The responses I have received have been mixed in their opinions. The points they have made have generally been good and varied on both sides of the issue. For those who have been in favor of the single size approach, most seemed to be worried about the problems created by trying to make images with antennas of two sizes. These problems include worries about the quality of the resulting images, the lack of adequate software for this purpose, and the complexity for the astronomer who must reduce the data and make the images.

Those in favor of the heterogeneous approach primarily had two general reasons. First, several think that antennas larger than 8 meters would be very difficult to use for their favorite experiments. Examples are solar, planetary, Sunyaev-Zeldovich, galactic objects requiring large mosaics, and submillimeter observations. Second, there are people who think that there are advantages for antennas of two sizes in making images of complex, extended sources.

In practice, this is a hard decision and I don't know if any of us can be completely sure about the right answer. Much depends on technical questions about the detailed properties of the antennas and how much they cost. We will know more over the next few months but not everything we might want to. Also we can provide some more information about the imaging quality, the software problem, and the expected sensitivity for various cases. However, we will not be able to do this exhaustively and in the end we are going to have to make educated guesses about how a given version of the instrument will work.

There are two sorts of considerations to consider 1) purely scientific issues, and 2) practical considerations

1) Science
To help with the scientific issues we are setting up a web page under the NRAO MMA site to provide more information for those interested in the detailed studies we are making. The location is

http://www.tuc.nrao.edu/mma/mmalsa

or one can just follow the path to it from the NRAO homepage

http://www.nrao.edu

clicking on "MMA" and then on "MMA-LSA Collaboration".

Included now are two memos: one discussing the sensitivity issues for various different regimes from low to high resolution and a second presenting the first few simulations with pointing errors of mosaicking observations with different size antennas and pointing errors. We will add to this when we have more material.

2) Practical Considerations
By this I mean issues involving funding, the timescale for the project and the wishes of the European community which might not always exactly correspond to the U.S. consensus.

a) Funding
The idea of the collaboration is for each side (US and European) to contribute half the resources necessary to build and operate the array. This requires both sides to produce their share of the money. This process is just beginning in Europe and might fail. Also, as all of us know, our funding from the US is not certain, although things seem to be going well at present. Thus both groups need a fallback position. Thus if we start down the road to a given project, perhaps both sides need a viable fallback project if the other side is forced to dropout. For the heterogeneous project that might be the 40 8m we originally proposed. For the homogeneous array that might be an array of 25-30 12m's. Are either or both of these acceptable ?

b) Timescale
Right now the U.S. plan for an 8m is fairly far along. The plans will be reviewed in any case given what we learn from the Europeans and this may slow us down a little bit. But the current feeling is that going to a 12m or larger antenna size may require a new design. The European's have discussed a two year design phase for their 15m concept. They also want to study scaling down this design to a 12m. Perhaps this could go faster but as we increase the size of the antenna, the complexity of the design, in particular the pointing system, increases. With the heterogeneous concept we can probably go ahead quickly with the 8m antenna and spend a bit more time perfecting the 15m design. The homogeneous concept with a 12m antenna, requires that the U.S. and Europeans converge on that design before we can start to build the project. This could slow down the project significantly. Is this acceptable ?

c) Consensus
The Europeans come into the project wanting significantly more collecting area than the original MMA plan. So far, the U.S. community, although certainly in favor of collecting area, has put more emphasis on quality imaging. Both groups now want a submillimeter capability. Thus we need to find a solution which does the best job of achieving all these goals. Which design is better at doing that ?

I welcome comments on these and other considerations for the combined project at mmalsa@nrao.edu

Frazer Owen
MMA Project Scientist