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
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.
We present details of the design and construction of a new pinhole Spatial Filtering system at the Cambridge Optical Aperture Synthesis Telescope and show the results from the first astronomical observations with the system. We discuss the advantages of using such a system to reduce uncertainties in visibility observations with an optical/IR interferometer. We compare simultaneous observations with and without the system and show the potential to improve both the quality and quantity of scientific output of an interferometer. We also show that filtered observations are much more robust to changes in atmospheric seeing than unfiltered observations, improving the accuracy of calibrated visibility measurements while reducing the need for repeated calibration. Given the simplicity and high throughput of pinhole spatial filters we suggest that they could be a valuable addition to many current and future arrays.
The first-generation COAST array is now primarily operated as a tool
for astrophysics, with any development work aimed at improving
observing efficiency and at prototyping hardware for future arrays. In this paper we summarize the full range of astrophysical results
obtained with COAST in the previous two years. Results of a
program to investigate hotspots on red supergiant stars are
presented in detail.
The Cambridge DIMMWIT was developed for the site of COAST (the Cambridge Optical Aperture Synthesis Telescope), a prototype optical interferometer. Unlike other differential image motion monitors, this design is portable in order to carry out seeing campaigns at the site of any optical inteferometer. Of particular interest is the site of a second-generation interferometer proposed by the MRO (Magdalena Ridge Observatory) consortium. The DIMMWIT design
has two objectives: to measure the Fried parameter <i>r</i><sub>0</sub> and the speckle lifetime tau<sub>0</sub>, and to be easily transportable. Here, we outline the theory of differential image motion, the design of the DIMMWIT, describe how turbulence parameters can be measured with COAST, and compare measurements of the seeing conditions made simultaneously by the monitor and COAST.
It is foreseen that the Gran Telescopio Canarias will include an Adaptive Optics (AO) system to provide diffraction-limited images in the near-IR. The preliminary scientific requirements are presented here and are used to determine the basic system specifications. The sky coverage is determined for the cases of natural and laser guide stars. The results of conjugating the deformable mirror to the entrance pupil and to the mean turbulence height are compared. The Gran Telescopio Canarias will employ a segmented primary mirror. This will introduce discontinuous wavefront errors due to possible effects of these errors on the AO performance. The resulting requirements for these errors are compared with those determined by the error budget for long-exposure image quality.