ON-SKY WIDE-FIELD ADAPTIVE OPTICS CORRECTION USING MULTIPLE LASER GUIDE STARS AT THE MMT

A wide-field adaptive optics system based on an adaptive secondary mirror (ASM) is one of a future plan for the next-generation Subaru adaptive optics system. The main application of ASM based AO will be a groundlayer adaptive optics (GLAO) with field-of-view larger than 10 arc minutes. The high Strehl-ratio of on-source correction by high-order ASM (expected to be about 1000) and the reduction of emissivity are also attractive points. In this paper, we report a preliminary result of simulations for the these applications of ASM to study conceptual design of the next-generation wide-field Subaru adaptive optics.

The Large Binocular Telescope (LBT) is now operating with the first of two permanently installed adaptive secondary mirrors, and the first of two complementary near-IR instruments called LUCIFER is operational as well. The ARGOS laser-guided ground-layer adaptive optics (GLAO) system, described elsewhere at this conference1, will build on this foundation to deliver the highest resolution over the 4 arc min wide-field imaging and multi-object spectroscopic modes of LUCIFER. In this paper, we describe a planned upgrade to ARGOS which will supplement the Rayleigh-based GLAO system with sodium laser guide stars (LGS) to fulfill the telescope's diffraction-limited potential. In its narrow-field mode of 30 arc sec, LUCIFER will deliver imaging at the Nyquist limit of the individual 8.4 m apertures down to J band and long-slit spectroscopy with resolution up to 40,000. In addition, the LBT Interferometer2 (LBTI) will cophase the two apertures, offering imaging at the diffraction limit of the 22.8 m baseline at wavelengths from 1.2 to 20 μm. In the first phase of the upgrade, a 10 W sodium LGS will be added to each half of the LBT, using the same launch telescopes mounted behind the two secondary mirrors as the Rayleigh LGS. The upgrade will rely on other components of the ARGOS infrastructure such as acquisition and guiding, and fast tip-tilt cameras. New wavefront sensors will be added to LUCIFER and LBTI. In the upgrade's second phase, the sodium and Rayleigh LGS will be used together in a hybrid tomographic sensing system. This configuration will offer the advantage that a single tip-tilt star will continue to be sufficient even for MCAO operation3, which is planned with LBT's LINC-NIRVANA instrument4,5.

The adaptive optics system for the 6.5 m MMT, based on a deformable secondary mirror, has been on the sky now for three commissioning runs totalling approximately 30 nights. The mirror has begun to demonstrate uniquely clean point-spread functions, high photon efficiency, and very low background in the thermal infrared. In this paper we review the lessons learned from the first few months of operation. Broadly, the hardware works well, and we are learning how procedures related to operation, system error recovery, and safety should be implemented in software. Experience with the MMT system is now guiding the design of the second and third adaptive secondaries, being built for the Large Binocular Telescope. In this context, we discuss the general requirements for retrofitting an adaptive secondary to an existing large telescope. Finally, we describe how the new technology can support the design of adaptive optics for 30-cm class telescopes, with particular attention to ground-layer adaptive optics (GLAO), where conjugation as close as possible to the turbulence is important.

