J. E. Baldwin

Diffraction-limited 800 nm imaging with the 2.56 m Nordic optical telescope.

A quantitative assessment is presented of diraction-limited stellar images with Strehl ratios of 0.25{0.30 obtained by selection of short-exposure CCD images of stars brighter than +6 m at 810 nm with the Nordic Optical Telescope.

The application of interferometry to optical astronomical imaging

In the first part of this review we survey the role optical/infrared interferometry now plays in ground–based astronomy. We discuss in turn the origins of astronomical interferometry, the motivation for its development, the techniques of its implementation, examples of its astronomical significance, and the limitations of the current generation of interferometric arrays. The second part focuses on the prospects for ground–based astronomical imaging interferometry over the near to mid–term (i.e. 10 years) at optical and near–infrared wavelengths. An assessment is made of the astronomical and technical factors which determine the optimal designs for imaging arrays. An analysis based on scientific capability, technical feasibility and cost argues for an array of large numbers of moderate–sized (2 m class) telescopes rather than one comprising a small number of much larger collectors.

The point spread function in Lucky Imaging and variations in seeing on short timescales

Aims. We investigate the properties of astronomical images made by combining the best images selected from a sequence of shortexposure frames (Lucky Imaging); we assess the match between modelling and observation and discover what variations in seeing occur on very short timescales. Methods. Numerical simulations of a random phase-changing screen passing across a telescope aperture with ideal optics are used to determine the expected point spread function and isoplanatic properties for a range of seeing conditions for comparison with observations. Results. All the model images comprise a diffraction-limited core with Strehl ratios from 0.05–0.5 and an underlying broad disk. The isoplanatic patch sizes are large and coherence times long. The observations are a close match to the models in most respects. Large variations in seeing occur on temporal scales as short as 0.2 s and spatial scales as small as 1 m.

Design and performance of COAST

The Cambridge Optical Aperture Synthesis Telescope, COAST, is a four-telescope array for high resolution imaging using measurements of complex visibilities and closure phases. This paper describes what its component parts are and why.

Noise-free detectors in the visible and infrared: implications for the design of next-generation AO systems and large telescopes

This paper is intended to discuss the impact of noiseless CCD detectors in three significant areas of the development of large telescopes and the instruments that go on them. These are (1) CCDs that have all the characteristics that we are used to seeing in CCDs can now be made with negligible readout noise even at higher pixel rates (> 10 MHz) and will allow rather different approaches to the design of instrument is generally, (2) the technique of achieving diffraction limited imaging in ground-based telescopes known as Lucky Astronomy in which images are taken at high speed has been demonstrated to work under a variety of different conditions and (3) some suggestions as to how these methods may be applied directly to much larger diameter telescopes in order to achieve high resolution imaging and spectroscopy without the expense of laser guide stars or multi-conjugate adaptive optics.

Current status of COAST

This paper reviews the current performance of the Cambridge Optical Aperture Synthesis Telescope as an imaging array. Tests of the hardware and methods of measuring fringe visibility and closure phase are described in the context of prospects for a Large Optical Array.

Diffraction-limited I band imaging with faint reference stars

The use of faint reference stars for the selection of good short exposure images has recently been demonstrated as a technique which can provide essentially diffraction-limited I band imaging from well-figured ground-based telescopes as large as 2.5 m diameter. The faint limiting magnitude and enhanced isoplanatic patch size for the selected exposures technique means that 20% of the night sky is within range of a suitable reference star for I-band imaging. Typically the 1%-10% of exposures with the highest Strehl ratios are selected. When these exposures are shifted and added together, field stars in the resulting images have Strehl ratios as high as 0.26 and FWHM as small as 90 milliarcseconds. Within the selected exposures the isoplanatic patch is found to be up to 50 arcseconds in diameter at 810 nm wavelength. Images within globular clusters and of multiple stars from the Nordic Optical Telescope using reference stars as faint as I~16 are presented. The technique relies on a new generation of CCDs which provide sub-electron readout noise at very fast readout rates. The performance of the selection technique for various astronomical programs is discussed in comparison with natural guide star Adaptive Optics (AO).

COAST: its current status, operation, and results

The Cambridge Optical Aperture Synthesis Telescope, COAST, now has the capacity to measure visibility amplitudes and closure phase for stellar sources. This paper summarizes the current status of the instrument and how the data is analyzed.

COAST: the current status

We present a summary of the status of the Cambridge Optical Aperture Synthesis Telescope (COAST). Since our last report we have concentrated on improving both the efficiency of use of the array and its astrophysical capabilities. In particular we have achieved useful improvements in throughput, detector sensitivity and the efficiency of securing measurements of visibility amplitudes and closure phases. With five telescopes fully operational, COAST is now being used routinely for parallel programs of astrophysics and as a technical test-bed for its proposed successor, the Large Optical Array--LOA.

An Interferometric Survey of Mira Variables and M Supergiants

We present preliminary results from an imaging survey of bright northern Mira variables and Supergiants. Diffraction-limited images of Mira exhibit the same elongation along PA 120° as observed in previous observations. Of the other Mira variables observed, R CasIs unambiguously detected as non-spherically symmetric. Our observations of α-Her are consistent with there being a ‘hot-spot’ on its surface, similar to those we have previously detected on Betelgeuse.