The fringe and flexure tracking system for LINC-NIRVANA: basic design and principle of operation

Abstract

ABSTRACT LINC-NIRVANA is the interferometric near-infrared imaging camera for the Large Binocular Telescope (LBT).Operating at JHK bands LINC-NIRVANA will provide an unique and unprecedented combination of high angularresolution ( 9 milliarcseconds at 1.25 µm ), wide “eld of view ( 100 arcseconds 2 at 1.25 µm ), and largecollecting area ( 100 m 2 ).One of the major contributions of the I. Physikalische Institut of the University of Cologne to this projectisthedevelopmentoftheFringeandFlexureTrackingSystem (FFTS). In close cooperation with the AdaptiveOptics systems of LINC-NIRVANA the FFTS is a fundamental component to ensure a complete and time-stable wavefront correction at the position of the science detector in order to allow for long integration times atinterferometric angular resolutions.Using a dedicated near-infrared det ector array at a combined focus close to the science detector, the Fringeand Flexure Tracking System analyses the interferometric point spread function (PSF) of a suitably brightreference source at frame rates of several hundred Hertz up to 1 kHz. By “tting a parameterized theoreticalmodel PSF to the preprocessed image-data the FFTS determines the amount of pistonic phase dierence andthe amount of an angular misalignment between the wavefronts of the two optical paths of LINC-NIRVANA.For every exposure the correcting parameters are derived in real-time and transmitted to the respective controlelectronics, or the Adaptive Optics syst ems of the single-eye telescopes, which will adjust their optical elementsaccordingly.In this paper we present the opto-mechanical hardware design, the principle of operation of the softwarecontrol algorithms, and the results of “rst numerical simulations and laboratory experiments of the performanceof this Fringe and Flexure Tracking System.Keywords: LBT, instrumentation, interferometry, fringe tracking, piston