Piero Salinari

Adaptive secondary mirror for the 6.5-m conversion of the Multiple Mirror Telescope: first laboratory testing results

We present the first results of test performed on a reduced size adaptive secondary prototype named P36. The full size unit, named MMT336, is ready to be assembled and it is planned to install it at the 6.5m conversion of the Multiple Mirror Telescope by the end of this year. The design of the final unit consists of: a convex thin deformable mirror whose figure is controlled by 336 electro-magnetic force actuators, a thick reference shell and a third aluminum shell used for actuator support and cooling. The force actuator response function is adjusted using both open and closed loop compensation to obtain an equivalent position actuator thanks to nearly co-located capacitive position sensors. The digital real-time control and the unit monitoring is done using custom-made electronics based on DSPs. The preliminary dynamical test aimed at identifying the P36 mirror response function to obtain a proper dynamics compensation were successful. In fact two main results have been obtained: 1) an accurate identification of the feedforward matrix used to control the mirror 2) settling time of approximately 0.5 ms, well within the specifications. We also complement these lab results with results obtained from simulations of the full size mirror dynamics.

The adaptive secondary mirror for the Large Binocular Telescope: results of acceptance laboratory test

The first of the two Gregorian Adaptive Secondary Mirror (ASM) units for the Large Binocular Telescope (LBT) has been fully integrated and tested for laboratory acceptance. The LBT unit represents the most advanced ASM device existing in hardware. The unit has 672 electro-magnetic force actuators to change the shape of the 1.6mm-thick and 911mm-diameter Zerodur shell. The actuators control the mirror figure using the position feedback from the internal metrology provided by co-located capacitive sensors. The on-board real-time control electronics has a parallel computational power of 163Gflop/s providing not only the internal control of the unit with a 72kHz loop but also the wavefront reconstruction for the 1kHz Adaptive Optics loop. The paper describes the final configuration of the system and reports the results of the characterization and optimization process together with the results of the laboratory acceptance tests.

Adaptive optics wavefront corrector using addressable liquid crystal retarders

For the 8m astronomical telescopes which will see first light in the 90s, interest in adaptive optics correction of the wavefront degraded by atmospheric Wrbulence is very great. A wavefront corrector is described in this paper, in which we present an approach based on liquid crystal material. Some of the properties in favor of using this technique are analyzed, together with some experimental results on a prototype. The great advantage of the liquid crystal approach is that a large number of actuators can be obtained in a 1-2 inch device, at a cost orders of magnitude lower than the conventional mirror-piezoactuator device.

AN ULTRA-LIGHTWEIGHT, LARGE APERTURE, DEPLOYABLE TELESCOPE FOR ADVANCED LIDAR APPLICATIONS

This work presents a new technological concept for large aperture, lightweight, telescopes using thin deployable active mirrors, currently under a feasibility study for spaceborne Lidars. The study is mainly addressed to a DIAL (Differential Absorption Lidar) at 935.5 nm for the measurement of water vapour profile in atmosphere, to be part of a typical small ESA Earth Observation satellite to be launched with ROCKOT vehicle. A detailed telescope optical design will be presented, including the results of angular and spatial resolution, effective optical aperture and radiometric transmission, optical alignment tolerances, stray-light and baffling. Also the results of a complete thermo-mechanical model will be shown, discussing temporal and thermal stability, deployment technology and performances, overall mass budget, technological and operational risk and system complexity.

Adaptive optics and site requirements for the search of Earth-like planets with ELTs

Since 1995, expolanets discoveries have triggered a renewal of the permanent question about the possible presence of life outside the solar system. Direct detection and characterization of earth-like extrasolar planets orbiting main-sequence stars are now among the most exciting and challenging astronomical topics where new major scientific results from space missions and also from ground-based ELT are expected. To scale the performances of an ELT for exoplanets searching, we examine the relative impact of three fundamental parameters (the actuator pitch, the telescope diameter and the site) on the image contrast. Then, we calculate the planet/star flux ratio needed to reach SNR=3 in 10h (only the photon noise is considered) from long-exposure AO-PSF computed with PAOLA (a fast analytical code developed by one of us: L.J.) for different ELT sizes and AO parameters under different observational conditions (atmospheric turbulenece, star magnitude) with or without coronagraphy. We find that an actuator pitch of 0.1 m is optimal for exoplanet searching in the visible and near-IR from 10 to 40 pc. Lastly, we emphasize that the site choice is as important as the telescope size choice is: a 15m telescope is just enough for coronagraphic search for exo-earths at 10pc (SNR=3 in 10h) from the promising Dome C (Antarctica), while a 30m ELT is needed to succeed the same observation in the same time from the Mauna Kea.

Adaptive optics wavefront corrector using addressable liquid-crystal retarders: II

We present recent results of our investigations into the feasibility of a wavefront corrector for adaptive optics, using nematic liquid crystal material. In particular, we are aiming at a small, flexible adaptive optics module, to be located at the focal plane of existing and new-generation telescopes in order to improve imaging quality. We address some of the specific problems investigated, concerning the theoretical behavior of the LC corrector, its comparison with other state-of-the-art wavefront correctors, modal and zonal, the speed and slew rate of the actuators, and a real-time capacitive servo loop scheme controlling the actuator retardance.