical transfer function optimized to scientific goals. The requirements and resulting system design are described. The system is also designed to be accommodated within the general scheme of the UK national AO program which was initiated in 1993. A brief discussion of the design issues involved is provided.

A fast method to measure the voltage characteristics of liquid crystals using Edser-Butler fringes

In a previous paper (1991) the authors described how Edser-Butler fringes could be used to assess the spatial optical structure of liquid-crystal cells and to determine dispersion characteristics and thickness. They explain how the same fringes may be used to easily assess how the extraordinary index changes with applied voltage and how this is affected by wavelength. The method is extremely quick, using only simple laboratory equipment. Results are given for liquid-crystal BDH-E44.

MARTINI: sensing and control system design

The Multiple Aperture Real Time Image Normalization Instrument (MARTINI) is an astronomical adaptive optics system for visible imaging and spectroscopic feedthrough at the 4.2-m William Herschel Telescope on La Palma. It consists of a six-subaperture, tip-tilt-piston, segmented mirror device and uses 4r(0) aperture-matching to provide optimum slope removal in zones large enough for operation in the visible and with reference objects fainter than V = 13 exp m. This limit is achieved by optimizing the use of reference light, by analyzing the information from a photon counting wavefront sensor using a non-flaming (i.e., irregular sampling) infinite impulse response filter for estimation and prediction of the wavefront slopes. The value of this approach is discussed along with its extension to higher-order correction schemes. Experimental evidence supporting the theoretical basis of the MARTINI system is also presented. The astronomical potential of such an approach, and the drawbacks, are outlined.

Optical design, simulation, and testing of an addressable 64 x 64 liquid crystal phase plate

The correction of phase aberrations in optical systems using an adaptive liquid crystal phase screen having 64 X 64 pixels is considered. These adaptive components have advantages over deformable mirror systems in that liquid crystal technology is relatively inexpensive and large pixelation densities can be achieved. In particular we address the application of these devices to astronomical adaptive optics. A simulator of the phase correction plate at work using time changing distorted wave fronts is described. Preliminary measurements of the optical flatness and response times of the prototype device are discussed.

Morphology & Kinematics of the ‘Born—Again’ Planetary Abell 78

Abell 78 is one of a group of planetaries having an old, H-rich nebula surrounding a hot star which has more recently ejected highly-processed, H-deficient material. The A78 central star has O VI emission lines and a 3700 km s-1 hot wind, and is surrounded by knots of very dusty, H-deficient material. These objects are thought to have suffered a late helium shell flash which resulted in the central star (then a white dwarf) returning to the AGB and ejecting highly-processed material.