We describe a back-end Adaptive Optics system for the CHARA Array called Lab-AO intended to compensate for non-common path errors between the AO system at the telescopes and the final beam combining area some hundreds of meters away. The system is an on-axis, very small field of view, low order system that will work on star light if enough is present, or will make use of a blue light beacon sent from the telescope towards the laboratory if not enough star light is available. The first of six of these system has been installed and has recently been tested on the sky. Another five will be built for the remaining telescopes later this year.
The CHARA array is an optical interferometer with six 1-meter diameter telescopes, providing baselines from 33 to 331 meters. With sub-milliarcsecond angular resolution, its versatile visible and near infrared combiners offer a unique angle of studying nearby stellar systems by spatially resolving their detailed structures. To improve the sensitivity and scientific throughput, the CHARA array was funded by NSF-ATI in 2011 to install adaptive optics (AO) systems on all six telescopes. The initial grant covers Phase I of the AO systems, which includes on-telescope Wavefront Sensors (WFS) and non-common-path (NCP) error correction. Meanwhile we are seeking funding for Phase II which will add large Deformable Mirrors on telescopes to close the full AO loop. The corrections of NCP error and static aberrations in the optical system beyond the WFS are described in the second paper of this series. This paper describes the design of the common-path optical system and the on-telescope WFS, and shows the on-sky commissioning results.
The CHARA Array is a six telescope optical/IR interferometer run by the Center for High Angular Resolution
Astronomy of Georgia State University and is located at Mount Wilson Observatory just to the north of Los Angeles
California. The CHARA Array has the largest operational baselines in the world and has been in regular use for
scientific observations since 2004. In 2011 we received funding from the NSF to begin work on Adaptive Optics for our
six telescopes. Phase I of this project, fully funded by the NSF grant, consists of designing and building wavefront
sensors for each telescope that will also serve as tip/tilt detectors. Having tip/tilt at the telescopes, instead of in the
laboratory, will add several magnitudes of sensitivity to this system. Phase I also includes a slow wavefront sensor in the
laboratory to measure non-common path errors and small deformable mirrors in the laboratory to remove static and
slowly changing aberrations. Phase II of the project will allow us to place high-speed deformable mirrors at the
telescopes thereby enabling full closed loop operation. We are currently seeking funding for Phase II. This paper will
describe the scientific rational and design of the system and give the current status of the project.
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.
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.
We have designed and constructed a prototype of Photometric Channels for Michigan Infrared Combiner (MIRC).
Photometric Channels provide direct real-time measurements of fluxes from individual telescopes, improving the
precision of visibility and close phase calibration. While this prototype has increased the MIRC data quality
since the commissioning in August 2009, it leaves several issues unresolved, such as the changing polarization
of starlight due to the CHARA beam train. We are planning to make an improvement of the prototype in
conjunction with the six-beam combiner upgrade in the summer 2011.
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.