We present a summary of activity at the Cambridge Optical Aperture Synthesis Telescope (COAST) group
during the period 2004-2006. Our main program has focused on technical design and prototyping for future
facility arrays such as the VLTI and Magdalena Ridge Observatory Interferometer, but with a small parallel
effort of focused astronomical observations with COAST, in particular multi-wavelength studies of supergiants.
We report on progress on these and other technical areas over the past 2 years.
Accurate knowledge of the spatial and temporal seeing has become increasingly important as AO systems move from being specialised instruments to standard equipment at large ground-based telescopes. While monitors that measure the spatial seeing scale are now commonplace, devices capable of measuring temporal seeing parameters are much rarer since the sampling requirements are severe. Nevertheless, such information is vital if the bandwidth and control requirements for active and adaptive systems at state-of-the-art telescopes and optical/IR interferometers are to be correctly specified. In this paper we describe a cheap, yet robust, Differential Image Motion Monitor Which Is Transportable (DIMMWIT) that can make both spatial and temporal seeing measurements. It samples starlight at rates up to 500Hz but contains no mechanical parts and uses only technology available to amateur astronomers. We review the design and performance of the device and present examples of results from routine use at the Cambridge Optical Aperture Synthesis Telescope (COAST) site in the UK. An identical system is also being tested at the Magdalena Ridge Optical Interferometer (MROI) site in New Mexico.
The DIMMWIT (Differential Image Motion Monitor, Which Is Transportable) is a portable DIMM that can measure the Fried
parameter r0 and the average wind speed of the turbulent layers. Analysing DIMM images to calculate r0 is a standard procedure, but wind speeds have rarely been calculated from differential image motion before. Here, we describe how wind speeds can be derived from either differential image motion power spectra or differential image velocities. The DIMMWIT wind speeds are then compared with a wind speed derived from the coherence times, t0, of interferometric fringes recorded simultaneously at COAST (Cambridge Optical Aperture Synthesis Telescope). Although t0, and hence the wind speed, is routinely measured by the interferometer at the COAST site, the Fried parameter had not been studied. The results of seeing campaigns at COAST and MROI (Magdalena Ridge Observatory Interferometer) are presented, along with a comparison of DIMMWIT r0 measurements with the FWHM of long exposure images recorded at the same time.
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
The astronomical site parameters for the Magdalena Ridge Observatory (MRO) are being studied from numerous aspects including meteorological, environmental, seismic and sky quality (e.g. "seeing", cloud cover). Results to date indicate that MRO is an excellent site for astronomical observing. Seeing measurements of less that 1 arc second in the optical are routinely obtained. Seismic conditions on the mountain ridge are below levels that will cause any major problems for construction and operation of an optical interferometer. Nighttime "allsky" camera imagery indicates a large percentage of clear nights.
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 r0 and the speckle lifetime tau0, 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.
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 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.