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We report the direct detection of cyclic diameter variations in the Mira variable χ Cygni. Interferometric observations made between 1997 July and 1998 September, using the Cambridge Optical Aperture Synthesis Telescope (COAST) indicate periodic changes in the apparent angular diameter with amplitude 45 per-cent of the smallest value.
The measurements were made in a 50 nm bandpass centred on 905 nm, which is only moderately contaminated by molecular absorption features. To assess the effects of atmospheric stratification on the apparent diameter measured in this band, we have also measured near-infrared diameters for a sample of five Miras, in both the J-band (1.3 μm) and Wing's (1971) 1.04 μm band, which is expected to isolate essentially pure continuum emission. We present J-band visibility curves which indicate that the intensity profiles of the stars in the sample differ greatly from each other.
In this paper I provide a brief introduction to astronomical
interferometry at optical and infrared wavelengths. Two key concepts,
central to understanding the basis and practice of interferometry are
introduced: image formation with conventional telescopes, in
particular the Fourier decomposition of images, and the nature and
utility of measurements of the coherence function or mutual intensity.
Thereafter I focus on optical/infrared interferometry, outlining how
measurements of the coherence function are made at these wavelengths,
how they can be used to interpret a source's structure, and what the
principles of interferometric imaging tell us about the limitations
expected for the current generation of arrays such as the VLTI.
In this second lecture I review some of the practical considerations
associated with developing and implementing ground-based
interferometry at optical and near-infrared wavelengths. Familiarity
with these logistical concerns is relevant not only for understanding
how arrays such as the VLTI actually operate but also for identifying
the most important questions that need to addressed when planning a
campaign of interferometric observations. I outline some of the
strategies that have been adopted to address these issues and point to
some areas ripe for future development.
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