The ASAC strongly supports the participation of Japan in the ALMA project as a full partner. This participation will enhance the baseline project in important ways. The ASAC considered the following enhancements: addition of the Atacama Compact Array (ACA), ensuring the full complement of receiver bands, additional antennas, and an enhanced spectroscopic capability. The ASAC was asked to prioritize these possible enhancements. While further definition is needed, at this point the prospect of the ACA and complete frequency coverage have the highest priority because they both enable new science to be done. The ACA, a separate array optimized for short spacings, will improve image fidelity for extended sources. Adding receiver bands will give access to all the available atmospheric millimeter and submillimeter windows. Additional array antennas and the enhanced correlator both improve the sensitivity of ALMA, making operation more efficient. The enhanced correlator would also add flexibility to spectroscopic observations in all the receiver bands.
The priorities for the receiver bands and the correlator will be discussed more fully in other sections of this report. Here, we discuss the ACA. The scientific case for the ACA is based on the fact that it will greatly improve the dynamic range and the image fidelity for extended sources. Indeed an array of smaller dishes will add the spacings that lie between the spacings well sampled by the array of 12-meter antennas and those sampled by the single dish, total-power measurements. This addition of spacings is best done with an array of dishes roughly half the diameter of the main array dishes. Based on studies by Guilloteau, Welch, and Morita, there is general agreement on the range of antenna number (10-16) and sizes (6-8 m) for the ACA. Generally, smaller diameters are better for imaging, as long as the minimum spacing is set by antenna size, while larger antennas are better for calibration. Because the receiver cabin must be able to contain the full ALMA receiver complement, it may be efficient to use the 12-m mount design with a smaller dish. Studies are needed to determine the dish diameter at which the minimum separation is limited by mount, rather than dish size. It is also generally agreed that the ACA must be able to operate as a stand-alone array. The ASAC recommends that the ALMA project ensures that the following studies are done in time for reports in January 2001, to be consistent with the timeline calling for a draft agreement for a three-way partnership by February 2001. The studies needed are these:
The ASAC should discuss the reports resulting from these studies and finalize scientific priorities for the enhanced ALMA before definition of the enhanced ALMA, currently scheduled for January 2001.