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.
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.
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.
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.
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 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.
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 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 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.
Proc. SPIE. 6268, Advances in Stellar Interferometry
KEYWORDS: Microelectromechanical systems, Optical imaging, Astronomy, Detection and tracking algorithms, Data modeling, Interferometry, Monte Carlo methods, Optical interferometry, Visibility, Algorithms
We present a flexible code created for imaging from the bispectrum and V2. By using a simulated annealing method, we limit the probability of converging to local chi-squared minima as can occur when traditional imaging methods are used on data sets with limited phase information. We present the results of our code used on a simulated data set utilizing a number of regularization schemes including maximum entropy. Using the statistical properties from Monte-Carlo Markov chains of images, we show how this code can place statistical limits on image features such as unseen binary companions.
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.
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
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.
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.
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.
We have built an infrared beam combiner for the GI2T/REGAIN interferometer of the Observatoire de la Cote d'Azur. The beam controller allows us to record spectrally dispersed Michelson interference fringes in the near-infrared J-, H- or K-bands. The beam combiner has the advantage that Michelson interferograms can simultaneously be recorded in about 128 different spectral channels. The tilt of the spectrally dispersed fringes is a measure of the instantaneous optical path difference. We present the optical design of the beam combiner and GI2T/REGAIN observations of the Mira star R Cas with this beam combiner in the spectral range of 2.00 micrometers - 2.18 micrometers (observations on 22 and 25 August 1999; variability phase 0.08; V-magnitude approximately 6; seven baselines between 12 m and 24 m; reference stars Vega and (beta) Peg).
After five years of development, the REGAIN project has obtained its first light during summer 1999. The main goals were improving the quality and quantity of data through a complete re-designing and re-building of the central beam combiner. The REGAIN interferometric bonnette delivers two coherent foci, one at visible wavelengths and one in the IR bands (J, H and K). The visible focus is equipped with a dedicated visible spectrograph and two photon counting detectors. The infrared focus can be equipped with different instruments. I will discuss the main technical issues that have been chosen.
The capabilities of GI2T-REGAIN (Grand Interferometre a 2 Telescopes-REcombinateur du GrAnd INterferometre) interferometer are well suited for studying and understanding the morphology and physical processes of multiple systems. Thus we present the observing program we intend to carry out with this instrument. It will lead to an accurate determination of their fundamental parameters such as masses and sizes. Our program on interacting binaries will be coupled to a mass transfer model which can benefit from these observations.
This paper reviews the scientific results obtained with the Grand Interferometre a 2 Telescopes (the GI2T interferometer) from 1990 to 1996. During this epoch, accurate spectroscopy coupled to interferometry were achieved on luminous and multiple stars. Subtle structures in circumstellar environments such as: jets in the binary system β Lyrae, dumpiness in the wind of P Cygni, a rotating arm in ζTau have been discovered. Measurements of angular diameter variability versus time and wavelength provide fundamental parameters which constraint δ Cephei and γCas models. In addition to GI2T results, we develop in our group hydrodynamic and radiative transfert models dedicated to the interpretation of interferometry results. These models can directly constrain luminous star physics through their observable parameters.
This paper presents the optical layout of the REGAIN beam combiner including the optical delay line LAROCA with its variable curvature mirror, the field rotator devices, the image and pupil tracking systems and the dedicated visible spectrography. Preliminary studies of foreseen improvements, such as adaptive optics, IR spectrograph and addition of a third telescope, will be discussed.