In the framework of a Memorandum of Understanding signed by ESO, OSO, NFRA and IRAM to jointly study concepts for a Large Southern Array (LSA) at (sub-) millimeter wavelengths, a Combined Report has been completed in April 1997 and widely distributed by ESO. If needed, additional copies can be obtained either through ESO or IRAM. The report summarizes various scientific and technical aspects, and also includes some site testing studies.
In the meantime, the LSA-project has been presented to the ESO Council as well as to IRAM's Executive Council, and encouragement was given by both to pursue this study into the next phase. In the European context, the study is timely in order to have the LSA project ready as a candidate for European funding after the VLT funding will be completed. In the international context, the study is timely because of the advanced planning state of the MMA project in the United States and of another millimeter array project by Japan.
Given this situation, the question arises, of course, whether the different projects will continue to be developed independently, or if one should not establish a common framework for all of them, including the possibility of even merging two or all three of them, last not least in view of the fact that all three projects have been considering high altitude sites in northern Chile.
On June 25 and 26, 1997, a delegation from Europe led by Riccardo Giacconi, Director General of ESO, and Roy Booth, Chairman of the Study Board for the LSA Project, met with a delegation from the US in Charlottesville to discuss the situation and possible future developments. The meeting adopted a resolution in which both sides agree ``to resolve to organize a partnership that will explore the union of the LSA and MMA into a single, common project to be located in Chile. Specifically, this partnership will study the technical, logistical, and operational aspects of a joint project. Of particular importance, the two antenna concepts currently under consideration will be studied to identify the best antenna size and design or combination of sizes to address the scientific goals of the two research communities. In doing so we will work through our observatories, utilizing the expertise in millimeter astronomy resident in research groups and institutions in our communities.''
It is in fulfillment of this agreed action that we are now starting a enquiry in both the European and the American communities to evaluate the respective merits of two possible options for the design of a joint array.
In Option 1, the combined project would consist of a hybrid array of 40 8-m
antennas and 25 to 35 15-m antennas. The 8-m antennas would have a
surface accuracy (25 
m rms) and pointing accuracy (1'' rms)
adequate for operation up to 800 GHz. The 15-m antennas would have
similar surface accuracy, but the pointing accuracy (1.5'' rms) may
only be adequate up to 350 GHz for high quality imaging by mosaicing.
In Option 2, the combined project would be an homogeneous array of
50 to 60 antennas of about 12 m diameter. The antennas would have
25 m surface accuracy and 1'' pointing accuracy, making them
suitable to high quality imaging of large fields up to 650 GHz.
In both options, the ultimate number of 15-m or 12-m antennas will depend on the cost.
A comparison of these two options shows that Option 1
Obviously the technical and scientific aspects of this alternative are quite complex; they are now subject of detailed discussions by specialized working groups.
Our purpose here is to obtain input from the European radio-astronomical community, on the scientific arguments in favor of one or the other of those two options. We would like you to examine quantitatively the respective merits of either option for the most demanding observing project(s) in your main field(s) of competence.
A typical list of questions, aimed at weighing the relative importance of high sensitivity and capability of wide field imaging, would be:
Please send your contributions by e-mail to: lsa@iram.fr.
Michael GREWING and Stéphane GUILLOTEAU