The project "Novel Astronomical Instrumentation based on photonic light Reformating" is a DFG-funded collaboration to exploit the recognized potential of photonics solutions for a radically new approach to astronomical instrumentation for optical/infrared high precision spectroscopy and high angular resolution imaging. We present a project overview and initial development results from our Adaptive Optics-photonic test bed, Ultrafast Laser Inscribed waveguides for interferometric beam combination and 3D printing structures for astronomical instrumentation. The project is expected to lead to important technological breakthroughs facilitating uniquely functionality and technical solutions for the next generation of instrumentation.
We present the results of the technology developments which lead to the manufacturing of the first integrated optics 2-telescope ABCD unit and 4-telescope DBC component for L-band, mid infrared stellar interferometry. All the samples were manufactured with ultrafast laser inscription in Gallium Lanthanum Sulfide and were characterized in near field with monochromatic light at the wavelength of 3.39 μm. The choice of the laser writing parameters and the control of long range stresses arising from the manufacturing process are discussed taking in consideration the measured interferometric calibration data.
Hi-5 is a high-contrast (or high dynamic range) infrared imager project for the VLTI. Its main goal is to characterize young extra-solar planetary systems and exozodiacal dust around southern main-sequence stars. In this paper, we present an update of the project and key technology pathways to improve the contrast achieved by the VLTI. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 μm) and advanced fringe tracking strategies. We also address the strong exoplanet science case (young exoplanets, planet formation, and exozodiacal disks) offered by this wavelength regime as well as other possible science cases such as stellar physics (fundamental parameters and multiplicity) and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed.
The astronomical J-band (1.25 micrometres) is a relatively untapped wave-band in long-baseline infrared interferometry. It allows access to the photosphere in giant and super-giant stars relatively free from opacities of molecular bands. The J-band can potentially be used for imaging spots in the 1350 nm ionised iron line on slowly rotating magnetically-active stars through spectro-interferometry. In addition, the access to the 1080 nanometres He I line may probe out flows and funnel-flows in T-Tauri stars and allow the study of the star-disk interaction.<p> </p> We present the progress in the development of a six-inputs, J-band interferometric beam combiner based on the discrete beam combiner (DBC) concept. DBCs are periodic arrays of evanescent coupled waveguides which can be used to retrieve simultaneously the complex visibility of every baseline from a multi-aperture interferometer. Existing, planned or future interferometric facilities combine or will combine six or more telescopes at the time, thus increasing the snapshot uv coverage from the interferometric measurements. A better uv coverage will consequently enhance the accuracy of the image reconstruction. DBCs are part of the wider project Integrated astrophotonics that aims to validates photonic technologies for utilisation in astronomy.<p> </p> Before manufacturing the component we performed extensive numerical simulations with a coupled modes model of the DBC to identify the best input configuration and array length. The 41 waveguides were arranged on a zig-zag array that allows a simple optical setup for dispersing the light at the output of the waveguides.<p> </p> The component we are currently developing is manufactured in borosilicate glass using the technique of multi-pass ultrafast laser inscription (ULI), using a mode-locked Yb:KYW laser at the wavelength of 1030 nm, pulse duration of 300 fs and repetition rate of 1 MHz. After annealing, the written components showed a propagation loss less than 0.3 dB/cm and a negligible birefringence at a wavelength of 1310 nm, which makes the components suitable for un-polarized light operation. A single mode fiber-to-component insertion loss of 0.9 dB was measured. Work is currently in progress to characterize the components in spectro-interferometric mode with white light covering the J-band spectrum.
