The next generation of optical interferometer arrays will require a large number of unit telescopes in the same manner as the VLA if meaningful scientific objectives are to be achieved. Studies based on the five element COAST array show that something like ten to fifteen telescopes are necessary. For such a project to be viable the unit telescopes must be designed from the outset for this task. The basic criteria are as follows: The wavefront quality and stability should be excellent, high optical throughput, autonomous automatic operation, couple efficiently into the beam transport and combination system, plus maintain acceptable unit cost. To achieve these goals a number of novel designs were considered and are described in this paper. Two of the most suitable designs and which had the least technological risk were studied in more detail by Telescope Technology Ltd. and are described in a separate paper.
We present a summary of the status of the Cambridge Optical Aperture
Synthesis Telescope, and review developments at the array through the
period 2000-2002. Summaries of the astronomical and technical
programmes completed, together with an outline of those that are
currently in progress are presented. Since our last report two years
ago in 2000, there have been significant changes in the context for
astronomical interferometry in the UK. We review these developments,
and describe our plans for the near and intermediate term at COAST,
and with colleagues in Europe at the VLTI and in the USA at the
Magdalena Ridge Observatory in New Mexico.
The resolution of a conventional telescope is determined by the spatial extent of the collecting surface, usually the primary mirror. Astronomical interferometers achieve increased fine detail by using unit telescopes spaced over large distances to increase the spatial extent. The required wavefront quality places very tight tolerances on the unit telescopes and they should be designed with the prime goal of meeting the wavefront specification. The unit telescope must be optimized for the role of a beam compressor rather than attempting to modify a conventional design.
Two alternative designs that minimize the number of reflections in the telescope will be considered, a crucial feature in obtaining the lowest possible wavefront error and maximizing throughput. The first, a siderostat has fixed imaging optics and a large steerable flat mirror to enable sky tracking. The second, an "Alt-Alt" system consists of two intersecting altitude axes in a "gyroscopic type" structure. A small flat lies at the intersection of the altitude axes to direct the starlight at a constant height and direction out of the telescope. The benefits and limitations of each are shown along with the key design issues that determine the most appropriate unit telescope for implementation in an interferometric telescope.
We present the latest astronomical results from the Cambridge Optical Aperture Synthesis Telescope (COAST). COAST is a first-generation stellar interferometer, which uses an array of small (40 cm) separated telescopes to perform high-resolution imaging at visible and near-infrared wavelengths. The new science results from COAST exploit two recently-added capabilities of the COAST array, namely the ability to observe in any over the infrared J, H and K bands as well as at visible wavelengths, plus operation with five telescopes. We present contemporaneous observations of the red supergiant Betelgeuse at three wavelengths in the red and near-infrared. These data show that the apparent symmetry of the stellar disk is a strong function of wavelength, but that the bright spots seen in visible light are consistent with a convective origin. Data obtained using all five array elements on the symbiotic star CH Cygni reveal an elliptical distortion of the disk of the red giant, possibly related to mass transfer of a compact companion.
At the Cambridge Optical Aperture Synthesis Telescope (COAST), first-generation photon counting avalanche photodiodes (APDs) have been used as the pupil-plane fringe detectors in the optical regime. These are being replaced with EG&G's super-low k (`SliK') APDs, which have an exceptionally low dark count (fewer than 100 counts per second) and high detection efficiency (up to 70% at 700 nm). The new detectors have increased the limiting magnitude of the telescope, enabling the observation of targets previously too faint to be seen. We shall discuss the operation of these devices at COAST and present new interferometric observations of stellar objects at visible magnitudes of eight and fainter.
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.
Visibility measurements obtained with optical astronomical interferometers are corrupted by random wavefront distortions of atmospheric origin. In this paper we discus how spatial filtering using pinholes can lead to increased measured visibility, improved signal-to-noise ratio and reduced dependence on seeing fluctuations. The potential for calibrating visibility measurements without resorting to a separate calibrator target is also discussed. Results of preliminary pinhole experiments carried out with the Cambridge Optical Aperture Synthesis Telescope are presented.
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.
One of the critical components of a separated element interferometer is the beam combiner. The initial alignment of the separate optical elements that make up this device and the maintenance of that alignment is usually problematic. Fiber optic devices provide an answer to the alignment difficulties but in single mode form have a restricted bandwidth. This paper discusses the design of a number of devices to overcome these short comings. These beam combiners can be small in size, their dimensions largely governed by the beam diameter. Large diameter beams are only necessary to reduce diffraction effects on the journey from the telescope to the beam combiner. On arrival the beam diameter can then be reduced to suit the beam combiner. Small, stable and low weight beam combiners are an advantage on the ground but even more so in space applications. Designs for combining the beams for large numbers of telescope are described.
There are difficult problems involved in building a near-infrared interferometer which uses more than two elements simultaneously. These problems have been overcome at the Cambridge Optical Aperture Synthesis Telescope(COAST). This has allowed us to make the first closure phase measurements on an astronomical source in the infrared.
We describe a scheme for fast, low noise readout of an infrared focal-plane array detector, capable of adequately sampling pupil plane fringes on three simultaneous baselines, as well as a procedure for aligning a many-component beam-combiner in the infrared. Finally, the performance of the working COAST infrared system is discussed.
A low-resolution CCD spectrometer has been installed at COAST to provide multi-wavelength fringe measurements across the band 650-950 nm. The measurements are based on the analysis of time-series of channeled spectra. Laboratory tests and stellar observations are presented. The advantages and limitations of the system are discussed.
The Cambridge optical aperture synthesis telescope (COAST) is a four element interferometer which measures visibility amplitudes and closure-phases. It produced its first images in 1995 and is now in a complete form, very similar to the original conception. In this paper we discuss the design and current status of the interferometer.
The Cambridge optical aperture synthesis telescope (COAST) has now been developed to the point where stellar images with a resolution of 20 mas can be produced in a routine manner. Based upon our experiences in the design and commissioning of COAST this paper discusses the possible design of a next generation interferometer.
In September 1995 the Cambridge optical aperture synthesis telescope (COAST) became the first optical interferometer to produce an image of a stellar source from phase-closure and visibility amplitude measurements. These observations demonstrated for the first time the feasibility of operating long-baseline optical/near-infrared interferometers for high dynamic range high-resolution imaging. Here we present these and subsequent observations made with COAST and describe the methods used to analyze such data.
In this paper we describe an instrument to obtain diffraction limited images on a large telescope using aperture masking. Detailed images of giant stars have been routinely obtained on the 4.2-m William Herschel Telescope (WHT) in La Palma with this technique. When using this method one of the major causes of data loss is the difficulty in detecting the secondary mirror support spiders crossing, and thus obscuring part of the aperture mask. To overcome this problem a system for continuously monitoring the front face of the aperture mask has been devised. Exposure times have been significantly reduced by placing the image in a corner of the CCD chip, the size of which is determined by the seeing, and by compressing the image to a single row of pixels. In order to perform this technique the fringes must be accurately aligned with the columns of the CCD chip. To eliminate the extra losses in throughput due to an optical image rotator the CCD camera, rather than the image, is rotated using a motorized mount. The aperture mask can be rotated independently to various position angles with respect to the object so that full UV coverage can be obtained.
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.
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.
The COAST group in the University of Cambridge have a lot of operational experience of using fast guiding systems to control the pointing of up to four separate telescopes simultaneously as well as a lot of operational experience of using array CCD systems and avalanche photodiode photon counting systems for signal detection. This paper looks at the advantages and disadvantages of these systems for fast guiding and wavefront sensing applications.