The Large Binocular Telescope Interferometer is a NASA-funded nulling and imaging instrument designed to coherently combine the two 8.4-m primary mirrors of the LBT for high-sensitivity, high-contrast, and highresolution infrared imaging (1.5-13 μm). PHASECam is LBTI's near-infrared camera used to measure tip-tilt and phase variations between the two AO-corrected apertures and provide high-angular resolution observations. We report on the status of the system and describe its on-sky performance measured during the first semester of 2014. With a spatial resolution equivalent to that of a 22.8-meter telescope and the light-gathering power of single 11.8-meter mirror, the co-phased LBT can be considered to be a forerunner of the next-generation extremely large telescopes (ELT).
Optical interferometry is a cost-effective means to extend the resolving power of astronomical instruments. Typically, the light from separate small and movable telescopes is brought through vacuum pipes to a central beam combiner. We are developing a new generation of AO systems to enhance the performance of interferometers in which the vacuum lines are replaced with optical fibers. The AO, included on each of the telescopes, concentrates light on the fiber inputs to achieve the greatest optical throughput. We describe the design approach to the AO systems, how their requirements differ from those of a traditional system, and how the addition of AO enables further enhancements to the design of optical interferometers.
Laser tomography capability using a multi laser guide star (LGS) system is being implemented at the 6.5 m MMT
telescope on Mt. Hopkins, AZ. The system uses five range-gated and dynamically refocused Rayleigh laser beacons to
sense the atmospheric wavefront aberration. Corrections are then applied to the wavefront using the 336-actuator
adaptive secondary mirror of the telescope. So far, the system has demonstrated successful control of ground-layer
aberration over a field of view substantially wider than is delivered by conventional adaptive optics. In this paper, we
report the latest results from this mode of operation, using for the first time a plate scale on our IR science camera that
samples the diffraction scale at the Nyquist limit. We also discuss findings for a reduction in the width of the on-axis
point-spread function from 1.07" to <0.2" in H band and present the progress achieved toward the implementation of
laser tomography. This will be attempted by means of a least squares reconstructor, which is obtained using
simultaneous measurements of the wavefronts from the LGS and an additional natural guide star.
The MMT observatory has recently implemented and tested an optimal wavefront controller for the NGS
adaptive optics system. Open loop atmospheric data collected at the telescope is used as the input to a
MATLAB based analytical model. The model uses nonlinear constrained minimization to determine controller
gains and optimize the system performance. The real-time controller performing the adaptive optics close loop
operation is implemented on a dedicated high performance PC based quad core server. The controller algorithm
is written in C and uses the GNU scientific library for linear algebra. Tests at the MMT confirmed the optimal
controller significantly reduced the residual RMS wavefront compared with the previous controller. Significant
reductions in image FWHM and increased peak intensities were obtained in J, H and K-bands. The optimal PID
controller is now operating as the baseline wavefront controller for the MMT NGS-AO system.
The Laser Adaptive Optics system of the 6.5 m MMT telescope has now been commissioned with Ground Layer
Adaptive Optics operations as a tool for astronomical science. In this mode the wavefronts sampled by each of five laser
beacons are averaged, leading to an estimate of the aberration in the ground layer. The ground layer is then compensated
by the deformable secondary mirror at 400 Hz. Image quality of
0.2-0.3 arc sec is delivered in the near infrared bands
from 1.2-2.5 μm over a field of view of 2 arc minutes. Tomographic wavefront sensing tests in May 2010 produced open
loop data necessary to streamline the software to generate a Laser Tomography Adaptive Optics (LTAO) reconstructor.
In addition, we present the work being done to achieve optimal control PID wavefront control and thus increase the
disturbance rejection frequency response for the system. Finally, we briefly describe plans to mount the ARIES near
infrared imager and echelle spectrograph, which will support the 2 arc min ground-layer corrected field and will exploit
the diffraction limit anticipated with LTAO.
We report closed-loop results obtained from the first adaptive optics system to deploy multiple laser guide beacons. The
system is mounted on the 6.5 m MMT telescope in Arizona, and is designed to explore advanced altitude-conjugated
techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm
integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a
unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement
the beacon range gate. Wavefront correction is applied with the telescope's unique deformable secondary mirror. The
system has now begun operations as a tool for astronomical science, in a mode in which the boundary-layer turbulence,
close to the telescope, is compensated. Image quality of 0.2-0.3 arc sec is routinely delivered in the near infrared bands
from 1.2-2.5 μm over a field of view of 2 arc min. Although it does not reach the diffraction limit, this represents a 3 to
4-fold improvement in resolution over the natural seeing, and a field of view an order of magnitude larger than
conventional adaptive optics systems deliver. We present performance metrics including images of the core of the
globular cluster M3 where correction is almost uniform across the full field. We describe plans underway to develop the
technology further on the twin 8.4 m Large Binocular Telescope and the future 25 m Giant Magellan Telescope.
At the MMT Observatory, the adaptive secondary system is producing well corrected images using natural guide stars.