We investigate the use of photonic lanterns in a fibre beam-combiner. One of the problems of fibre combiners is that the single-mode fibres coupling is affected by the atmospheric seeing. This effect could be potentially mitigated using a "photonic lantern". A photonic lantern converts the multi-mode input at the focal plane of a telescope to N single mode outputs. In presence of atmospheric seeing the light of the telescope will couple with one or more of the single-mode outputs unlike the current interferometers where coupling can be sporadic in bad seeing conditions. In a stellar interferometer each telescope focal plane could be coupled with a photonic lantern. After beam combination each single-mode outputs of the lantern will interfere with all the others. N single mode outputs from each lantern will make N non independent closure phases that can be averaged. Using this method the noise on the closure phase on independent baseline triangles could in principle be reduced by the square root of N.
We demonstrate a fast two-color widefield fluorescence microendoscopy system capable of simultaneously detecting several disease targets in intact human ex vivo lung tissue. We characterize the system for light throughput from the excitation light emitting diodes, fluorescence collection efficiency, and chromatic focal shifts. We demonstrate the effectiveness of the instrument by imaging bacteria (Pseudomonas aeruginosa) in ex vivo human lung tissue. We describe a mechanism of bacterial detection through the fiber bundle that uses blinking effects of bacteria as they move in front of the fiber core providing detection of objects smaller than the fiber core and cladding (∼3 μm). This effectively increases the measured spatial resolution of 4 μm. We show simultaneous imaging of neutrophils, monocytes, and fungus (Aspergillus fumigatus) in ex vivo human lung tissue. The instrument has 10 nM and 50 nM sensitivity for fluorescein and Cy5 solutions, respectively. Lung tissue autofluorescence remains visible at up to 200 fps camera acquisition rate. The optical system lends itself to clinical translation due to high-fluorescence sensitivity, simplicity, and the ability to multiplex several pathological molecular imaging targets simultaneously.
Cophasing six telescopes from the CHARA array, the CHARA-Michigan Phasetracker (CHAMP) and Michigan
Infrared Combiner (MIRC) are pushing the frontiers of infrared long-baseline interferometric imaging in key
scientific areas such as star- and planet-formation. Here we review our concepts and recent improvements on
the CHAMP and MIRC control interfaces, which establish the communication to the real-time data recording
& fringe tracking code, provide essential performance diagnostics, and assist the observer in the alignment and
flux optimization procedure. For fringe detection and tracking with MIRC, we have developed a novel matrix
approach, which provides predictions for the fringe positions based on cross-fringe information.
Michigan InfraRed Combiner (MIRC) is a near-infrared image-plane combiner at the CHARA array which
consists of six 1-m diameter telescopes with the longest baseline of 330m. MIRC was upgraded from a 4-beam
to a full 6-beam combiner in July 2011, which now records interferometry data of 15 baselines and 20 triangles
simultaneously. The improved snapshot UV coverage has greatly boosted the ability for imaging complicated
targets such as the asymmetry of circumstellar disks, interacting binaries and the surfaces of spotted stars. In
addition, the Photometric Channels subsystem, which directly measures the real time flux of individual beams,
has been upgraded to increase the light throughput to improve the visibility and closure phase calibration. The
system sensitivity has been improved as well to allow fainter objects such as Young Stellar Objects (YSOs) to be observable with MIRC for the first time. Our presentation will conclude with first preliminary results of imaging
two Be binaries observed by the upgraded MIRC.
The CHARA Array possesses the longest baselines in the world for infrared and visible interferometry, while the Michigan Infrared Combiner (MIRC) is the most advanced beam combiner for imaging. CHARA+MIRC has allowed imaging the surfaces of rapid rotators, interacting binary stars, and magnetically-active stars all for the first time. In this presentation, I will give an overview of the discoveries made by MIRC over the past five years and discuss technical and scientific lessons learned.
The CHARA-Michigan Phasetracker (CHAMP) successfully tracks fringes in 4-telescope and 6-telescope modes when observing high-visibility targets. We have found that our primary targets (Young Stellar Objects) have unexpectedly low visibility fringes (<20%) for most baselines at CHARA, generally below our tracking thresholds. We have undertaken an upgrade cycle in 2011-2012 to re-optimize CHAMP to allow group-delay tracking on the faintest fringes possible. We describe our multi-pronged strategy using special dicroics, new piezo scanners, and our first attempts to explore CHARA J-band made possible by using special metrology-blocking laser filters. CHAMP can now be used with all the combiners at CHARA.
