When we last reported the status of the U.C. Berkeley Infrared Spatial
Interferometer (ISI) in 2002, we presented simulations, based upon our two-telescope experience, for the expected performance of a three-telescope array that would be capable of measuring three simultaneous visibilities and one closure phase at mid-infrared wavelengths.
The ISI is now fully operational as an imaging array and is routinely making fringe visibility and closure phase measurements of late-type stars in the 9 to 12 micron wavelength region. We describe here the technology which is currently in use, along with actual measurements and preliminary 11.15 micron (one-dimensional) image reconstructions.
The U.C. Berkeley Infrared Spatial Interferometer is a two telescope
stellar interferometer operating in the 9-12 micron atmospheric
window, utilizing heterodyne detection with CO2 laser local oscillators. Science with the ISI has been focused on the measurements of the spatial distribution of dust and molecules around mass-losing late type stars, and more recently precision measurements of stellar diameters in the mid-infrared avoiding molecular lines. During the past few years, a National Science Foundation sponsored program of expansion from two to three telescopes has been underway. This expansion will allow the ISI to make visibility observations on three simultaneous baselines and a measure a closure phase. The third telescope was completed last year and shipped to Mt. Wilson, and more recently a Central Control Facility and Master Laser Oscillator Facility were also completed and recently shipped to Mt. Wilson. In this paper we report progress on this program and highlight some of the most recent astrophysical
results.
The size and variability of the photospheres of several late-type stars has been probed using 11 micron heterodyne interferometry. High resolution observations performed during the years 1999 - 2001 yielded diameter measurements accurate to about 1% for α Ori and o Cet, an supergiant and a mira variable. Narrow bandwidths (0.17 cm-1) and high resolution spectra were used to avoid molecular lines. Observations were made at several different wavelengths, sometimes purposely overlapping an observed spectral feature. In all cases, the 11 micron sizes are larger than previously measured visible and near-infrared diameters. The discrepancies will be discussed. In addition, a variation of the diameter of Mira with phase has been observed.
Previous measurements of atmospheric density fluctuations have shown that a substantial fraction of seeing fluctuations occur within 100 feet of the ground, and that the power spectrum of path length fluctuations through the atmosphere has a somewhat smaller slope than that predicted by the Kolmogorov-Taylor approximation. To provide some possibility of appreciable path-length corrections, the ISI has assembled a system capable of measuring temperature changes at fifteen foot intervals of heights up to 70 feet from the ground. Analysis of temperature measurements made under a variety of conditions confirms previous results concerning the decrease in the magnitude of the fluctuations with altitude near the ground: the rms magnitude of the temperature fluctuations at an elevation of 70 feet is, on average, 52% of the mean rms value at 9 feet. However, these new measurements made at point locations show a power spectrum close to the Kolmogorov-Taylor prediction at frequenices up to 1.0 Hz, for average wind speeds above 2 m/s. In addition, correlation analysis between sensors located at the same elevation but separated by a given distance shows up to 50% correlation out to separations as large as 24 meters with wind speeds of a few meters per second, and indicate that Taylor's approximation applies over spatial distances in the range of 24 - 85 meters, or on timescales as large as ten seconds, and perhaps as large as 14 or 15 seconds. This makes path length corrections possible by temperature measurements at nearby locations.
The U.C. Berkeley Infrared Spatial Interferometer (ISI), previously
described by Hale et al. as a two-telescope stellar interferometer operating in the mid-infrared regime of 9-12 μm, is now testing a recently constructed third telescope and centralized laser local oscillator and beam combining facilities for operation of a
three-telescope system with phase closure. This new system will allow the ISI Array to make measurements of stellar photospheric and dust asymmetries in the mid-IR, measurements which have previously been lacking. The new facilities and instrumentation required for three-way heterodyne beam combination and fringe detection, together with techniques for measuring the closure phase, are described. Both hardware and software methods of beam combination are used for producing three simultaneous fringe visibilities and a closure phase measurement. Additionally, the phase closure signal-to-noise of pairwise and of all-in-one beam combination are contrasted.
A novel, fast-switching filterbank system has been incorporated into the Infrared Spatial Interferometer to allow high spectral resolution observations of mid-infrared spectral lines, while precisely calibrating atmospheric fluctuations. This system allows the complex visibility in a spectral line to be measured relative to the continuum with high precision, about 1% in visibility amplitude and a few degrees in visibility phase. The high spectral resolving power (R approximately 100000) and spatial resolution (1/10 arcsecond) allow the formation radii of various polyatomic molecules (e.g., ammonia and silane) to be directly measured. Initial results for carbon star IRC + 10216 and red supergiant VY CMa are presented.
The Infrared Spatial Interferometer (ISI) is an interferometer installed on Mt. Wilson and operating in the 10 μm wavelength region, using heterodyne detection and two movable 1.65 m telescopes. Its general technology and characteristics, recent changes, and observational results are broadly discussed. Some compensation for atmospheric path length fluctuations is demonstrated. Stellar observations show, among other characteristics, that many stars emit gas and dust episodically with times of 10-100 years between events, and that stellar diameters measured in the mid-infrared region are about 10 percent larger than those measured with interferometry using visible light.
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