Guido Brusa

Adaptive secondary mirrors for the Large Binocular Telescope

The two adaptive secondary (AS) mirrors for LBT (LBT672) represent the new generation of the AS technology. Their design is based on the experience earned during the extensive tests of the previous generation unit (the MMT AS mirror). Both the mechanics and the electronics have been revised, improving the stability, reliability, maintenance and computational power of the system. The deformable mirror of each unit consists of a 1.6mm-thick Zerodur shell having a diameter of 911mm. The front surface is concave to match the Gregorian design of the telescope. Its figure is controlled by 672 electro-magnetic force actuators that are supported and cooled by an aluminum plate. The actuator forces are controlled using a combination of feed-forward and de-centralized closed loop compensation, thanks to the feedback signals from the 672 co-located capacitive position sensors. The surface reference for the capacitive sensors is a 50mm-thick Zerodur shell faced to the back surface of the thin mirror and rigidly connected to the support plate of the actuators. Digital real-time control and unit monitoring is obtained using new custom-made on-board electronics based on new generation 32bit floating-point DSPs. The total computational power (121 Gflop/s) of the LBT672 units allows using the control electronics as wave-front computer without any reduction of the actuator control capability. We report the details of the new features introduced in the LBT672 design and the preliminary laboratory results obtained on a prototype used to test them. Finally the facility in Arcetri to test the final LBT672 units is presented.

Large Binocular Telescope Adaptive Optics System: new achievements and perspectives in adaptive optics

The Large Binocular Telescope (LBT) is a unique telescope featuring two co-mounted optical trains with 8.4m primary mirrors. The telescope Adaptive Optics (AO) system uses two innovative key components, namely an adaptive secondary mirror with 672 actuators and a high-order pyramid wave-front sensor. During the on-sky commissioning such a system reached performances never achieved before on large ground-based optical telescopes. Images with 40mas resolution and Strehl Ratios higher than 80% have been acquired in H band (1.6 μm). Such images showed a contrast as high as 10-4. Based on these results, we compare the performances offered by a Natural Guide Star (NGS) system upgraded with the state-of-the-art technology and those delivered by existing Laser Guide Star (LGS) systems. The comparison, in terms of sky coverage and performances, suggests rethinking the current role ascribed to NGS and LGS in the next generation of AO systems for the 8-10 meter class telescopes and Extremely Large Telescopes (ELTs).

The adaptive secondary mirror for the Large Binocular Telescope: optical acceptance test and preliminary on-sky commissioning results

The Large Binocular Telescope (LBT) has two adaptive secondary mirrors based on 672 voice-coil force actuators. The shape of the mirror is controlled using internal metrology based on co-located capacitive sensors. The first mirror unit is currently mounted on LBT for on-sky commissioning as part of the First Light Adaptive Optics System (FLAO). During spring-time 2009 the optical acceptance test was performed using the 14-m optical test tower at the Osservatorio Astrofisico di Arcetri (INAF) showing the capability of flattening the shell at the level of 14nm rms residual surface error. This paper reports the optical layout, calibration procedures and results of the optical acceptance test. Moreover we report the first results obtained during the early runs of FLAO commissioning showing the ability of the mirror to compensate for atmospheric turbulence with extremely high Strehl ratio values (better than 80% in H-band) as permitted by the largest number of correcting degrees of freedom currently available on-sky for astronomical telescopes.

Natural guide star adaptive optics systems at LBT: FLAO commissioning and science operations status

This paper summarizes the activities and the principal results achieved during the commissioning of the two Natural Guide Star (NGS) AO systems called FLAO#1 & 2 installed at the bent Gregorian focal stations of the 2x8.4m Large Binocular Telescope (LBT). The commissioning activities of FLAO#1 took place in the period February 2010 - October 2011, while FLAO#2 commissioning started in December 2011 and should be completed by November 2012. The main results of the commissioning campaign are presented in terms of the H-band Strehl Ratio values achieved under different observing conditions. We will also describe the automatic procedures to configure and set-up the FLAO systems, and in particular the modal gain optimization procedure, which has been proven to be a very important one in achieving the nominal performance. Finally, some of the results achieved in two science runs using the near infra-red camera PISCES are briefly highlighted.

Observations of Herbig Ae disks with nulling interferometry

We present the results of 10 � m nulling interferometric observations of 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 m telescopes. A portion of the observations was completed with the adaptive secondary at the MMT. We have conclusively spatially resolved 3 of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter two recently resolved using adaptive optics in combination with nulling interferometry. For the resolved objects we find that the 10 � m emitting regions have a spatial extent of 15Y30 AU in diameter. We also have some evidence for resolved emission surrounding an additional two stars (V892 Tau and R CrA). For those objects in our study with mid-IR SEDs in the classification of Meeus and coworkers, we find that the group I objects (those with constant to increasing mid-IR flux) are more likely to be resolved, within our limited sample. This trend is evident in correlations in the inferred disksizes versus the submillimeter SED slope and disk sizeversus fractional infrared luminosity of the systems. We explore the spatial distribution and orientation of the warm dust in the resolved systems and constrain physical models that are consistent with their observational signatures.

