Report for ALMA Calibration and Imaging

                        August- September 2002

                       NA Progress Report

Overview

     During August, Science Group activities centered around arrival of the Vertex antenna, defining the configuration for ALMA, and the ASAC face-to-face meeting early in September. Wootten devised a budget and manpower plan for construction in consultation with van Dishoeck.

     During September, Science IPT activity centered on final definition of the ALMA Configuration, providing ASAC support for their face-to-face meeting, which occurred in Socorro 6-8 September, and on Calibration Group activities. In September, van Dishoeck was appointed IPT Lead for Science.

Figure 1 ASAC admires the VertexRSI antenna.

Several Science IPT staff telecons were held, on 3 and 24 August 2002, and on 3 September 2002, at which configuration progress and preparations for the ASAC face-to-face meeting were discussed, and on 24 September 2002, when the ACC/EACC meeting results were discussed, along with the Maastricht 2GC minutes, the upcoming Configuration Review and NRAO Director's Frontend Review, as well as calibration group progress and milestones for the group. The lack of 90-degree phase switching in the 2GC specifications is a cause for concern for ALMA Science.

All members of the NA Science IPT, as well as EU Science IPT leader van Dishoeck and Project Scientist Guilloteau, attended the ASAC Face-to-face meeting. Planning for the Science IPT during the construction phase of ALMA was laid out during that meeting, and continued during discussions

between van Dishoeck and Guilloteau in Munich the following week.

Anticipated problems:

Various discussions were held to plan for the prototype interferometer. Although most elements will be in place at the ATF by early 2004, the prototype receivers may not arrive until mid 2005 for testing late that year. The Scienct IPT believes that this must be done earlier. In particular, for one Level 2 milestone to be achieved:

December 2004: Review of tests of calibration strategies on

                prototype interferometer complete

the prototype receiver delivery should be moved up by a year. Differences between the evaluation receiver interfaces and those of the prototype receiver suggest that substantial work would be required to implement the evaluation receivers for the prototype interferometer. The project should

construct a plan for component verification at the prototype interferometer as soon as possible.

ASAC Meeting

Wootten prepared the agenda for the meeting, contacting speakers and inviting guests in accord with the plans made by ASAC Chair Pierre Cox and Vice Chair Christine Wilson. Butler handled organization of the meeting, including all local aspects. Radford, Holdaway and Mangum made presentations to the ASAC at the meeting.

Configuration

During this period, Configuration leader Conway delivered a plan for configurations covering less than 4km in extent to the JAO. Holdaway also received this and began designing the interface from the largest Conway array to the more extended Y+ configurations. Butler also worked on the

configuration design during this period.

Status of Y+ Configuration

The principles behind the Y+ configuration have been laid out in ALMA Memo 419. Arguments for the Y+ configuration, including operational simplicity, the natural intermediate resolution arrays, the sometimes superior imaging quality, and the potential to expand to longer baselines in the future, were compelling enough to move the ALMA project management to procure land to accommodate the Y+ configuration instead of the 14~km ring array, even though only a single rough study had been performed at that time.

  The Y+ configuration set is built out of the largest Conway spiral configuration by moving antennas six at a time out onto three wide arms which roughly make a ``Y''. The resolution is improved by about 30% with each such incremental reconfiguration until the antennas begin to be crammed against the outer boundary of the ALMA concession. This scheme permits either 36 antennas to be moved to achieve a 300~GHz resolution of about 16 mas, or 42 antennas to be moved to achieve a 300~GHz resolution of 13~mas. In either case, the maximum baseline lengths are about 18~km.

Large scale imaging simulations were performed using the 14~km ring array (14~mas), the Y+ incrementally optimized 36 antenna move (16~mas) and 42 antenna move (13~mas) arrays, and the Y+ ``fully optimized'' 42 antenna moves (12~mas) array. A small amount of data from the 4~km spiral array was added to provide superior short spacing coverage. These simulations indicate that the Y+ configurations are competitive with or better than the ring array for high fidelity imaging of complex sources. The imaging simulations also indicated that there was no loss in imaging capability by using a narrow-armed mask (leading to fewer roads and less cabling) instead of the

loose-armed mask. An imaging simulation document will appear in due course.

Questions were raised at the Sept 6-7 ASAC Meeting concerning the Y+ configuration's ability to perform adequately at its highest resolutions. Very simple imaging cases were investigated (ie, two point sources a variable distance apart, or two point sources with a moderately extended low brightness Gaussian component), and Holdaway investigated different array configurations' abilities to accurately determine the point source positions and fluxes. The Y+ configuration with 36 moved antennas does not compete very well with the 14~km ring (or other Y+ options) for sources separated by less than 20 mas (about 1.5 times the synthesized beam). When the point sources were separated by more than 20~mas, all of the configuration options did more or less similarly well (though the ring array did get into intermittent trouble as the point sources were further separated and they were sitting in each other's side lobes). Much of the difference among configurations at

the highest resolutions went away when I used the somewhat more complex image structure with the low brightness Gaussian, or when I added realistic phase errors (random residual phase errors, as might exist after fast switching phase calibration, and a much smaller systematic phase term). Nonetheless, some of the criticism of the Y+ configuration with regards to the highest resolution was justified. The 42 move Y+ configuration, with higher resolution, was more competitive with the 14~km ring, and I will adopt a 42 move Y+ configuration.

