Peter A. Schuller

Ten-micron instrument MIDI: getting ready for observations on the VLTI

The mid-infrared interferometric instrument MIDI is currently undergoing testing in preparation for commissioning on the Very Large Telescope Interferometer VLTI at the end of this year 2002. It will perform interferometric observations over the 8 μm - 13 μm wavelength range, with a spatial resolution of 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 4 mag or 1 Jy for self-fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument and the performance during laboratory tests, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations.

The limb-darkened Arcturus: imaging with the IOTA/IONIC interferometer

Aims. We undertook an H band interferometric examination of Arcturus, a star frequently used as a spatial and spectral calibrator. Methods. Using the IOTA 3 telescope interferometer, we performed spectro-interferometric observations (R ≈ 35) of Arcturus. Atmospheric models and prescriptions were fitted to the data to derive the brightness distribution of the photosphere. Image reconstruction was performed using two software algorithms: Wisard and Mira. Results. An achromatic power law proved to be a good model of the brightness distribution, with a limb darkening compatible with the one derived from atmospheric model simulations using our marcs model. A Rosseland diameter of 21.05 ± 0.21 was derived, corresponding to an effective temperature of Teff = 4295 ± 26 K. No companion was detected from the closure phases, with an upper limit on the brightness ratio of 8 × 10^−4 at 1 AU. The dynamic range at such distance from the photosphere was established as 1.5 × 10^−4 (1σrms). An upper limit of 1.7×10^−3 was also derived for the level of brightness asymmetries present in the photosphere.

Study of molecular layers in the atmosphere of the supergiant star mu Cep by interferometry in the K band

Infrared interferometry of supergiant and Mira stars has recently been reinterpreted as revealing the presence of deep molecular layers. Empirical models for a photosphere surrounded by a simple molecular layer or envelope have led to a consistent interpretation of previously inconsistent data. The stellar photospheres are found to be smaller than previously understood, and the molecular layer is much higher and denser than predicted by hydrostatic equilibrium. However, the analysis was based on spatial observations with medium-band optical filters, which mixed the visibilities of different spatial structures. This paper reports spatial interferometry with narrow spectral bands, isolating near-continuum and strong molecular features, obtained for the supergiant µ Cep. The measurements confirm strong variation of apparent diameter across the K-band. A layer model shows that a stellar photosphere of angular diameter 14.11±0.60 mas is surrounded by a molecular layer of diameter 18.56±0.26 mas, with an optical thickness varying from nearly zero at 2.15 µm to >1 at 2.39 µm. Although µ Cep and α Ori have a similar spectral type, interferometry shows that they differ in their radiative properties. Comparison with previous broad-band measurements shows the importance of narrow spectral bands. The molecular layer or envelope appears to be a common feature of cool supergiants.

IOTA: Recent Technology and Science ⁄

The Sydney University Stellar Interferometer (SUSI) is a long-baseline optical interferometer operating at an observatory near Narrabri in Australia. SUSI features a 640 m long North-South array with 11 fixed siderostat stations. New science from the Blue (400-500 nm) and from the recently commissioned Red (500-950 nm) fringe detectors will be presented. Recent technological developments, mainly associated with the new Red detection system, encompassing wavefront correction, fringe encoding, wavelength switching and data analysis strategies, are described.Closure-phase science and technology are dominant features of the recent activity at IOTA. Our science projects include imaging several spectroscopic binary stars, imaging YSOs including Herbig AeBe stars, detecting asymmetries in a large sample of Mira stars, and measuring water shells around Miras. Many technology projects were pursued in order to make these science observations possible. These include installation of a third-generation integrated-optics 3-beam combiner (IONIC), completion of the real-time control system software, installation of fringe-packet tracking software, use of narrow sub-H band filters, validation of the phase-closure operation, development of CPLD control of the science camera (PICNIC) and star-tracker camera (LLiST), installation of a new star-tracker camera, expansion of the observing facility, and installation of new semi-automated optical alignment tools.

Last technology and results from the IOTA interferometer

The infrared optical telescope array (IOTA), one of the most productive interferometers in term of science and new technologies was decommissioned in summer 2006. We discuss the testing of a low-resolution spectrograph coupled with the IOTA-3T integrated-optics beam combiner and some of the scientific results obtained from this instrument.

IOTA: Recent technology and science

Closure-phase science and technology are dominant features of the recent activity at IOTA. Our science projects include imaging several spectroscopic binary stars, imaging YSOs including Herbig AeBe stars, detecting asymmetries in a large sample of Mira stars, and measuring water shells around Miras. Many technology projects were pursued in order to make these science observations possible. These include installation of a third-generation integrated-optics 3-beam combiner (IONIC), completion of the real-time control system software, installation of fringe-packet tracking software, use of narrow sub-H band filters, validation of the phase-closure operation, development of CPLD control of the science camera (PICNIC) and star-tracker camera (LLiST), installation of a new star-tracker camera, expansion of the observing facility, and installation of new semi-automated optical alignment tools.

LLiST - a new star tracker camera for tip-tilt correction at IOTA ∗

The tip-tilt correction system at the Infrared Optical Telescope Array (IOTA) has been upgraded with a new star tracker camera. The camera features a backside-illuminated CCD chip offering doubled overall quantum efficiency and a four times higher system gain compared to the previous system. Tests carried out to characterize the new system showed a higher system gain with a lower read-out noise electron level. Shorter read-out cycle times now allow to compensate tip-tilt fluctuations so that their error imposed on visibility measurements becomes comparable to, and even smaller than, that of higher-order aberrations.

Stabilising a nulling interferometer using optical path difference dithering: an update

Nulling interferometry has been suggested as the underlying principle for an instrument which could provide direct detection and spectroscopy of Earth-like exo-planets, including searches for potential bio-signatures. This paper documents the potential of optical path difference (OPD) stabilisation with dithering methods for improving the mean nulling ratio and its stability. The basic dithering algorithm, its refined versions and parameter tuning, are reviewed. This paper takes up the recently presented results1 and provides an update on OPD-stabilisation at significantly higher levels of nulling performance.