An Adaptive Optic (AO) system is a fundamental requirement of 8m-class telescopes. We know that in order to obtain the maximum possible resolution allowed by these telescopes we need to correct the atmospheric turbulence. Thanks to adaptive optic systems we are able to use all the effective potential of these instruments, drawing all the information from the universe sources as best as possible. In an AO system there are two main components: the wavefront sensor (WFS) that is able to measure the aberrations on the incoming wavefront in the telescope, and the deformable mirror (DM) that is able to assume a shape opposite to the one measured by the sensor. The two subsystem are connected by the reconstructor (REC). In order to do this, the REC requires a “common language" between these two main AO components. It means that it needs a mapping between the sensor-space and the mirror-space, called an interaction matrix (IM). Therefore, in order to operate correctly, an AO system has a main requirement: the measure of an IM in order to obtain a calibration of the whole AO system. The IM measurement is a 'mile stone' for an AO system and must be done regardless of the telescope size or class. Usually, this calibration step is done adding to the telescope system an auxiliary artificial source of light (i.e a fiber) that illuminates both the deformable mirror and the sensor, permitting the calibration of the AO system. For large telescope (more than 8m, like Extremely Large Telescopes, ELTs) the fiber based IM measurement requires challenging optical setups that in some cases are also impractical to build. In these cases, new techniques to measure the IM are needed. In this PhD work we want to check the possibility of a different method of calibration that can be applied directly on sky, at the telescope, without any auxiliary source. Such a technique can be used to calibrate AO system on a telescope of any size. We want to test the new calibration technique, called “sinusoidal modulation technique”, on the Large Binocular Telescope (LBT) AO system, which is already a complete AO system with the two main components: a secondary deformable mirror with by 672 actuators, and a pyramid wavefront sensor. My first phase of PhD work was helping to implement the WFS board (containing the pyramid sensor and all the auxiliary optical components) working both optical alignments and tests of some optical components. Thanks to the “solar tower” facility of the Astrophysical Observatory of Arcetri (Firenze), we have been able to reproduce an environment very similar to the telescope one, testing the main LBT AO components: the pyramid sensor and the secondary deformable mirror. Thanks to this the second phase of my PhD thesis: the measure of IM applying the sinusoidal modulation technique. At first we have measured the IM using a fiber auxiliary source to calibrate the system, without any kind of disturbance injected. After that, we have tried to use this calibration technique in order to measure the IM directly “on sky”, so adding an atmospheric disturbance to the AO system. The results obtained in this PhD work measuring the IM directly in the Arcetri solar tower system are crucial for the future development: the possibility of the acquisition of IM directly on sky means that we are able to calibrate an AO system also for extremely large telescope class where classic IM measurements technique are problematic and, sometimes, impossible. Finally we have not to forget the reason why we need this: the main aim is to observe the universe. Thanks to these new big class of telescopes and only using their full capabilities, we will be able to increase our knowledge of the universe objects observed, because we will be able to resolve more detailed characteristics, discovering, analyzing and understanding the behavior of the universe components.

We present the results of the laboratory characterization of the ARGOS LGS wavefront sensor (LGSW) and dichroic units. ARGOS is the laser guide star adaptive optics system of the Large Binocular Telescope (LBT). It implements a Ground Layer Adaptive Optics (GLAO) correction for LUCI, an infrared imager and multi-object spectrograph (MOS), using 3 pulsed Rayleigh beacons focused at 12km altitude. The LGSW is a Shack-Hartman sensor having 15 × 15 subaspertures over the telescope pupil. Each LGS is independently stabilized for on-sky jitter and gated to reduce spot elongation. The 3 LGS pupils are stabilized to compensate mechanical flexure and are arranged on a single detector. Two units of LGSW have been produced and tested at Arcetri Observatory. We report on the results obtained in the pre-shipment laboratory test: internal active flexure compensation loop performance, optomechanical stability under different gravity conditions, thermal cycling, Pockels cells performance. We also update on the upcoming installation and commissioning campaign at LBT.

A future plan for the next-generation Subaru adaptive optics, is a system based on an adaptive secondary mirror. A ground-layer adaptive optics combined with a new wide-field multi-object infrared camera and spectrograph will be a main application of the adaptive secondary mirror. A preliminary simulation results show that the resolution achieved by the ground-layer adaptive optics is expected to be better than 0.2 arcsecond in the <i>K</i>-band over 15 arcminutes field-of-view. In this paper, the performance simulation is updated taking dependence on observation conditions, the zenith angle and the season, into account.

