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ARENA project

Non-belspo (2006-2008)

ARENA - Design and construction of two small size telescopes in Antarctica

Project description

1. Overview of the project:
The Antarctic Plateau, especially the Dome C area, seems to provide unique environmental conditions for astronomical observations. Therefore, astrophysicists from all over the world are seriously considering Antarctica as a possible site for the construction of a new large observatory, mainly dedicated to infrared and high angular resolution observations to investigate extra-solar planets, their formation and evolution. Due to the high infrared transparency and low background noise, Antarctica will provide excellent observational conditions for various astrophysical topics such as stellar evolution, galaxies and cosmological surveys.

In this context, the Institute of Astrophysics and Geophysics (Liège University, IAGL) has become an active member of the EII (European Interferometry Initiatives, 1/1/2004 - ) and of the ARENA (Antarctic Research, a European Network for Astrophysics, 1/1/2006 - ) projects that have been selected under the EU FP6. In particular, J. Surdej is the leader of the EII Work Package “Developing the vision for a next-generation interferometric facility” and J.-P. Swings is the leader of the ARENA Network Activity “TOWARD LARGE ASTRONOMICAL INSTRUMENTS IN ANTARCTICA”. Furthermore, the IPY (International Polar Year) program committee has endorsed the “AstroPoles” proposal submitted by a large international consortium of which IAGL is one of the member institutes. We attach a copy of the letter that we have received from the ICSU/WMO Joint Committee for the International Polar Year 2007-2008 with the expectation that it will help to support the present application for funding from BELSPO.

In the present application, we propose that BELSPO finance the design and construction of two small size telescopes that will allow 1) to directly image with a simple coronagraph the surroundings of bright stars with the possible detection of new disk features and young giant planets around nearby stars, 2) to calibrate at various heights above the ground (0-40 m) the atmospheric turbulence residuals, 3) to perform relevant tests concerning the injection of stellar light in an optical fiber that will be part of an interferometer project at Dome C and, finally, 4) to combine with optical fibers the light from the two unit telescopes in the interferometric mode.

The total cost estimate of this project is 1.65 million €.

2. Scientific and technological aims of the ARENA Consortium:
ARENA is a consortium of some 20 Research Laboratories, Universities, Funding Agencies, Polar Station Operators and Industrial Companies from 7 countries of the EU, and Australia, involving more than 100 collaborators. It is the first European initiative to coordinate astrophysical programmes and related infrastructure implementation in Antarctica. It is essentially aimed at fostering cooperation between small groups pertaining to entities that are already involved in existing Antarctic astronomical programs, or planning to start new programs. In this framework, IAGL proposes to construct two small size telescopes in order to further characterize Dome C in the heart of Antarctica as well as to perform challenging astronomical observations of bright stars in the coronagraphic and interferometric modes.

Two member states, Italy and France, have recently completed the all-year round operation at the CONCORDIA station at Dome C. The ARENA project’s aim is to structure, reinforce and network several European laboratories (among which IAGL) and operators involved or willing to be involved in Antarctic astronomical investigations with the final objective to set up new large instruments and focal plane equipments in the coming decade. The network consists of 4 activities in addition to the overall management, NA1 : i) NA2 concerns the site qualification, ii) NA3 aims at identifying the technical constraints on the design of large astronomical instruments for polar environment, emphasizing robotization (J.-P. Swings is the leader of this NA), iii) NA4 will foster collaborations between the operators of CONCORDIA and their users to add value to CONCORDIA, iv) NA5 will focus on the selection of appropriate key scientific programs, implemented in synergy with other ground based observatories and space missions. The funds granted by EC will contribute to the currently on-going site qualification, and will help in structuring the still dispersed and scarce teams of European astronomers through organizing specialized workshops putting operators, users and industrial partners in close contact, and annual large international Conferences. The networks will deliver a detailed report on the site qualification over a period of 5 years, industrial reports and studies addressing the problems of building large instruments at Dome C, a book of carefully selected scientific key-programmes, and recommendations on instrumental development. IAGL proposes to contribute to this effort by providing two small size telescopes.

