Configurations
Al reported on the Imaging and Calibration Group configuration meeting of 9 June. Present at this meeting were T. Helfer, L. Kogan, M. Yun, B. Butler, S. Radford, F. Viallefond, J. Richer, A. Wootten, and Morita. No definite recommendations have been agreed upon within the committee. Simulations are proceeding for several configurations. A priority of this committee is to assemble a set of images that represent the range of imaging requirements likely to be required of ALMA. Suggested images are the VLA CasA and M51 images; the Horse Head Nebula, Orion and other HST images, the Orion Ridge image (Bally). Any others that members of the MAC or the community would like to submit to this committee would be welcome. They will use these to simulate the response of different configurations to large dynamic ranges, scale sizes, and faithfulness of reproducing the input brightness distribution. The effects of different UV coverages, sidelob response, etc. on the accuracy of reproducing the input image are being examined. THIS COMMITTEE WOULD BE GLAD TO RECEIVE IMAGES FOR THIS PURPOSE. Send them to either Brian Butler or Tamara Helfer.

There will be further discussion of this at the URSI Meeting on Aug. 14th. Those of you who plan to attend the URSI meeting should put this on your list of sessions to attend. This is a very important issue that deserves our full attention.

Water Vapor Radiometery
Al summarized the current efforts to study the ability to correct for atmospheric phase fluctuations using water vapor radiometry (wvr). Programs are actively being pursued at OVRO (22 GHz line), BIMA/U.Md (22GHz line), VLA (22 GHz line), AT (22 GHz line), IRAM (simultaneous 220 GHZ and 3 mm continuum differential radiometry), NRO (22 GHz line and continuum differential radiometry), CSO-JCMT (183 GHz line), Chajnantor (183 GHz line), and SMA (183 GHz line, Chris Wilson). This summary can be found at: http://www.cv.nrao.edu/~alwootten/mmaimcal/wvrnotes.html

Since water vapor has a dispersive component above about 400 GHz, the question of whether wvr can be reliably used at the highest frequency bands of ALMA was raised. If so, this might permit some relaxation of the specs for fast switching which could save substantial dollars. Richard Hills has suggested that the dispersive component can probably be modeled with adequate accuracy to be useful even at 850 GHz (see Richard's email of 7/30). Richard pointed out that present experiments using the 183 GHz line on Chajnantor with a 2000 K (DSB) mixer with 3 channels appropriately offset from line center appears to be giving corrections close to what is needed. Clearly, this issue can only be resolved with much more experimental work which we hope to hear more about in the coming months.

NSF Audit
Bob Brown gave a summary of the NSF Audit of the "US Reference Project" which took place in Charlottesville on 7-9 July. Although it was recognized by everyone that noone expects this particular facility to be built, the main reason for doing this was to review the process proposed to manage and build the project by experts who have managed other major projects. An expert review can provide confidence in the project, isolate weak points in the project plan,and highlight the strengths of the project.

Day 1 was spent describing how the telescope would be built; on day-2 the 11 division heads were broken out into to separate groups to describe their work, the result of which was a written report by each; on day-3 variances from the division reports were discussed. Two main concerns of the committee were system integration and management. Their concern about system integration was that they felt not enough thought had been given to how the system would be brought on-line in Chile. Their concern with management was that not enough checks are built in to recognize potential problems early so that appropriate action can be taken to avoid or minimize them. The latter will require recruitment of the right people.

The committee was very impressed with the technical areas of the project and the details of the project definition. They judged that the technical risk for the project was very low. They also felt the costing of the project was good. The committee raised the question of how the project would be descoped if the budget were cut. The response was that the two 3 mm recievers (67-89 GHz FET, and 89-116 GHz SIS) would be combined into a single receiver of somewhat reduced bandwidth coverage (75- 116 GHz) and consideration would be given to eliminating the Q-band (7mm) receiver. These suggestions, of course, should be discussed with the community before any such decisions are made, as these are issues that affect the scientific capability of the facility. John Carlstrom and others in the cosmology community are concerned about the possibility of eliminating the Q-band capability of ALMA. Carlstrom pointed out the following advantages of an ALMA Q-band system:

• ALMA has half the VLA collecting area but about twice the aperture efficiency, making it about equally sensitive as the VLA for point sources but much faster for surveys due to its broader beam.
• Sensitivity should be about 3 microK per hour for continuum, making it excellent for distant clusters, etc.
• ALMA's southern hemisphere coverage compliments the VLA's northern coverage.
• The cost should be only about $50K/antenna with compressor for a separate dewar. This is only 1% or less of the cost of the project.
• ALMA will have many shorter baselines than the VLA can provide, offering a distinct imaging advantage for extended objects.
• A report of the audit is being written by John Peoples and Gary Sanders. In their summary they emphasize that the major issues now are:

1. How to work with the Europeans and Japanese to bring the ALMA project into being.
2. To determine as quickly as possible how much money will be available for construction and to define the project within this budget. This, of course, will have to be done in collaboration with the European partners (and the Japanese assuming that they will eventually become a partner).
3. To put management in place for the project as quickly as possible.

The consensus was that the audit will benefit the project in the long run.

THE NEXT MAC TELECON WILL BE ON WED. AUG. 25TH, SAME TIME AND NUMBER.