With the exception of the two prime focus cameras, all the instruments on the Large Binocular Telescope (LBT) use the two advanced 672-voice-coil-actuators adaptive secondary mirrors (ASMs). Based on 10 years of ASMs operation experience, and taking advantage of new technology developments, we decided to upgrade some of the ASMs hardware, to increase their performance and reliability. In this paper, we describe these improvements, i.e., upgrade of the power backplanes used in the ASM electronic control system, integration of an accelerometer-based tip-tilt vibration suppression system and upgrade of the optical calibration interferometers.

The long commissioning of the Adaptive Optics Facility (AOF) project has been completed shortly after this conference, providing AO correction to two Very Large Telescope (VLT) foci supported by an adaptive secondary mirror and four laser guide stars. Four AO modes are delivered: a Single Conjugate AO (SCAO) system for commissioning purpose, wide field and medium field Ground Layer AO (GLAO) for seeing improvement and narrow field Laser Tomography AO (LTAO) for ultimate performance. This paper intends to describe the implemented AO baseline and to highlight the most relevant results and lessons learned. In particular, it will address the control and reconstruction strategy, the wavefront sensing baseline and the online telemetry used to optimize the system online, estimate the turbulence profile and calibrate the misregistrations. Focusing on the LTAO mode, we will describe the tomography optimization, by exploring the reconstruction parameter space. Finally, on sky performance results will be presented both in terms of strehl ratio and limiting magnitude.

We present laboratory results of the closed-loop performance of the Magellan\nAdaptive Optics (AO) Adaptive Secondary Mirror (ASM), pyramid wavefront sensor\n(PWFS), and VisAO visible adaptive optics camera. The Magellan AO system is a\n585-actuator low-emissivity high-throughput system scheduled for first light on\nthe 6.5 meter Magellan Clay telescope in November 2012. Using a dichroic\nbeamsplitter near the telescope focal plane, the AO system will be able to\nsimultaneously perform visible (500-1000 nm) AO science with our VisAO camera\nand either 10 micron or 3-5 micron science using either the BLINC/MIRAC4 or\nCLIO cameras, respectively. The ASM, PWS, and VisAO camera have undergone final\nsystem tests in the solar test tower at the Arcetri Institute in Florence,\nItaly, reaching Strehls of 37% in i'-band with 400 modes and simulated\nturbulence of 14 cm ro at v-band. We present images and test results of the\nassembled VisAO system, which includes our prototype advanced Atmospheric\nDispersion Corrector (ADC), prototype calcite Wollaston prisms for SDI imaging,\nand a suite of beamsplitters, filters, and other optics. Our advanced ADC\nperforms in the lab as designed and is a 58% improvement over conventional ADC\ndesigns. We also present images and results of our unique Calibration Return\nOptic (CRO) test system and the ASM, which has successfully run in closed- loop\nat 1kHz. The CRO test is a retro reflecting optical test that allows us to test\nthe ASM off-sky in close-loop using an artificial star formed by a fiber\nsource.\n

The adaptive secondary mirrors (ASMs), being an integral part of the telescope optical train, have huge advantages over other wave-front correctors. At the same time, because of this specific characteristic, high performance and high reliability is required from an ASM. In this paper, we report on algorithm/software developments at the Large Binocular Telescope designed to improve performance and reliability of the two 672-voice-coil-actuators ASMs. By making use of the ASMs telemetry, we are able to intervene when critical condition occurs, monitor the daily operation of the ASMs and review the long-term trends to plan future maintenance activities.

Improved Adaptive Optics (AO) systems continue to revolutionize ground-based astronomy. Key to understanding a new AO systems capability is a quantitative measurement of performance, such as the Strehl Ratio (SR). At the Large Binocular Telescope (LBT) we have undertaken a program to monitor the performance of our AO system in regular use for science operations. Input to our analysis includes data taken specifically for this purpose during twilight and on engineering nights. We report our findings thus far and in particular discuss the challenges of collecting consistent data sets in twilight, the methods we used to overcome those challenges, and preliminary results from data collected so far.

