Antennas and Total Power

The prototype antenna contractors have been selected. We therefore concentrated on recommendations for testing procedures and antenna issues that impact other areas. We considered the priorities when testing the prototype antennas. For the prototype tests, we stress the following points.

• It is extremely important to test whether and under what conditions the pointing specifications (06) are met. Developing observational strategies aimed at optimizing the pointing is an important goal. In particular, one should examine the possibility of installing optical telescopes on all antennas, together with a servo system allowing real time pointing corrections. It seems likely that such systems are only effective if they are planned as part of the system and the committee recommends therefore that a system of this type is considered soon.
• It is also very important to have some method of recovering zero spacing flux using all or part of the array operated in single dish mode (see Appendix D). The committee recommends that a detailed comparison be made of the relative merits of using nutators switching rapidly (10 Hz) and On-The-Fly (OTF) mapping. A decision on the best strategy for ALMA should be made subsequent to these tests. In particular, one should test whether rapid OTF mapping (e.g 30 scans in 1 sec with 1 second turn-around) is feasible and whether gain stability ($\Delta G/G$) of order 10^-4 per second can be attained. Tests should also be made with the water vapor radiometer (WVR) in order to assess the ability of the WVR to monitor atmospheric emission fluctuations. Analogous studies are needed to test how effectively chopping with a simple nutator eliminates atmospheric fluctuations. Equipping each prototype antenna with a nutator will facilitate these studies. With this information in hand, it should be possible to decide whether nutators are, or are not, necessary for the array antennas. The general opinion of the ASAC was that if one could reach the scientific goals without using nutators, this was preferable. Thus one should aim at a system that could do an OTF map with all 64 antennas simultaneously.
• Polarization measurements are also sensitive to missing zero spacing flux (see Appendix C), and thus it should be possible to do polarization OTF at at least 2 ALMA frequencies. The decision discussed above (OTF versus nutators) may be different if one is measuring polarized flux and thus a test of polarization OTF is desirable.