----------------------------------------------------------------------------------------- ALMA Science Advisory Committee Berkeley, 9-10 September 2000 Draft Minutes Participants: Baars, Bachiller, Baudry, Blake, Booth, de Vos, Bronfman, Brown, Chikada, Cox, Crutcher, Emerson, Evans, Fukui, Guilloteau, Gurwell, Hasagawa, Ishiguru, Kawabe, Kurz, Mangum, Menten, Morita, Nakai, Okamura, Richer, Shaver, van Dishoeck, Walmsley, Welch, Wilson, Woody, Wootten, Yun 1. Welcome and Introduction K. Menten opened the meeting, welcomed all participants, and reviewed the agenda and plans for reports from the meeting. 2. Interim Report on Definition of Phase 2, Decision Process, Schedule, Cost R. Brown reported that the ALMA Agreement is currently in draft form, and will be discussed by the ACC at its October meeting. It is the foundation document for the ALMA partnership, and will include as Annexes the project book, table of tasks and values, management plan, etc. An agreed draft should be ready by December 2000, and the document should be signed by the end of 2001. He also reported that a formal petition to the Chilean government covering site access and the legal aspects is being prepared, with the objective of approval by January 2002. The Phase 2 cost estimate remains the same ($552 million (2000)), following minor adjustments both ways, and there is a strong disincentive to change. The funding process in the U.S. is underway now. 3. Report from ALMA Liaison Group, including Compact Array R. Kurz reviewed the ALG issues. Following the ASAC input, the June ALG meeting defined the interim scope: 78 x 12m antennas, 8 frequency bands plus WVR, and, pending further study and definition, a compact array and an enhanced correlator. Work is currently underway on the costs of the enhancements. The schedule to a 3-way Agreement is as follows: Japanese review of the Europe/US agreement (ASAP), final definition of enhanced ALMA (Jan 2001), initial draft of 3-way Agreement (Feb 2001), final draft (June 2001), formal Japanese FY2002 budget request (July 2001). The tasks may be somewhat re-distributed when Japan enters the partnership. It was commented that there must be ASAC input to the 3-way Agreement. Y. Fukui reported that the Japanese funding prospects are quite positive, but stressed that a high visibility for the enhanced ALMA is necessary, and that Japan must be a full, equal partner, providing an equal number of antennas. S. Guilloteau discussed aspects of the Compact Array. He concluded that the ACA should be able to work as a stand-alone array. 6m antennas would provide better UV coverage but worse sensitivity, while 8m antennas would permit use of the same receivers, mount etc. To match the brightness sensitivity of ALMA 12 x 8m antennas or 16 x 6m antennas would be required in the ACA. J. Welch stressed the problem of single-dish pointing errors, which argues for smaller antennas. He concluded that 6m antennas would be optimal, with the number of antennas = (12/D) x 6, where D is the antenna diameter in meters. K.-I. Morita discussed the sensitivity of the ACA, and presented imaging simulations. N. Evans chaired a discussion on the enhanced ALMA. Options include (a) increasing the number of 12m antennas beyond 64, (b) the ACA, (c) inclusion of most (at least 8) receiver bands, and (d) the enhanced correlator. The possibility of longer baselines (20-25 km) was also mentioned. There was general agreement that (b) and (c) have high priority. 4. Correlator Plans A. Wootten summarized plans for the baseline correlator. The present design can accommodate 64 antennas, and the final quadrant will be delivered to Chajnantor in 2006. The Canadian WIDAR being developed for the EVLA will be discussed in the correlator group. A. Baudry discussed the future correlator currently under study. The goals are higher efficiency, sensitivity and flexibility, and optimized performance/cost. It would benefit from rapid technological advances. A report is due at the end of 2001, a prototype at the end of 2003, and a decision to proceed in 2003-2004. M. de Vos provided some details of the current design concept. A. Baudry raised several issues, including the adequacy of 5-10 kHz maximum resolution, minimum dump time required, number of channels, need for on-line WVR correction, requirements for sub-arraying, importance of lower correlator power consumption. S. Okamura described the new Japanese FX correlator proposed for ALMA. It accommodates both wide-band and high-resolution observations simultaneously. The main change from the former design is the application of flexible frequency-channel smoothing for the correlated data, which would reduce the cost of the correlator by a factor of two. General comments about the ALMA correlator included the need for high spectral resolution to study cloud cores and the need for high time resolution (~ 1 msec) for solar observations. On a suggestion by C. Wilson, it was agreed that a clear comparison between the various correlator projects should be provided. This will be coordinated by the two project scientists. 