We describe results from the first multi-laser wavefront sensing system designed to support tomographic modes of adaptive optics (AO). The system, now operating at the 6.5 m MMT telescope in Arizona, creates five beacons 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 Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. A separate high-order Shack-Hartmann sensor records simultaneous measurements of wavefronts from a natural star. From open-loop measurements, we find the average beacon wavefront gives a good estimate of ground layer aberration. We present results of full tomographic wavefront analysis, enabled by supplementing the laser data with simultaneous fast image motion measurements from three stars in the field. We describe plans for an early demonstration at the MMT of closed-loop ground layer AO, and later tomographic AO.

For a non-cooperative target, a laser beacon is created by illuminating the target with a beacon beam. When a beacon beam propagates though deep turbulence, turbulence spreads the beam. A conventional phase conjugate adaptive optics (AO) system is not efficient in the presence of Beacon Anisoplanatism when the beacon beam spot size at the target includes many isoplanatic patch sizes. We introduce a concept of the wavefront-based stochastic parallel gradient decent (WSPGD) AO system, which uses an off-axis wavefront sensor to provide feedback for the beam control algorithm. This concept is based on the finding that the phase aberrations of laser return from the target contain information about beam spot size at the target, and that correction of a limited number of low-order Zernike modes increases on-axis intensity and power in the bucket at the target. We evaluated the WSPGD AO system performance in simulation for two tactical engagement scenarios in the presence of strong turbulence. We found that that the WSPGD AO system can efficiently compensate the effects of strong turbulence including Beacon Anisoplanatism, even when the beam spot size at the target includes up to 20 isoplanatic patch sizes and the isoplanatic angle is by a factor of 2.6 less than the diffraction limit. The Strehl ratio gain for this scenario is 1.6 - 2.5, and the maximum Strehl ratio is achieved after 15-20 iterations. A laboratory demonstration performed under a separate program confirmed our theoretical predictions.

It is well-known that cone effect or focus anisoplanatism is produced by the limited distance of a laser guide star (LGS) which is created within the Earth atmosphere and consequently located at a finite distance from the observer. In this paper, the cone effect of the LGS for different vertical profiles of the refractive index structure constant is numerically investigated by using a revised computer program of atmospheric propagation of optical wave and an adaptive optics (AO) system including dynamic control process. According to the practice, the overall tilt for the tilt-correction mirror is obtained from a natural star and the aberrated wavefront for phase correction of the deformable mirror is obtained from a LGS in our numerical simulation. It is surprisingly found that the effect of altitude of the LGS on the AO phase compensation effectiveness by using the commonly-available vertical profiles of C²_n and the lateral wind speed in the atmosphere is relatively weak, and the cone effect for some C²_n profiles is even negligible. It is found that the cone effect does not have obvious relationship with the turbulence strength, however, it depends on the vertical distribution profile of apparently. On the other hand, the cone effect depends on the vertical distribution of the lateral wind speed as well. In comparison to a longer wavelength, the cone effect becomes more obvious in the case of a shorter wavelength. In all cases concerned in this paper, an AO system by using a sodium guide star has almost same phase compensation effectiveness as that by using the astronomical target itself as a beacon. Effect of dynamic control process in an AO system on the cone effect is studied in this paper for the first time within our knowledge.

It is well known that using two or more deformable mirrors (DMs) can improve the compensation ability of an adaptive optics (AO) system. However, to keep the stability of an AO system, the correlation between the multiple DMs must be suppressed during the correction. In this paper, we proposed a slope-based decoupling algorithm to simultaneous control the multiple DMs. In order to examine the validity and practicality of this algorithm, a typical woofer-tweeter (W-T) AO system was set up. For the W-T system, a theory model was simulated and the results indicated in theory that the algorithm we presented can selectively make woofer and tweeter correct different spatial frequency aberration and suppress the cross coupling between the dual DMs. At the same time, the experimental results for the W-T AO system were consistent with the results of the simulation, which demonstrated in practice that this algorithm is practical for the AO system with dual DMs.