The Michigan Young Star Imager at CHARA (MYSTIC) is a K-band interferometric beam combining instrument funded by the U.S. National Science Foundation, designed primarily for imaging sub-au scale disk structures around nearby young stars and to probe the planet formation process. Installed at the CHARA Array in July 2021, with baselines up to 331 m, MYSTIC provides a maximum angular resolution of λ / 2B ∼ 0.7 mas. The instrument injects phase-corrected light from the array into inexpensive, single-mode, polarization maintaining silica fibers, which are then passed via a vacuum feedthrough into a cryogenic dewar operating at 220 K for imaging. MYSTIC uses a high frame rate, ultra-low read noise SAPHIRA detector and implements two beam combiners: a six-telescope image plane beam combiner, based on the MIRC-X design, for targets as faint as 7.7 Kmag, as well as a four-telescope integrated optic beam-combiner mode using a spare chip leftover from the GRAVITY instrument. MYSTIC is co-phased with the MIRC-X (J + H band) instrument for simultaneous fringe-tracking and imaging and shares its software suite with the latter to allow a single observer to operate both instruments. We present the instrument design, review its operational performance, present early commissioning science observations, and propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future.
The Center for High Angular Resolution Astronomy (CHARA) Array is a six-element interferometer with baselines ranging from 34 to 331 m. Three new beam combiners are entering operation: MYSTIC is a 6-telescope combiner for K-band; SPICA is a 6-telescope combiner for the visible R-band; and SILMARIL is a 3-telescope combiner for high sensitivity in H and K-bands. A seventh, portable telescope will use fiber optics for beam transport and will increase the baselines to 1 km. Observing time is available through a program funded by NSF. The programs are solicited and peer-reviewed by NSF’s National Optical-Infrared Astronomy Research Laboratory. The open community access has significantly expanded the range of astronomical investigations of stars and their environments. Here we summarize the scientific work and the on-going technical advances of the CHARA Array.
Much research has been done to show the possibilities of using long transport fibers in optical interferometry. The CHARA Michelson Array Pathfinder will extend the spatial coverage of the CHARA Array by adding a mobile 1-meter telescope connected by optical fibers. The pathfinder will operate in H-band and will explore baselines up to approximately 1 km, giving an angular resolution of 0.2 mas. The new telescope will be placed at short baselines to image the surfaces of large stars and at long baselines to resolve small stars. Here we describe the project and our progress on various subsystems.
Optical interferometry is a powerful technique to achieve high angular resolution. However, its main issue is its lack of sensitivity, compared to other observation techniques. Efforts have been made in the previous decade to improve the sensitivity of optical interferometry, with instruments such as PIONIER and GRAVITY at VLTI, or MIRC-X and MYSTIC at CHARA. While those instruments pushed on sensitivity, their design focus was not the sensitivity but relative astrometric accuracy, imaging capability, or spectral resolution. Our goal is to build an instrument specifically designed to optimize for sensitivity. This meant focusing our design efforts on different parts of the instrument and investigating new technologies and techniques. First, we make use of the low noise C-RED One camera using e-APD technology and provided by First Light Imaging, already used in the improvement of sensitivity in recent new instruments. We forego the use of single-mode fibers but still favor an image plane design that offers more sensitivity than a pupil plane layout. We also use a minimum number of optical elements to maximize the throughput of the design, using a long focal length cylindrical mirror. We chose to limit our design to 3 beams, to have the capability to obtain closure phases, but not dilute the incoming flux in more beam combinations. We also use in our design an edge filter to have the capability to observe Hand K-band at the same time. We use a low spectral resolution, allowing for group delay fringe tracking but maximizing the SNR of the fringes for each spectral channel. All these elements will lead to a typical limiting magnitude between 10 and 11 in both H- and K-bands.