5. Calibration schemes Various possibities for calibration were discussed by S. Guilloteau, who drew the following conclusions: ambient loads should be used, semi-transparent vanes should be developed, and an FTS tau-meter located in the middle of the array should be used with secondary calibrators. J. Welch described a possible subreflector calibration system, in which a mirror rotates about its axis in the subreflector hole. S. Guilloteau expressed concern about the problem of reliability with such a system. J. Mangum provided an analysis of the chopper wheel calibration technique (ALMA memo 318), and concluded that the best case may provide a calibration accuracy of about 1%. The uncertainty in atmospheric opacity should also be < 1%. N. Evans commented that such accuracies are far from current possibilities. A possibility being developed for passband calibration was outlined by D. Emerson. An artificial (photonic) narrow-band signal is injected, from a log-periodic radiator at the subreflector (which could fit in with the subreflector system described by J. Welch). It would cover 100-1000 GHz, and could also be used for diagnostics. R. Crutcher commented that it could be very useful for polarization calibration, and C. Wilson added that it could be used to measure sideband ratios. Such passband calibration would take just a few seconds. It would be calibrated with an astronomical source about once per week. A prototype is under development and a full system will be available for the test interferometer in April 2004. It is part of the baseline project, and is in the project book. R. Brown said that ASAC recommendations are needed on calibration. K. Menten replied that this must be further discussed, although there seemed to be agreement against cold loads, and in favour of starting the development of semi-transparent vanes. 6. Report on Configurations M. Yun reported that the concepts of the doughnut configuration (new memo 320) and the spiral zoom have essentially merged, and it is now largely a matter of operations. He presented simulation studies on the ACA plus total power, using M51 as an example. He concluded that total power is essential, and more important than the choice between 6m and 8m diameter antennas. E. van Dishoeck commented that it would be useful to compare with the 64 x 12m ALMA + total power. R. Booth suggested that the spiral zoom be taken as the final choice (for ease of operation), but others felt that more study was needed. Concerning the longest baselines, it was agreed that 14 km is reasonable, and there is no need to consider longer baselines at present. 7. Report on Spectrum Protection, Site The very significant advance in spectrum protection made at the recent WARC was applauded by the ASAC, and the next steps in this area were briefly discussed. A. Wootten discussed site issues, and stressed the great importance of safety. The CBI operational experience was reviewed; the major operational issue was snow, and access to and from the site. The CBI group was reported as saying that the two major factors for success on this high site were (1) organization, and (2) testing and building at low altitude. The recent environmental statistics were reviewed. A new memo (322) gives a comparison of Chajnantor with Pampa la Bola, although there was uncertainty because the statistics on the two sites do not cover exactly the same periods of time. Concerning the Lascar eruption, there is already a geologist’s report. A plug formed in the volcano, causing the eruption; this happens about every seven years. The plume went to the east with the prevailing winds with no effect on the site, although seismic effects could not be determined as the seismograph was out of operation at the time. 8. Polarization Specifications A Wootten read a summary of the specifications proposed by S. Myers and L. D’Addario. R. Crutcher repeated that the passband calibration system described by D. Emerson would be very good for polarization, and said that linear polarization could then be used instead of circular. 