LBC onboard LBT: the optomechanics
LBT is a binocular telescope whose first light instrument are a pair of Prime Focus stations. These, operating at the exceptionally fast F/1.14 focus of the two 8.4m diameters mirrors, are the fastest wide field camera for 8m class telescopes. Moreover, because of the unique twin mirrors configuration, the choice of splitting in wavelengths the two cameras makes possible to design a superb Blu Prime focus station. This, with lenses in fused silica, allows for excellent optical quality in the U band. This is, so far, the only U-band wide field camera for an 8m class telescope. The optical and optomechanical design, the procurement of blanks, their polishing, the procurement of the mechanics assembly, the integration, shipment and alignment at the telescope, has been full responsibility of the group now based in Padova.
The project involves four Observatories where the CCD package, including the electronics, further to the management of the project, has been in the hands of the PI Emanuele Giallongo at the Observatory of Rome. The design is originally inspired to the classic three lenses Wynne design, although the challenging specifications requires to expand the design to six lenses, with a strong aspheric surface. The main lens is in excess of 800mm in diameter and its weight exceed 100kgs. The design included optimization for the blue channel with all lenses in Fused silica for the nominal wavelength range from U to B, and the red channel with cheaper BK7 glass for the remaining V, R and I bands, although with modification of the last (cryostat) lens and with a smaller reduction in the covered Field of View, the optomechanics is suitable to cover the very Near InfraRed bands from J to H.
The optomechanical designs included a number of system for supporting the big lenses, including some thermal compensating devices and some leverage mechanisms in order to ensure the lenses are safely kept in position within their tight tolerances in all the conditions of temperature and gravity load that encounters in their life. The lenses, manufactured by Reosc-Sagem, in France, has been shipped to Italy with a special box we designed, and then integrated in clean room on the barrel. Integration included an interferometric alignment of the whole set of lenses. Shipped to Arizona in a special container with a damping system the Prime Focus units have been mounted on the telescope and aligned with the main mirror. Finally, an Active Optics system has been developed in order ot maintain the quality of the system constant under different operational scenarios. Images quality as good as less than half arcsec in the U band has been successful demonstrated on the sky. Nominal optical quality of the instrument has been mantained to the level of 80% of the Encircled Energy into a single pixel.
People: A. Baruffolo, J. Farinato, G. Gentile, R. Ragazzoni
Collaboration: INAF OA Roma, INAF OA Bologna, INAF OA Firenze, INAF OA Trieste, LBT Observatory, Arizona