3. Scientific and technological objectives achievable with the proposed small size telescopes
Thanks to the very low atmospheric turbulence above the ground layer (~40m) at Dome C, small telescopes will be diffraction-limited in the near-infrared.
The best resolution achievable with a 50-cm telescope working in the near-infrared H band (1.6 micron) is therefore around 0.6 arcsec. In the R and I bands (0.7 – 0.9 micron), a simple tip-tilt correction within the instrument can provide sufficient image quality for the coronagraphic studies of faint features around bright stars. Thanks to the low wave front aberrations, new coronagraphs like the Four Quadrant-Phase Mask (Riaud et al. 2003, PASP 115) or the Annular Groove Phase Mask (AGPM; Mawet et al. 2005, ApJ 633) could achieve their true potential in terms of dynamic. Their very low Inner Working Distance should allow peering into the central stellar peak well under the diffraction limit with a contrast at the 103 level. The full discovery space of the AGPM could unveil circumstellar features like debris disks around bright young stars. Detection of substellar companions like brown dwarfs and young extrasolar planets should therefore be feasible.

Manufacturing and implementing new generation phase mask coronagraphs on fairly conventional telescopes will technologically pave the way towards bigger projects like the Extremely Large Telescopes (ELT). Moreover, coronagraphs are currently being envisaged as new concepts for wave front sensing systems. Research in this domain would benefit a lot from an actual implementation.

In addition to these scientific objectives, the 50-cm telescopes are intended to work as a test facility for future interferometric projects in Antarctica. The main goal is to validate the essential subsystems of future interferometers, i.e., starlight collection with an off-axis telescope, subsequent injection into a single-mode optical fibre and beam combination (Absil & Mawet 2006, Proc. JENAM). Beam transport by means of single-mode fibres is currently considered as the most promising technique for future long-baseline interferometers, for which beam transport in open air would raise severe issues such as diffraction and ground-layer turbulence. Studying the variability of the injection efficiency will also tell us whether real-time intensity compensation is necessary to achieve high stability and thus high performance in future interferometers, which aim at extrasolar planet detection down to a few Earth masses. Interferometric observations of bright stars will be achieved by combining the two unit telescopes with optical fibres and an integrated optics beam combiner (currently under development). Once validated, the small interferometer will eventually aim at the detection of faint circumstellar features such as debris disks, brown dwarfs or planets, benefiting from the exceptionally low turbulence at Dome C.

4. Technical proposition
The scientific and technological objectives described above will become achievable thanks to the operation of two identical small size telescopes mounted on a movable tower 40m high. This tower will be provided by members of the ARENA Consortium.

The unit telescopes will be of the Cassegrain type ( parabola-hyperbola combination) or Ritchey-Chrétien (hyperbola-hyperbola combination) with a reduced field of view of 1 arcmin., suitable for the proposed coronagraphic and interferometric mode observations, and a primary mirror of 500 mm made of zerodur. The focal length of each telescope will be 4m (f#8: scale of 0.05 arcsec/micron) for their use with a mono mode optical fiber of 4 microns. It will be possible to change the focal length (up to 12.5m, f#25) thanks to a coudé train for its use with a VIS camera (scale of 0.15 arcsec/pixel with a pixel size of 9 microns) or IR (scale of 0.45 arcsec/pixel with a pixel size of 27 microns).

The telescopes will be equipped with an Alt-Az mount. They will be optimized for interferometric observations in a very low temperature environment alike that prevailing at Dome C in Antarctica. The operation of the telescopes will be fully automatic.

The cost estimate for the construction of the two telescopes is 1 MEuros for the first one and 650 000 Euros for the second one. Total cost is thus 1.65 MEuros.

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