MMT adaptive secondary: first AO closed-loop results

The adaptive secondary for the MMT is the first mirror of its kind. It was designed to allow the application of wavefront corrections (including tip-tilt) directly at the secondary mirror location. Among the advantages of such a choice for adaptive optics operation are higher throughput, lower emissivity, and simpler optical setup. Furthermore, this specific implementation provides capabilities that are not found in most correctors including internal position feedback, large stroke (to allow chopping) and provision for absolute position calibration. The mirror has now been used at the MMT during several runs where it has performed reliably. In this paper we discuss the mirror operation and AO performance achieved during these runs in which the adaptive secondary has been operating in conjunction with a Shack-Hartmann wavefront sensor as part of the MMT adaptive optics system. In particular we mention a residual mirror position error due to wind buffeting and other errors of ≈ 15 nm rms surface and a stable closed loop operation with a 0dB point of the error transfer function in the range 20-30 Hz limited mainly by the wavefront sensor maximum frame rate. Because of the location of the adaptive secondary with respect to the wavefront sensor camera, reimaging optics are required in order to perform the optical interaction matrix measurements needed to run the AO loop. This optical setup has been used in the lab but not replicated at the telescope so far. We will discuss the effects of the lack of such an internal calibration on the AO loop performances and a possible alternative to the lab calibration technique that uses directly light from sky objects.

MMT adaptive secondary: performance evaluation and field testing

The adaptive secondary for the MMT (called MMT336) is the first mirror of its kind. It was designed to allow the application of wavefront corrections (including tip-tilt) directly at the secondary mirror location. Among the advantages of such a choice for adaptive optics operation are higher throughput, lower emissivity, and simpler optical setup. The mirror also has capabilities that are not found in most correctors including internal position feedback, large stroke (to allow chopping) and provision for absolute position calibration. The 336 actuator adaptive secondary for MMT has been used daily for over one year in our adaptive optics testing facility which has built confidence in the mirror operation and allowed us to interface it to the MMT adaptive optics system. Here we present the most recent data acquired in the lab on the mirror performance. By using interferometer measurements we were able to achieve a residual surface error of approximately 40nm rms. Coupling the mirror with a Shack-Hartmann wavefront sensor we obtained a stable closed loop operation with a -3dB closed loop bandwidth of approximately 30Hz limited by the wavefront sensor frame rate. We also present some preliminary results that show a 5Hz, 90% duty cycle, ±5 arcsec chopping of the mirror. Finally the experience gained and the problems encountered during the first light adaptive optics run at the telescope will be briefly summarized. A more extensive report can be found in another paper also presented at this conference.

Nulling Data Reduction and On-sky Performance of the Large Binocular Telescope Interferometer

National Aeronautics and Space Administration, Exoplanet Exploration Program; National Aeronautics and Space Administration; European Union through ERC [279973]

MMT-AO: two years of operation with the first adaptive secondary

The Multiple Mirror Telescope (MMT) adaptive optics system (MMT-AO) has been operated in a campaign mode for the last two years. In total seven runs, each lasting about two weeks, have been carried out. During these observational runs a large amount of data have been collected. These data allow us to draw some preliminary conclusions about the overall system performances. In this paper we discuss in detail the achieved performances of the MMT-AO system which is equipped with the first adaptive secondary ever developed. The performances are examined both in terms of number of corrected modes and control bandwidth achieved. We also discuss our attempts to improve the system calibration. This is done by modulating the internal slope offsets while the system is operating in closed loop on the sky.

Mid-Infrared Imaging of the Post-Asymptotic Giant Branch Star AC Herculis with the Multiple Mirror Telescope Adaptive Optics System*

We utilized the unique 6.5m MMT deformable secondary adaptive optics system to produce highresolution (FWHM=0.3), very high Strehl mid-infrared (9.8, 11.7 & 18 μm) images of the post-AGB star AC Her. The very high (98± 2%) Strehls achieved with Mid-IR AO led naturally to an ultra-stable PSF independent of airmass, seeing, or location on the sky. We find no significant difference between AC Her’s morphology and our unresolved PSF calibration stars (μ UMa & α Her) at 9.8, 11.7, & 18 microns. Our current observations do not confirm any extended Mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 micron images which suggested the presence of a resolved ∼ 0.6 edge-on circumbinary disk. We conclude that AC Her has no extended Mid-IR structure on scales greater than 0.2 (R < 75 AU). These first results of Mid-IR AO science are very encouraging for future high accuracy Mid-IR imaging with this technique. Subject headings: instrumentation: adaptive optics — binaries: general — stars: evolution — stars: formation — stars: AGB, Proto-Planetary NebulaeWe utilized the unique 6.5 m Multiplie Mirror Telescope deformable secondary adaptive optics (AO) system to produce high-resolution (FWHM = 03), very high Strehl mid-infrared (9.8, 11.7, and 18 μm) images of the post-asymptotic giant branch star AC Her. The very high (98% ± 2%) Strehls achieved with mid-IR AO led naturally to an ultrastable point-spread function (PSF) independent of air mass, seeing, or location on the sky. We find no significant difference between AC Hers morphology and our unresolved PSF calibration stars (μ UMa and α Her) at 9.8, 11.7, and 18 μm. Our current observations do not confirm any extended mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 μm images that suggested the presence of a resolved ~06 edge-on circumbinary disk. We conclude that AC Her has no extended mid-IR structure on scales greater than 02 (R < 75 AU). These first results of mid-IR AO science are very encouraging for future high-accuracy mid-IR imaging with this technique.