To date, about 17 hot Jupiters have been directly detected by photometric and/or spectroscopic observations.
Only 2 of them, however, are non-transiting hot Jupiters and the rest are all transiting ones. Since non-transiting
hot Jupiter systems are analogs of high contrast binaries, optical/infrared long baseline interferometers can resolve
them and detect the planets if highly stable and precise closure phase measurements are obtained. Thus, this is
a good opportunity for optical/infrared interferometers to contribute to the field of exoplanet characterization.
To reach this goal, detailed calibration studies are essential. In this paper, we report the first results of our
closure phase calibration studies. Specifically, we find strong closure phase drifts that are highly correlated with
target positions, i.e., altitude and azimuth angle. The correlation is stronger with altitude. Our experiments
indicate that the major cause of the drifts is probably longitudinal dispersion. We are able to find a strategy with
multiple approaches to reduce this effect, and are able to model the closure phase drift with a quadratic function
of both altitude and azimuth. We then use this model to calibrate the drifts, and test this new calibration scheme
with the high contrast binary ε Per. Although we can find a better orbital solution with this new method, we
have also found difficulties to interpret the orbit of ε Per, which may stem from possible mis-calibrations or the
influence of the third component in the system. More investigations are definitely necessary to address this issue
and to further confirm our calibration strategy.
Based on the success of four-telescope imaging with the Michigan Infrared Combiner (MIRC) on the CHARA
Array, our Michigan-based group will now upgrade our system to combine all six CHARA telescope simultaneously.
In order to make this observationally efficient, we have had to improve a number of subsystems and
commission new ones, including the new CHAMP fringe tracker, the introduction of photometric channels, the
upgrading of the realtime operating systems, and the obvious hardware and software upgrades of the control
system and the data pipeline. Here we will discuss the advantages of six-telescope operation, outline our upgrade
plans and discuss our current progress.
Using the sub-milli-arcsecond resolution of the CHARA interferometer array and
combining light with the 2-telescope combiner CHARA Classic, we have detected
strong near-infrared (NIR) emission interior to the dust-sublimation radius of
Herbig Ae stars MWC275 and AB Aur. The large contribution of this emission
component, which we argue to be hot gas, to the total NIR spectral energy distribution
(SED) is not predicted by current models of the dust evaporation front,
indicating that the NIR disk is more complicated than expected. Furthermore,
we demonstrate that the structure of the evaporation front in MWC275 is time
variable, making single epoch, large uv coverage observations critical to decoding
front geometry. With the commissioning of CHARA Michigan Phase Tracker
in the summer of 2008, the Michigan Infrared Combiner (a 6 telescope combiner
at CHARA) will become an ideal instrument for studying the evaporation
front, achieving the required sensitivities to begin the first "true" interferometric
imaging of the gas-dust transition region in young stellar objects (YSOs).
Here, we summarize results on the evaporation front structure obtained with
CHARA Classic and describe future prospects with CHARA MIRC in elucidating
morphology of the gas-dust transition region.
The CHARA Michigan Phase-tracker (CHAMP) is a real-time fringe tracker for the CHARA Array, a six-telescope
long baseline optical interferometer on Mount Wilson, California. CHAMP has been optimized for
tracking sensitivity at J, H, or K bands and is not meant as a science instrument itself. This ultimately results
in maximum sensitivity for all the science beam combiners that benefit from stabilized fringes. CHAMP was
designed, built, and tested in the laboratory at the University of Michigan and will be delivered to the CHARA
Array in 2008. We present the final design of CHAMP, highlighting some its key characteristics, including a novel
post-combination transport and imaging system. We also discuss testing and validation studies and present first
closed-loop operation in the laboratory.
