10. Calibration, Imaging and Science September 2000 Report Major accomplishments September 2000 I. Imaging - Configuration Studies, Site and Water Vapor Radiometry The imaging and calibration group held several phone meetings during September, with the agenda, indeces and linked documents to be found at: http://www.cv.nrao.edu/~awootten/mmaimcal/ A. Configuration Studies The simulation efforts for the primary ALMA array have continued during September. Steve Heddle in the UK continued to progress on the imaging simulation, producing 3km Array snapshots and 4 hour tracks for simulations of objects in the image library using both the Kogan double ring and zoom spiral configurations (http://www.heddle97.freeserve.co.uk/ALMA/CLEANIND.HTM). Min Yun reported to the ASAC on the progress to date at their face-to-face meeting in Berkeley on 9-10 September. During September, Min took up a position as Assistant Professor at the University of Massachusetts. He plans to continue his ALMA activities as he can. Kogan remains as an active member of the configuration group. B. Atacama Compact Array Definition of and imaging test for the Atacama Compact Array (ACA) continued with several reports issued, notably the ALMA Scientific Advisory Committee (ASAC) endorsement of the ACA as a desired component of the enhanced ALMA. During September, Welch produced a memo for the ASAC on the number and size of the antennas. He concluded that for the compact array to contribute a point source sensitivity matching that of the more closely spaced 12m antennas, the necessary number of small antennas is approximately (12/$D_C$)$\times$6, where $D_C$ is the diameter of the compact array antenna. A 6m antenna should provide good imaging characteristics and suitable calibration sensitivity. The ASAC concluded that '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.' They called for: a study of how minimum separation depends on dish size for dishes mounted on the standard 12-m mount; a study of the effects of dish size, between 6 and 8-m dishes, and of the number and layout of dishes on imaging of several of the standard test images. and stated that the imaging study should include the effects of errors due to noise, calibration, and pointing and be done at representative frequencies where these errors have different relative sizes. Jaap Baars intitiated the first of these studies, and produced a draft recommendation by month's end. Wootten provided a memo on the need for nutating subreflectors in the compact array, concluding that a nutator on at least one antenna should be planned for high frequency operation of the array. Morita and Yun pursued their studies of the imaging characteristics of the array. Morita has been using the SDE package for his simulations, while Yun has used AIPS and investigated the use of AIPS++. C. Site Wootten delivered a report to the ASAC on the monitoring of the site. Visits by ASAC members and others resulted in reports from CBI observers on their experiences in making observations from the site. Bronfman reported that the weather has been poor throughout the year in Chile in general, though the site monitoring data suggest it remains better than during the El Nino year. The CBI observers report 67% usable nights during non-summer months, but had some access problems owing to snow which would not have been a problem at Pampa La Bola. Radford initiated a detailed study of contemporaneous tipping radiometer data at the two sites to determine how the transparency differed between the sites. Sakamoto, Radford, Butler and others had found in ALMA Memo No. 322 issued last month some suggestions of more variable wind direction at the lower Pampa La Bola site, so Butler initiated a study of contemporaneous phase comparisons from the site testing interferometers located on both sites. Radford updated Project Book Chapter 14 (Site Characterization) and updated reduction of site data through 24 August. Plans for visiting prospective pad locations during upcoming visits to the site were discussed. Deployment of the Inmarsat M4 terminal at Chajnantor will be delayed until December at the earliest owing to equipment failure. II. Calibration A. Interferometer/Antenna Amplitude Calibration Mangum presented his investigation of amplitude calibration schemes to the ASAC. Welch described the two-load chopping scheme being investigated by the MDC at BIMA, along with other ideas in a recent memo by Plambeck. Guilloteau compared relative merits of several schemes. Emerson presented a scheme for bandpass calibration which is planned for implementation. The ASAC endorsed continued investigation of the two-load scheme, along with development of the use of a semitransparent vane and the use of coherent signals as described by Emerson. Mangum initiated study of the semitransparent vane and of implementation issues for the ALMA adaptation of the BIMA prime focus amplitude calibration system. Mangum visited Hat Creek to get a first-hand look at the BIMA apex amplitude calibration system. Discussions with Jack Welch regarding the implementation of this system in the ALMA antennas indicated that we should have no problem making the slight modifications needed to make them fit in the apex hole of the Vertex antenna. Simon Radford continues work on the ALMA nutator design incorporating this system. B. Pointing Calibration Mangum continued work on his memo on pointing of the ALMA antennas. III. Science A. ALMA studies, including the ASAC At an NRAO-wide retreat in Socorro, members of the Imaging and Calibration Group got to meet face-to-face with each other, with others in the observatory and with other members of the project, discussing a wide range of subjects. Several presentations were made at the retreat. Wootten discussed use of ALMA antennas as single antenna telescopes during early phases of the project. Cleanup work continued on the volume summarizing ALMA Science from the meeting last October. Plans for the face to face ASAC meeting in Berkeley, including the agenda and reading material were finalized by Wootten. The meeting occurred with good progress and by month's end the ASAC report was reaching its final form. Wootten also attended the correlator meeting held just after the ASAC meeting and reported on that meeting in the ALMA 'virtual hallway'. He presented a report and wrote a section on the ALMA correlator on the 'baseline ALMA correlator' for the ASAC report. Preparations were made for the face-to-face ALMA SSR meeting in Berkeley during mid-October. Rough drafts of use case observing modes will be discussed there. B. Imaging and Calibration Plans for next month Mangum and new SSR appointee Steve Myers will attend the ALMA SSR meeting in Berkeley. The ASAC report will be disseminated to the project. Configuration work will continue, with a focus on finishing a set of the simulations, extending them to more compact arrays and distilling the results into a recommendation on array design. A phone meeting of the configuration group is planned for October. Hopefully, with the impending release of AIPS++ mosaicing will become a part of the simulations soon. Work also continues on the characterization of the ACA. Issues and concerns Advertising for a replacement for Min Yun has commenced.