Shortly after the system came online, however, it was found that vibrations in the telescope structure were limiting the
Strehl of the corrected image. The worst of these vibrations are at 19 Hz, which puts them just outside of the system
correction bandwidth. The laser guide star system at the MMT is also impacted by a different vibration mode at 14 Hz
that affects the pointing of the laser beacon on the sky. To correct these errors, accelerometers were installed on the
secondary mirror to measure its motion. The measured motion was then used to generate a feed-forward correction term
which has already been proven in on-sky testing to work for the LGS case. The NGS case is more difficult and attempts
to correct image motion have failed due to excessive feedback.
The Multiple Mirror Telescope (MMT), upgraded in 2000 to a monolithic 6.5m primary mirror from its original array of six 1.8m primary mirrors, was commissioned with axis controllers designed early in the upgrade process without regard to structural resonances or the possibility of the need for digital filtering of the control axis signal path. Post-commissioning performance issues led us to investigate replacement of the original control system with a more modern digital controller with full control over the system filters and gain paths. This work, from system identification through
controller design iteration by simulation, and pre-deployment hardware-in-the-loop testing, was performed using latest-generation
tools with Matlab® and Simulink®. Using Simulink's Real Time Workshop toolbox to automatically generate C source code for the controller from the Simulink diagram and a custom target build script, we were able to deploy the new controller into our existing software infrastructure running Wind River's VxWorks™real-time operating system. This paper describes the process of the controller design, including system identification data collection, with discussion of implementation of non-linear control modes and disturbance decoupling, which became necessary to obtain acceptable wind buffeting rejection.
A multi-laser adaptive optics system, at the 6.5 m MMT telescope, has been undergoing commissioning in
preparation for wide-field, partially corrected as well as narrow-field, diffraction limited science observations in
the thermal and near infrared. After several delays due to bad weather, we have successfully closed the full high
order ground-layer adaptive optics (GLAO) control loop for the first time in February 2008 using five Rayleigh
laser guide stars and a single tilt star. Characterization and automated correction of static aberrations such
as non-common path errors were addressed in May 2008. Calibration measurements in preparation for laser
tomography adaptive optics (LTAO) operation are planned for the fall of 2008 along with the start of shared-risk
GLAO science observations.
We present the results of GLAO observations with the PISCES imager, a 1 - 2.5 &mgr;m camera with a field of
view of 110 arc seconds. The status of the remaining GLAO commissioning work is also reviewed. Finally, we
present plans for commissioning work to implement the LTAO operating mode of the system.
We describe the conceptual design of an advanced laser guide star facility (LGSF) for the Large Binocular Telescope
(LBT), to be built in collaboration with the LBT's international partners. The highest priority goal for the facility is the
correction of ground-layer turbulence, providing partial seeing compensation in the near IR bands over a 4' field. In the
H band, GLAO is projected to improve the median seeing from 0.55" to 0.2".
The new facility will build on the LBT's natural guide star AO system, integrated into the telescope with correction by
adaptive secondary mirrors, and will draw on Arizona's experience in the construction of the first multi-laser adaptive
optics (AO) system at the 6.5 m MMT. The LGSF will use four Rayleigh beacons at 532 nm, projected to an altitude of
25 km, on each of the two 8.4 m component telescopes. Initial use of the system for ground layer correction will deliver
image quality well matched to the LBT's two LUCIFER near IR instruments. They will be used for direct imaging over
a 4'×4' field and will offer a unique capability in high resolution multi-object spectroscopy.
The LGSF is designed to include long-term upgrade paths. Coherent imaging at the combined focus of the two apertures
will be exploited by the LBT Interferometer in the thermal IR. Using the same launch optics, an axial sodium or
Rayleigh beacon can be added to each constellation, for tomographic wavefront reconstruction and diffraction limited
imaging over the usual isoplanatic patch. In the longer term, a second DM conjugated to high altitude is foreseen for the
LBT's LINC-NIRVANA instrument, which would extend the coherent diffraction-limited field to an arcminute in
diameter with multi-conjugate AO.
The Large Binocular Telescope (LBT) features dual 8.4 m diameter mirrors in a common elevation-over-azimuth mount. The LBT moves in elevation on two large crescent-shaped C-rings that are supported by radial hydrostatic bearing pads located near the four corners of the rectangular azimuth frame. The azimuth frame, in turn, is supported by four hydrostatic bearing pads and uses hydrodynamic roller bearings for centering. Each axis is gear driven by four large electric motors. In addition to precision optical motor encoders, each axis is equipped with Farrand Inductosyn strip encoders which yield 0.005 arcsecond resolution. The telescope weighs 580 metric tons and is designed to track with 0.03 arcsecond or better servo precision under wind speeds as high as 24 km/hr. Though the telescope is still under construction, the Mount Control System (MCS) has been routinely exercised to achieve First Light. The authors present a description of the unique, DSP-based synchronous architecture of the MCS and its capabilities.