Although direct direction of light from hot Jupiters has recently been achieved by measurements from the
Spitzer Space Telescope and the Hubble Space Telescope, information on those hot Jupiters are still not enough
to break all the model degeneracies and provide detailed conclusions. More detections that can measure the
astrometric orbits and flux variations of hot Jupiters, especially in the near-IR, are necessary. One promising
way to reach this goal is to use precision closure phase measurements obtained with ground-based long baseline
optical interferometers. Here we present our preliminary closure phase studies on the nearby hot Jupiter system
υ And b using CHARA-MIRC. Our data analysis shows our closure phase precisions are at ~ 0.4σ and ~ 0.6σ
level of the required signal for detections for the short and long triangles of CHARA respectively. In order to
make real detections, we have several improvements in the future to increase the signal-to-noise of the data. Once
these improvements are realized, our goal of directly detecting light from υ And b will be feasible to achieve.
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
We report the first scientific results from the Michigan Infrared Combiner (MIRC), including the first resolved
image of a main-sequence star besides the Sun. Using the CHARA Array, MIRC was able to clearly resolve the
well-known elongation of Altair's photosphere due to centrifugal distortion, and was also able to unambiguously
image the effect of gravity darkening. In this report, we also show preliminary images of the interacting binary
β Lyr and give an update of MIRC performance.
The Michigan Infrared Combiner (MIRC) has been designed for two primary goals: 1) imaging with all six CHARA telescopes simultaneously in the near-infrared, 2) direct detection of "hot Jupiter" exoplanets using precision closure phases. In September 2005, MIRC was commissioned on-sky at the CHARA Array on Mt. Wilson, CA, successfully combining light from 4 telescopes simultaneously. After a brief overview of MIRC features and design philosophy, we provide detailed description of key components and present results of laboratory tests. Lastly, we present first results from the commissioning run, focusing on engineering performance. We also present remarkable on-sky closure phase results from the first night of recorded data with the best-ever demonstrated closure phase stability and precision (ΔΦ = 0.03 degrees).
We describe a set of general purpose utilities for visualizing and manipulating optical interferometry data stored in
the FITS-based OIFITS data format. This class of routines contains code like the OiPlot navigation/visualization
tool which allows the user to extract visibility, closure phase and UV-coverage information from the OIFITS files
and to display the information in various ways. OiPlot also has basic data model fitting capabilities which can
be used for a rapid first analysis of the scientific data. More advanced image reconstruction techniques are part
of a dedicated utility. In addition, these routines allow data from multiple interferometers to be combined and
used together. Part of our work also aims at developing software specific to the Michigan InfraRed Combiner
(MIRC). Our experience designing a flexible and robust graphical user interfaced based on sockets using python
libraries has wide applicability and this paper will discuss practicalities.
We present the design for a near-infrared (JHK) fringe tracker to be used at the CHARA Array, a long baseline optical interferometer located at Mount Wilson Observatory. The CHARA Michigan Phase-tracker (CHAMP) is being fabricated and tested at the University of Michigan and will be transported to the CHARA Array for general use. CHAMP is separate from the science combiners and can therefore be optimized for fringe tracking. It will modulate around fringe center by 1-2λ at up to 500 Hz and calculate phase offsets in real-time using a modified 'ABCD' method . Six pair-wise Mach-Zehnder combiners will phase the entire Array. We give an overview of the optical layout and discuss our design strategy. Components such as the path-length modulators, low-OH fiber transport system, 1024x1024 HAWAII-1 detector, and control computer are discussed.
This paper wants to be a practical example in building a real-time data-acquisition and control system from scratch using relatively non-expensive PC hardware and open-source software. The practical example of building the control system for the Michigan Infrared Combiner (MIRC) at the CHARA interferometer will be used to give the reader a 'hands-on' experience in installing and configuring the RTAI-Fusion real-time operating system and developing a complete control system with it.
