NIRVANA onboard LBT: the wavefront sensing package
|Figure 1: Opto-mechanical layout of NIRVANA|
NIRVANA stands for Near InfraRed and Visibile Adaptive iNterferometer for
Astronomy. It is the instrument that, mounted on the Large Binocular Telescope
(LBT), will provide imaging and, potentially, spectroscopic capabilities, with
a resolution equivalent to the longest baseline of the LBT telescope, amounting
to roughly 24m. As the two mirrors of LBT are mounted on the same mechanical
structure and NIRVANA will be rigid with respect to these, the main task of
this instrument is to clear out the collected starlights from the aberrations
introduced by the atmospheric turbulence and to combine interferometrically the
two beams, making these forming an interference fringe pattern inside a
cryogenic camera. The instrument is designed to take advantage of the unique
features of LBT. In fact it will use the two secondary Adaptive Mirrors, that
will introduce the compensation for the ground layer turbulence. A further
correction for the high altitude turbulence will be introduced by two
piezo-stack deformable mirror mounted aboard the instrument.
The two arms of NIRVANA, in fact, will employ, each, a so-called Multi-Conjugated Adaptive Optics system (MCAO) allowing for atmospheric turbulence compensation over a Field of View as large as nearly 2 arcmin in diameter. The two beams are then co phased by the means of a fast recombination mirror, able to take into account the difference in path-length introduced in the atmosphere by the two different volumes encountered by the starlight in its journey to the telescope's mirrors, and will provide a characteristic fringe pattern, equal for all the stars in the Field of View. This optical configuration, called Fizeau interferometer, is also unique in the realm of large (8m-class) telescopes since that, usually, other interferometers exhibit a Field of View that barely exceeds one arcsec.
|Figure 2: Nirvana bench|
In order to produce the above described compensation a combination of two wavefront sensing units, one for each arm, is foreseen. The basic choice is to avoid the use of laser guide stars, both for budget and for technological reasons. The compensation will be made using as references up to 12 natural guide stars to be found in an annular Field of View of 6 arcmin in the outer diameter and 2 arcmin in the inner one. Compensation of the high altitude layers will be provided by two dedicated wavefront sensing units, looking to the central 2 arcmin Field of View, where up to 8 reference stars can be used. These are operated in a remote way, such that the environment is kept unperturbed by all these complex operations. The co-phasing sensor will be located inside the cryogenic camera, such that differential flexures that are hard to trace down due to differential flexures and/or thermal expansion are easily controllable.
|Figure 3: Ground Layer Wavefromt Sensor handling device to be used in Padova|
It is foreseen the possibility to implement more than one way (for instance
through a fiber-fed set of Integral Field Units) for the upgrading of the
instrument with spectroscopic capabilities.
The instrument successfully passed the Conceptual Design Review Phase (early
2003) the Final Design Review phase in spring 2004, and it is in its
construction phase. As the instrument is a collaboration between Italy and
Germany, the Italian responsibility is mostly focused on the areas where its
expertise is internationally recognized. In fact most of the Italian
contribution is mainly concerning the MCAO system and the opto-mechanics of the
wavefront sensing parts.
In particular the four units of Wavefront sensing (two per arms) are taking advantage of the conceptually similar systems developed in collaboration with the European Southern Observatory for the Very Large Telescope. Further to take advantage from such a project, the Italian contribution pushes forward the technological development in such a field, as, in fact, the Field of View where natural stars can be searched is enlarged in area by almost an order of magnitude, as well as the pupil sampling, and the interferometric capabilities are explicitly considered in the design of such challenging sensing units. The Principal Investigator and the Project Manager of the Italian institutes involved (The Observatories of Padova, Bologna, Rome, Firenze and the University of Genova) belong to Padova Observatory, which is also charged of the construction of the two Ground Layer Wavefront Sensors for NIRVANA.
In 2008 the mechanical design of the Ground-Layes Multi-Conjugate Adaptive-Optics sensor (GWS) has been finalized. In OAPd laboratories the optical components received up to now (pyramids, lenses) have been tested and the integration of the opto-mechanics has started.