Study and Prototyping of a Super Station for LOFAR in Nançay – LOFAR

We propose the detailed study and prototyping of a Super-Station for the LOFAR radiotelescope (LSS). France is inthe process of purchasing a LOFAR station to be installed in Nançay.; it thus participates to the europeanization of this radiotelescope of new generation, precursor of SKA and other instruments of the future. This acquisition will permit french scientists to join the scientific exploitation of this instrument, which consists of tens of stations in Holland and other european countries. The proposed study will permit to consider a new and entirely original development, that would strongly reinforce the scientific weight and recognition of the french scientific and technical participation in LOFAR: rather than, as our british or german partners, seeking to multiply the number of "standard" stations in France, our concept would consists in a radical transformation of the Nançay station. This possibility relies on the existence of a national funding agency such as ANR, rather than individual institutes buying individual stations as in other countries. A "standard" LOFAR station includes an electronic back-end to which are connected 96 low-frequency and 96 high-frequency antennae, which it uses alternatively. Our concept consists in adding a new option with 96 mini-arrays, with analogic phasing, of N antennae (with N~10), with each of the arrays connected to one channel of the back-end (which would thus need little or no modification). For a limited cost, since the antennae are simple and cheap, we could thus multiply bu N the effective area of the standard station. This would permit, on one hand, to strongly increase the sensitivity of LOFAR (the whole instrument) for weak sources at small angular scales (long baselines); on the other hand, in the frequent periods when LOFAR uses only the stations of the dutch core, 1) a stand-alone use, giving an instrument with very interesting possibilities (e.g. for the detection of new pulsars, cf. associated scientific document) ; 2) innovative observing modes, forming a second core that can be correlated with the other european stations (the Blue Gene supercomputer in Gröninen will have the capability of correlating this new instrument in parallel with the correlation of the dutch core); 3) visibility measurements over short baselines (large angular scales) not accessible to the standard LOFAR, by correlating the signals from LSS individual mini-arrays. This concept, presented during international workshops in Paris (1/2008) and Hambourg (9/2008) has had an extremely positive reception from our european partners (6 countries represented); some of them (Ukraine, Austria) might join our developments and are associated to the present proposal. The concept leaves a wide freedom in the choice of the antennae, their frequency range and the geometry of the arrays. An important advantage for us is that, in addition to their recognized technical excellence, the Nançay engineers already have a strong expertise of the LOFAR back-end, a version of which is used for the EMBRACE demonstrator of SKA-DS. Our proposal thus covers: (a) a specific science case, which should confirm the interest of this development, (b) a detailed study of the concept (selection of antennas, optimisation of array distribution/geometry, phasing, command and control electrnics), (c) the realization of a prototype formed of 2 to 3 mini-phased arrays of N antennae, to qualify the concept, (d) a detailed study of the new observing modes, and of the calibration methods. These studies will benefit, in particular, from the contribution of post-doctoral researchers who will have had the opportunity of participating in the first stages of the scientific exploitation of LOFAR, starting in 2009. The main results of our work, if the conclusions of our studies (feasibility and scientific interest) are positive, will be, at 2 years: a full costed project of a LSS in Nançay, to be proposed to the funding agencies on the basis of an associated science case, the 2 to 3 prototype modules; at the end of the project, a detailed technical design of the LSS, an observation programme and a precise proposal to our dutch and other european partners for specific observing modes, a negociation with ASTRON on the corresponding increase of our participation in the scientific return of LOFAR. The proposed development relies strongly on the recognized expertise of the Nançay engineers, to whom it offers a new investment in the radiotelescopes of the future. For a limited cost (that we estimate lower than that of an additional standard station) il will permit to multiply the scientific and technical impact of the french scientists in the european LOFAR collaboration, and in the worldwide community that actively prepares the Square Kilometer Array.