We present a brief review of recent scientific and technical advances at the Infrared Optical Telescope Array (IOTA). IOTA is a long-baseline interferometer located atop Mount Hopkins, Arizona. Recent work has emphasized the use of the three-telescope interferometer completed in 2002. We report on results obtained on a range of scientific targets, including AGB stars, Herbig AeBe Stars, binary stars, and the recent outburst of the recurrent nova RS Oph. We report the completion of a new spectrometer which allows visibility measurements at several high spectral resolution channels simultaneously. Finally, it is our sad duty to report that IOTA will be closed this year.
An automatic fringe tracking system has been developed and implemented at the Infrared Optical Telescope Array (IOTA). In testing during May 2002, the system successfully minimized the optical path differences (OPDs) for all three baselines at IOTA. Based on sliding window discrete Fourier transform (DFT) calculations that were optimized for computational efficiency and robustness to atmospheric disturbances, the algorithm has also been tested extensively on off-line data. Implemented in ANSI C on the 266 MHz PowerPC processor running the VxWorks real-time operating system, the algorithm runs in approximately 2.0 milliseconds per scan (including all three interferograms), using the science camera and piezo scanners to measure and correct the OPDs. Preliminary analysis on an extension of this algorithm indicates a potential for predictive tracking, although at present, real-time implementation of this extension would require significantly more computational capacity.
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.
We describe the fringe-packet tracking software installed at the infrared optical telescope array (IOTA). Three independently developed fringe-packet tracking algorithms can be used to equalise the optical path lengths at the interferometer. We compare the performance of these three algorithms and show results obtained tracking fringes for three independent baselines on the sky.
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.
We are working towards imaging the surfaces and circumstellar envelopes of Mira stars in the near-infrared, using the IOTA interferometer and the IONIC integrated-optics 3-beam combiner. In order to study atmospheric structures of these stars, we installed 3 narrow-band filters that subdivide H-band into 3 roughly equal-width sub-bands - a central one for continuum, and 2 adjacent ones to sample Mira star's (mostly water) absorption-bands. We present here our characterization of the IOTA 3-Telescope interferometer for closure-phase measurements with broad and narrow-band filters in the H atmospheric window. This includes characterizing the stability, chromaticity, and polarization effects of the present IOTA optics with the IONIC beam-combiner, and characterizing the accuracy of our closure phase measurements.
Single-mode fibers have been used in the near-infrared to dramatically reduce calibration error for long-baseline interferometry. We have begun an effort to apply the advantages of spatial filtering at visible wavelengths for precision measurements of pulsating Cepheids using the IOTA interferometer. Rather than employing photometric taps to calibrate fluctuating coupling efficiency, we are using an "asymmetric" coupler which allows this calibration to be done without losing photons. The Single-Mode Asymmetric Recombination Technique (SMART) experiment has finished lab-testing, and has been installed at IOTA for full commissioning in Summer 2002. We report the results of lab characterization and first sky tests, as well as first fringes on a star using a visible-wavelength single-mode coupler. With both lab and sky experience using unpolarized light, we have found that circular silica fibers are quite practical for precision interferometric measurements. We conclude that circular fibers (as opposed to polarization maintaining fibers) have an undeserved poor reputation and that birefringence effects pose no significant difficulty.
We report the first long-baseline interferometric observations of R CrB. The observations were carried out at the Infrared Optical Telescope Array (IOTA), using our new JHK beam combiner which enables us to record fringes simultaneously in the J-, H-, and K-bands. The circumstellar envelope of R CrB is resolved at a baseline of 21 m, and the <i>K</i>-band visibility is derived to be 0.61 ± 0.03 along a position angle of about 170 degrees. The visibility obtained with IOTA, as well as speckle visibilities with baselines up to 6 m and the spectral energy distribution (SED), are fitted with 2-component models consisting of the central star and an optically thin dust shell. The <i>K</i>-band visibilities predicted by the models are about 10% smaller than the visibility obtained with IOTA. However, given the simplifications adopted in our models and the complex nature of the object, this can be regarded as rough agreement. As a hypothesis to explain the small discrepancy, we propose that there might be a group of newly formed dust clouds, which might appear as a third visibility component.