9. Receivers The scheduled telecon with W. Wild and J. Payne did not take place, and J. Richer gave a brief summary of some recent receiver developments. The receiver group had agreed to extend band 2 to 84 GHz. They feel some specs are too tight, and would like a revision to Table 2 in the specs document. A discussion of receiver band priorities was led by E. van Dishoeck, who presented the following tentative list: Top priority: Band 3 (84-116 GHz): many molecular lines, redshifted CO, 86 GHz SiO Band 6 (211-275 GHz): “ “ “ , “ “ , dust cont. Band 7 (275-370 GHz): “ “ “ , “ “ , “ “ Band 9 (602-720 GHz): prime submm band; CO 6-5, warm dense gas, etc Second priority: Band 4 (125-163 GHz): redshifted CO Band 1 (31-45 GHz): S-Z measurements, free-free and synchrotron emission Band 10 (787-950 GHz): [CI] 810 GHz, CO 7-6, dust continuum Band 8 (385-500 GHz): [CI] 492 GHz Third priority: Band 5 (163-211 GHz): H2O 183 GHz Band 2 (67-84 GHz): redshifted CO N. Nakai showed plots of redshifted CO lines and far infrared lines vs. frequency, and argued that bands 8 and 10 are also important. It was agreed that band 10 has a high scientific priority, but would probably come later to allow for technical development. A. Wootten made a case for putting band 10 on the ACA early. Finally, the relative priorities between bands 1, 4, 8, 10 were not assigned; for now all four are just considered to be together in the second overall priority group. However, if decisions are eventually to be made within these broad priority groups, the ASAC should be involved. The possibility of S-Z measurements with the ACA was noted. 10. Water Vapor Radiometry Reports D. Woody reported on the 22 GHz work at OVRO. The water line monitors are stable, there is good correlation with phase, and coherence correction is easy – the method works. Future plans include improved observations, a cool front end amplifier, and a 16-channel spectrometer, with an expectation of < 50 micron delay error. J. Richer reported on the 183 GHz work, referring to the recommendations from March. The progress at Chajnantor is slow, owing to the site problems. In a new proposal, there will be two different systems (Cambridge, Onsala), with a decision in 2002. The advantages of a cooled system (outside the main dewar) were repeated. The possibility of doing tilt corrections for anomolous refraction was raised again, and it was suggested that this be re-considered by the ASAC. Another factor of 10 in sensitivity is required, calling for a good cooled receiver. S. Guilloteau commented that a measure of the seeing on the site was needed. The CBI data would be useful, and J. Richer volunteered to get the required data by the next face-to-face ASAC meeting. C. Wilson said that the Canadian 183 GHz radiometer is almost complete, and is going to the SMA receiver cabin. Tests (also using the CSO radiometer for interferometer tests) will begin in Feb/Apr 2001. This work is complementary to the Cambridge/Onsala work. The 20 micron Infrared Radiometer for Millimeter Astronomy (IRMA) is being operated at the JCMT over the next few months. There is funding for a second unit, so eventually there will be two-unit interferometer tests. The matching of the IRMA beam to the mm beam was mentioned as a concern. 11. Report on Antennas and Test Interferometer D. Emerson and J. Baars summarized recent developments on the two prototype antennas. Both are still due for delivery in Oct/Nov 2001. The foundations will be prepared in April 2001. J. Mangum reported on the Test Interferometer. A final report on the Test Interferometer results should be ready by April 2003, as input for the selection process. He mentioned some issues for ASAC consideration, including the importance of WVR tests and the priority of an amplitude calibration system. The polarization properties are considered important, and concern was expressed that there is no polarization spec. 12. Preparation of Reports Reports are being prepared on the following topics. In each case the first-named person has prime responsibility. They are due for submission by 20 September, and the full report will be completed by 22 September. * Enhanced ALMA, ACA --- N. Evans, P. Cox * Correlator --- R. Bachiller, S. Guilloteau, A. Wootten * Calibration --- J. Welch, J. Richer, S. Guilloteau, J. Mangum * Configurations --- M.-S. Yun, R. Booth * Site --- A. Wootten * Polarization --- R. Crutcher * Receivers --- E. van Dishoeck, G. Blake, M. Walmsley, D. Woody (7 mm band) * Water Vapor Radiometer --- J. Welch * Antennas/Test Interferometer --- K. Menten (general statements) 13. Election of new Vice-Chairperson E. van Dishoeck was elected as the new vice-chairperson of the ASAC. J. Welch replaces K. Menten as chairperson now, and E. van Dishoeck will become chairperson following the March 2001 ASAC meeting. 14. Next Meetings The next face-to-face meeting of the ASAC will take place in Florence, in March 2001 (exact date TBD). The next ASAC teleconference is scheduled for Monday 9th October.