Roger C. Boysen

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.

A blind detection of a large, complex, Sunyaev–Zel’dovich structure★

We present an interesting Sunyaev–Zel’dovich (SZ) detection in the first of the Arcminute Microkelvin Imager (AMI) ‘blind’, degree-square fields to have been observed down to our target sensitivity of 100µJy beam^(-1). In follow-up deep pointed observations the SZ effect is detected with a maximum peak decrement greater than eight times the thermal noise. No corresponding emission is visible in the ROSAT all-sky X-ray survey and no cluster is evident in the Palomar all-sky optical survey. Compared with existing SZ images of distant clusters, the extent is large (≈10 arcmin) and complex; our analysis favours a model containing two clusters rather than a single cluster. Our Bayesian analysis is currently limited to modelling each cluster with an ellipsoidal or spherical β model, which does not do justice to this decrement. Fitting an ellipsoid to the deeper candidate we find the following. (a) Assuming that the Evrard et al. approximation to Press & Schechter correctly gives the number density of clusters as a function of mass and redshift, then, in the search area, the formal Bayesian probability ratio of the AMI detection of this cluster is 7.9 × 10^4:1; alternatively assuming Jenkins et al. as the true prior, the formal Bayesian probability ratio of detection is 2.1 × 10^5:1. (b) The cluster mass is M_(T,200) = 5.5_(-1.3)^(+1.2) x 10^(14)h^(-1)_(70) M_☉. (c) Abandoning a physical model with number density prior and instead simply modelling the SZ decrement using a phenomenological β model of temperature decrement as a function of angular distance, we find a central SZ temperature decrement of -295_(-15)^(+36) µK – this allows for cosmic microwave background primary anisotropies, receiver noise and radio sources. We are unsure if the cluster system we observe is a merging system or two separate clusters.

Design of the MROI delay line optical path compensator

The delay lines for the Magdalena Ridge Observatory Interferometer in New Mexico are required to provide up to 380m optical path delay with an OPD jitter of better than 15nm, in vacuum, using a single adjustable stroke. In order to meet these demanding requirements in a cost-effective manner a unique combination of techniques has been used in the design and construction of the delay line trolley which operates continuously within 190m of evacuated pipe. These features include contactless delivery of power and control signals, active control of the cats eye optics and the use of composite materials to achieve good thermal stability. A full-size prototype trolley has been built and fully tested and the first production trolley is under construction. We describe the systems key design features and review the construction and alignment of the delay line trolley. Results obtained with the trolley operating in an evacuated 20m-long test rig under the full range of conditions required for successful astronomical observations are presented. An OPD jitter of typically 10nm is achieved over the total tracking velocity range from 0 to 15mm/s.

Fringe envelope tracking at COAST

We report on a new fringe envelope tracking system installed at the Cambridge Optical Aperture Synthesis telescope (COAST). This currently uses the existing photon-counting avalanche photo diode (APD) detector system to allow real-time fringe tracking on up to 3 baselines simultaneously. This system has been recently tested on the sky and has proved to successfully track the fringe envelope on a 38m baseline. The algorithm based on an envelope method has also been implemented and tested at the Infrared-Optical Telescope Array (IOTA) interferometer.

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.

Software and control for the Magdalena Ridge Observatory interferometer delay lines

The delay lines for the Magdalena Ridge Observatory Interferometer (MROI) will provide remote control of optical delays of up to 380m with sub-wavelength precision in vacuum. The delay-line prototype is now fully functional, all features having been demonstrated in a 20m long evacuated test rig. We describe the architecture, design and performance of the delay line software: this features distributed real-time control and flexible remote logging of diagnostic data from the delay line hardware components at up to 5 kHz.

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.

The long-stroke MROI vacuum delay lines: from concept to production

We report on test results on the delay line system for the MRO Interferometer, currently under construction in Cambridge, UK. The delay lines are designed to provide 380 metres of vacuum path delay in a single stage, offering rapid star-to-star slews, high throughput and high transmitted wavefront quality. Details of the final design adopted for these delay lines are presented, together with lessons learnt from successful performance tests of the full-scale prototype trolley in a 20-metre long vacuum test rig. Delivery of the first production trolley is expected in New Mexico in early 2009.

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.

COAST: recent technology and developments

We present a summary of the activity of the Cambridge Optical Aperture Synthesis Telescope (COAST) team and review progress on the astronomical and technical projects we have been working on in the period 2002--2004. Our current focus has now moved from operating COAST as an astronomical instrument towards its use as a test-bed for strategic technical development for future facility arrays. We have continued to develop a collaboration with the Magdalena Ridge Observatory Interferometer, and we summarise the programmes we expect to be working on over the next few years for that ambitious project. In parallel, we are investigating a number of areas for the European Very Large Telescope Interferometer and these are outlined briefly.