We report here the first visibility and closure-phase measurements done with the IONIC instrument at the IOTA interferometer. The IONIC
instrument is presented and preliminary analysis of the results
discussed. Future improvements of IONIC are envisioned.
We present observations of the symbiotic star CH Cyg with a new JHK-band beam combiner mounted to the IOTA interferometer. The new beam combiner consists of an anamorphic cylindrical lens system and a grism, and allows the <i>simultaneous</i> recording of spectrally dispersed J-, H- and K-band Michelson interferograms. The observations of CH Cyg were conducted on 5, 6, 8 and 11 June 2001 using baselines of 17m to 25m. From the interferograms of CH Cyg, J-, H-, and K-band visibility functions can be determined. Uniform-disk fits to the visibilities give, e.g., stellar diameters of (7.8 ± 0.6) mas and (8.7 ± 0.8) mas in H and K, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of Mira star models. The available HIPPARCOS parallax of CH Cyg allows us to determine linear radii. For example, on the basis of the K-band visibility, Rosseland radii in the range of 214 to 243 solar radii can be derived utilizing CLVs of different fundamental mode Mira models as fit functions. These radii agree well within the error bars with the corresponding theoretical model Rosseland radii of 230 to 282 solar radii. Models of first overtone pulsators are not in good agreement with the observations. The wavelength dependence of the stellar diameter can be well studied by using visibility ratios V(λ<sub>1</sub>)/V(λ<sub>2</sub>) since ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. We found that the 2.03 μm uniform disk diameter of CH Cyg is approximately 1.1
times larger than the 2.15 μm and 2.26 μm uniform-disk diameter.
We describe the new control system for the PICNIC near-infrared camera and the visible star tracker, implemented at the IOTA interferometer, based on the ALTERA Complex Programmable Logic Device (CPLD) technology. These digital components provide an adaptive interface between the control system and the cameras used at IOTA, allowing flexibility when connecting very different devices. In particular the clocking and processing circuits used for the PICNIC camera can be changed in milliseconds during normal operation. The camera can then switch between full quadrant readout mode used for alignment and diagnostics, and a N pixel readout mode used for science operation.
Several scientific topics linked to the observation of extended structures around astrophysical sources (dust torus around AGN, disks around young stars, envelopes around AGBs) require imaging capability with milli-arcsecond spatial resolution. The current VLTI instruments, AMBER and MIDI, will provide in the coming months the
required high angular resolution, yet without actual imaging. As a rule of thumb, the image quality accessible with an optical interferometer is directly related to the number of telescopes used simultaneously: the more the apertures, the better and the faster the reconstruction of the image. We propose an instrument concept to
achieve interferometric combination of N telescopes (4 ≤ <i>N</i> ≤ 8) thanks to planar optics technology: 4 x 8-m telescopes in the short term and/or 8 x 1.8-m telescopes in the long term. The foreseen image reconstruction quality in the visible and/or in the near infrared will be equivalent to the one achieved with millimeter radio interferometers. Achievable spatial resolution will be better than the one foreseen with ALMA. This instrument would be able to acquire routinely 1 mas resolution images. A 13 to 20 magnitude sensitivity in spectral ranges from 0.6 to 2.5 <i>μm</i> is expected depending on the choice of the phase referencing guide source. High dynamic range, even on faint objects, is achievable thanks to the high accuracy provided by integrated optics
for visibility amplitude and phase measurements. Based on recent validations of integrated optics presented here an imaging instrument concept can be proposed. The results obtained using the VLTI facilities give a demonstration of the potential of the proposed technique.
