The adaptive secondary mirrors for the Large Binocular Telescope: a progress report A. Riccardi a , G. Brusa b , M. Xompero a , D. Zanotti a , C. Del Vecchio a , P. Salinari a , P. Ranfagni a , D. Gallieni c , R. Biasi d , M. Andrighettoni d , S. Miller e , P. Mantegazza f a INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy b Steward Observatory - University of Arizona, 933 North Cherry Avenue, 85721 Tucson (AZ), U.S.A. c ADS International srl, Via Roma 87, 23868 Valmadrera (LC), ITALY d Microgate srl, Via Stradivari 4, 39100 Bolzano, ITALY f Mirror Lab of Steward Observatory, 257 National Champion Drive, 85721 Tucson (AZ), U.S.A. e Dipartimento di Ingegneria Aerospaziale - Politecnico di Milano, Campus Bovisa Sud, Via La Masa 34, 20156 Milano, ITALY ABSTRACT The two 911mm-diameter adaptive secondary (AS) mirrors for the Large Binocular telescope (LBT) are currently under manufacturing process. Each unit has 672 electro-magnetic force actuators. They control the figure of the Gregorian secondary 1.6mm-thick mirrors with an internal loop using the signal of co-located capacitive sensors. The obtained computational power of the on-board control electronics allows to use it as real-time computer for wavefront reconstruction. We present the progress in manufacturing and assembling of the first telescope unit, the progress in software production, the status of the testing facilities and an update on the latest modification of the design. Keywords: adaptive optics, wavefront correctors, adaptive secondary 1. INTRODUCTION Large Binocular Telescope 1 (LBT) is equipped with two gregorian adaptive secondary (AS) units (LBT672a and LBT672b). Each one joins the functionalities of a conventional secondary mirror for seeing limited observations, of an active optics and field stabilization compensator, of a chopper for Mid-IR observations and, last but not the least, of a 672-actuator deformable mirror for Adaptive Optics correction. Because of the location of the ASs, they represent the common AO deformable mirror for all the 4+4 focal stations of the telescope. In particular they perform AO correction for the the first light adaptive optics system 2 (W@LBT) in the front bent-gregorian focal stations serving the spectro-imager Lucifer, 3 for the IR interferometer with nulling capabilities 4 (LBTI) in the central bent-gregorian focal stations and they act as ground layer correctors for the visible-NIR multi-conjugated interferometer 5 (NIRVANA) located in the back bent-gregorian focal stations. The adaptive secondary (AS) mirror technology is an on-sky proven technology since late 2001 with the first light of the 336-actuator AS unit for MMT 6, 7 (MMT336). CAAO (University of Arizona, AZ, USA) routinely produced with MMT336 more than one year of scientific results. 8 The two 672-actuator AS units for LBT (LBT672a and LBT672b) represent the new generation of the current AS technology. Their design is based on the experience gained with the MMT unit, but both optical manufacturing procedure and electro-mechanics design have been revised, improving performances, stability, reliability, maintenance and computational power of the system. 9, 10 The partners involved in the project are almost the same team of companies and institutes of the MMT336 project. Microgate (Italy) is in charge of the design and production of the electronics and the related microcode. Further author information: (Send correspondence to A.R.) A.R.: E-mail: riccardi@arcetri.astro.it