The CHARA Array is the longest baseline optical interferometer in the world. Operated with natural seeing, it has delivered landmark sub-milliarcsecond results in the areas of stellar imaging, binaries, and stellar diameters. However, to achieve ambitious observations of faint targets such as young stellar objects and active galactic nuclei, higher sensitivity is required. For that purpose, adaptive optics are developed to correct atmospheric turbulence and non-common path aberrations between each telescope and the beam combiner lab. This paper describes the AO software and its integration into the CHARA system. We also report initial on-sky tests that demonstrate an increase of scientific throughput by sensitivity gain and by extending useful observing time in worse seeing conditions. Our 6 telescopes and 12 AO systems with tens of critical alignments and control loops pose challenges in operation. We describe our methods enabling a single scientist to operate the entire system.
The CHARA Array is an optical/infrared interferometer that combines the light from six 1-meter telescopes. With baselines ranging from 34 to 331 meters, CHARA provides sub-milliarcsecond resolution to measure stellar diameters, image stellar surfaces, resolve close binary companions, and study circumstellar environments. In this paper, we present recent highlights from the CHARA Array, focusing on the implementation of adaptive optics, the ongoing development of next generation beam combiners, an update on the community access program, and a discussion about future developments.
MIRC-X is a six telescope beam combiner at the CHARA array that works in J and H wavelength bands and provides an angular resolution equivalent to a B=331m diameter telescope. The legacy MIRC combiner has delivered outstanding results in the fields of stellar astrophysics and binaries. However, we required higher sensitivity to make ambitious scientific measurements of faint targets such as young stellar objects, binary systems with exoplanets, and active galactic nuclei. For that purpose, MIRC-X is built and is offered to the community since mid-2017. MIRC-X has demonstrated up to two magnitudes of improved faint magnitude sensitivity with the best-case H <= 8. Here we present a review of the instrument and present early science results, and highlight some of our ongoing science programs.
The CHARA array is an optical/near infrared interferometer consisting of six 1-meter diameter telescopes, the longest baseline of which is 331 meters. With sub-millisecond angular resolution, the CHARA array is able to spatially resolve nearby stellar systems and reveal their detailed structures. To improve the sensitivity and scientific throughput, the CHARA array was funded by NSF-ATI in 2011 for an upgrade of adaptive optics (AO) systems to all six telescopes. This first grant covered Phase I of the adaptive optics system, which includes an on-telescope Wavefront Sensor (WFS) and non-common-path (NCP) error correction. Phase II of the program was funded by the NSF/MRI in 2016, and includes purchasing and installing the deformable mirrors at each telescope to complete the system. In this paper we will discuss both phases of the program, how the challenge of AO differs for interferometry, and the first results of the full system.
The CHARA Array is a six-element, optical/NIR interferometer, which currently has the largest operational baselines in the world. The Array is operated by Georgia State University and is located at the Mount Wilson Observatory in California. The Array thrives thanks to members of the CHARA consortium that includes LESIA (Observatoire de Paris), Observatoire de la Cote dAzur, University of Michigan, Sydney University, Australian National University, and University of Exeter. Here we give a brief introduction to the Array infrastructure with a focus on a developing Adaptive Optics (AO) program, the new community access program funded by the NSF, and recent science results.
The ability to use single mode (SM) fibers for beam transport in optical interferometry offers practical advantages over conventional long vacuum pipes. One challenge facing fiber transport is maintaining constant differential path length in an environment where environmental thermal variations can lead to cm-level variations from day to night. We have fabricated three composite cables of length 470 m, each containing 4 copper wires and 3 SM fibers that operate at the astronomical H band (1500-1800 nm). Multiple fibers allow us to test performance of a circular core fiber (SMF28), a panda-style polarization-maintaining (PM) fiber, and a lastly a specialty dispersion-compensated PM fiber. We will present experimental results using precision electrical resistance measurements of the of a composite cable beam transport system. We find that the application of 1200 W over a 470 m cable causes the optical path difference in air to change by 75 mm (+/- 2 mm) and the resistance to change from 5.36 to 5.50Ω. Additionally, we show control of the dispersion of 470 m of fiber in a single polarization using white light interference fringes (λc=1575 nm, Δλ=75 nm) using our method.
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