We report on a new fringe envelope tracking system installed at the Cambridge Optical Aperture Synthesis telescope (COAST). This currently uses the existing photon-counting avalanche photo diode (APD) detector system to allow real-time fringe tracking on up to 3 baselines simultaneously. This system has been recently tested on the sky and has proved to successfully track the fringe envelope on a 38m baseline. The algorithm based on an envelope method has also been implemented and tested at the Infrared-Optical Telescope Array (IOTA) interferometer.
The design and scientific objectives of a near infrared channeled spectrometer planned at the IOTA interferometer are discussed. The spectrometer has the flexibility to reconfigure easily for conventional broadband operations in addition to multi-channel mode. This instrument makes use of the existing PICNIC camera at the IOTA in order to be cost efficient. The spectrometer has been designed specifically for studying Mira stars. However, it will find its application in other areas of astrophysical interests such as studies of circumstellar disks around young stars and binary stars.
Our new IOTA JHK-band beam combiner allows the <i>simultaneous</i> recording of spectrally dispersed J-, H- and K-band Michelson interferograms. In this paper we present our IOTA observations of the Mira star T Cep with this beam combiner (observations in June 2001; four baselines in the range of 14 m to 27 m). The beam combiner optics consists of an anamorphic cylindrical lens system and a prism. From the interferograms of T Cep we derive the visibilities and the J-, H-, and K-band uniform-disk diameters of 14.0 ± 0.6 mas, 13.7 ± 0.6 mas and 15.0 ± 0.6 mas, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of different Mira star models. The available HIPPARCOS parallax (4.76 ± 0.75 mas) of T Cep allows us to determine linear radii. For example, from the K-band visibility we derive a Rosseland radius of 329<sub>-50</sub>/<sup>+70</sup> solar radii if we use the CLVs of the M-models as fit functions. This radius is in good agreement with the theoretical M-model Rosseland radius of 315 solar radii. The comparison of measured stellar parameters (e.g. diameters, effective temperature, visibility shape) with theoretical parameters indicates whether any of the models is a fair representation of T Cep.
The ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. Therefore, we use the visibility ratios V(λ<sub>1</sub>)/V(λ<sub>2</sub>) to investigate the wavelength dependence of the stellar diameter. We find that the 2.03 μm uniform-disk diameter of T Cep is about 1.26 times larger than the 2.26 μm uniform-disk diameter.
The IOTA (Infrared Optical Telescope Array) has been routinely
operating with two-telescopes since 1994, a mode destined to become
obsolete following its recent conversion to a three-telescope
array. In two-telescope mode, the IOTA has made numerous
scientific and technical contributions, see e.g. our list of
publications at http://cfa-www.harvard.edu/cfa/oir/IOTA/PUBLI/publications.html.
We present preliminary results on three different topics using recent
data from the two-telescope IOTA: (1) measurements of Mira star
diameters simultaneously in three different near-infrared spectral
bands, (2) measurement of the characteristic size and shape of the
source of near-infared emission in the x-ray binary system CI Cam, and (3) aperture synthesis of the Carbon star V Hydrae combining data from the IOTA and from aperture masking at the Keck-I telescope.
New beam combination techniques, using two and three telescopes, have been the focus of activity at IOTA during the past two years since our last update. In particular, we have added a third telescope, made closure-phase measurements, demonstrated two- and three-beam combination with integrated optics combiners, demonstrated two-beam combination with an asymmetric coupler, and made simultaneous JHK visibility measurements with an image-plane combiner.
We present the general architecture of the GI2T/REGAIN control system. Based on a Graphical User Interface and different client-server communications, the system has to control both telescopes, the delay line, the beam-combiner, the data acquisition system and the real-time processing used as fringe tracker. We also describe in details the implementation of a real-time fringe tracker based on 4 monochromatic images and which used the fractional excess algorithm. Numerical simulations are shown. The control system is also dedicated to the acquisition of all the relevant data for the visibility calibration. We will also describe in details the data reduction package that provides the corrected visibilities. This architecture is very general and robust and has been developed having in mind that GI2T/REGAIN should be used by a wide community of astronomers.