Where does the work stand now?

Since the incremental Y+ configurations grow out of the spiral configuration, the details of the Y+ configuration depend upon the details of the Conway spiral configurations. Since receiving that,

Holdaway has been working on the final Y+ configuration. This work differs from the previous version of the Y+ configuration set in that - more care has gone into the topographical mask. A new mask reflects the ALMA concession boundaries as well as the prescription used by Conway

   for the inner configut ations.

 - Not enough attention was paid to short baselines in the previous version of the Y+ configuration set. Two 40~m baselines allows good flux calibration. Good imaging requires that

   reasonable short baseline coverage between 40~m and 500~m be preserved.

 - The long baseline coverage of the Y+ configuration is being carefully redesigned. As stated, we will be forced to go to a 42 move Y+ configuration in order to do a good job with the highest resolution imaging.

Calibration

Butler, as leader of the Calibration Group, initiated memo review process for it. He also organized and moderated first ALMA Calibration Group phone telecon, on 27 August. He completed an independent review of ALMA Memo No. 372. These reviews–process and early submissions-- were discussed during the telecon. The photonic calibration device status was also addressed, as well as the milestones for calibration, under construction by the Science IPT.

Butler also pursued BIMA efforts at absolute calibration, led by Jack Welch. Using a comparison between a calibrated horn and an antenna, the BIMA group has been using Jupiter and MWC349 to improve the standards for absolute calibration, currently at 30 GHz. Butler and Perley will compare

with VLA measurements of MWC349.

Holdaway worked a bit on the single laser ranging method for gaining information about the shape of the prototype antenna's surface.

Mangum reviewed of ALMA Memos 372 and 422 for the Calibration Group. He also authored and submitted ALMA Memo 434 on "Load Calibration at Millimeter and Submillimeter Wavelengths" to the memo series.

In conjunction with his antenna work, Mangum held extensive discussions with Angel Otarola and Albert Greve regarding detailed plans for use of API 5D measurement system at the ATF. He also discussed some of the finer details of the "Monitoring and Diagnostics" task for the Vertex antenna

testing with Albert Greve.

Mangum also worked on a dormant memo begun with Robert which describes interferometric holography.

Holdaway also worked on the dynamic scheduling simulations a bit.

Site Characterization

     One of the ASAC charges from the ACC dealt with site characterization. Radford, worked with REU student Selby Cull to provide analysis of the site data to the ASAC. Both are coauthors of the draft report presented to the ASAC by working group chair Neal Evans.

Radford prepared and shipped to Chile for deployment in early November two new webcams, replacement computers for the 225 GHz tipper, the 11.2 GHz interferometer, and the seismometer. He verified the tipper software functions and also updated interferometer software.

Radford processed data from the instruments at Chajnantor and updated the web pages to show the new data.

Science Software Requirements

       Myers continued to work with the Science Software Requirements group on audit requirements for aips++. Mangum also works with this group, as well as project scientists.

Outreach and Public Education

            Wootten, van Dishoeck and Guilloteau planned telecons for the Science IPT leadership (commencing 8 Oct), and for the Science IPT in general once per month (commencing 22 October). A calibration telecon is planned for mid-October, to discuss reviews of various memos in the ALMA Memo Series and other matters. Most of the NA Science IPT will meet with Project

Scientist Guilloteau in Tucson 21 October just before the Site Infrastructure Review.

     Wootten revised a paper on ALMA submitted to the SPIE symposium in August to reflect current AEG Policy toward antenna testing.

            Before the SPIE meeting, Wootten toured the Smithsonian Millimeter Array with Jim Moran. The 183 GHz radiometer, pioneering the way for ALMA, was inspected, and an antenna was tuned to this line. Although 3-4mm of water were present, the sky was not as black as needed with the 5 Ghz IF (i.e. in the line wings) to explore optics misalignment with a cold load. The antenna and receiver functions are controlled through a special Palm Pilot program Todd Hunter wrote. He also toured the Arizona Radio Observatory (12m and Submillimeter Telescope) as well as the Caltech Submillimeter Observatory, as part of an NSF site team visit. He wrote the team report for the CSO, submitted to NSF.

            Mangum wrote a guide to the ATF to facilitate visits to the site. He also wrote an article for the internal NRAO Newsletter ‘Point Source’ about the ATF.

Science IPT ALMA Papers

ALMA Memo 434 on "Load Calibration at Millimeter and Submillimeter Wavelengths"

Science IPT Meetings during September

NA Science IPT Telecon 6 August 2002 News; Delgado; Configuration Final Review Dates, Site Review Dates,Sensitivities--CARMAand Early ALMA, SPIE

NA Science IPT Telecon 27 August 2002 News; Configuration Progress; aips++, ALMACal Meeting

NA Science IPT Telecon 3 September 2002 (Agendas on Science IPT WWW site)

NA Science IPT Telecon 24 September 2002 (Agendas on Science IPT WWW site)

Science IPT Telecon 27 Aug Calibration Group Meeting

ASAC Telecon 7 August 2002

ASAC NA-only Telecon 3 Sept 2002

ASAC Face-to-face Meeting 6-9 September 2002 (Agenda, Presentations on

Sitescape)

NACC, ACC and EACC Meetings 16-18 September 2002.