ALMA Science Advisory Committee
Florence, 23-24 February 2001
Draft Minutes
Participants: J. Baars, R. Bachiller, A. Baudry, G. Blake, A. Bos, L. Bronfman, R. Brown, Y. Chikada, P. Cox, R. Crutcher, M. de Vos, R. Dickman, D. Emerson, N. Evans, Y. Fukui, S. Guilloteau, M. Gurwell, J. Hamaker, T. Hasagawa, M. Ishiguro, R. Kawabe, R. Kurz, R. Lucas, H. Matsuo, K. Menten, M. Momose, N. Nakai, S. Okumura, J. Richer, S. Sakamoto, N. Scoville, P. Shaver, K. Tatematsu, E. van Dishoeck, M. Walmsley, W. Welch, W. Wild, C. Wilson, A. Wootten, M. Yun
J. Welch welcomed all participants, and the plan for writing the report from the meeting was briefly discussed.
M. Ishiguro reviewed the enhancements and contributions that Japan was interested in making to the project, including its share of antennas and infrastructure, new receiver bands, photo-mixers for the photonic LO system, high-speed samplers, enhanced correlator, and computing. He also reviewed the major design & development activities in Japan. The project structure will be similar to that in the U.S. and Europe, with NAOJ as executive. He expressed Japan’s unhappiness with the proposed 20% cut to the project.
R. Brown introduced this topic by commenting that Japanese entry enhances the project, reduces the technical risk, and strengthens the project politically. The bilateral agreement is now in final form, and easy to expand to a third partner. According to the current timetable the AEC will complete the revised scope by June 2001 for submission to the ACC. The Japanese budget request is to be submitted in July 2001, and the goal is a three-way agreement ready for signature by the end of 2001. He reported that the ACC, in its teleconference the previous day (22 February), instructed the AEC to focus on what will be possible with a 20% reduction (maintaining an equal split between the three partners).
R. Kurz summarized the management and operations planning. The project will be organized around Integrated Product Teams (IPTs). The Management IPT will be co-located, and in place from the start of Phase 2; it will eventually be in Chile. It is not a legal entity, and has no budget of its own. The other IPTs will not necessarily be co-located. The IPT concept is needed for a distributed project such as this. It does increase management costs somewhat (which are normally about 12%), but it is known to work and has been used in NASA projects (although ALMA is unusual as a collaboration of equals).
Operations will be based on a number of operations centers: the AOS (Array Operations Site at Chajnantor), the OSF (Operations Support Facility near San Pedro), the SOC (Science Operations Center in Santiago), and the RDCs (Regional Data Centers in Europe, the U.S. and Japan). R. Kurz gave an overview of the operations plan that is being prepared as part of the European ALMA proposal. It is intended to have a continuing upgrades and development budget of about $10 million per year (a similar concept is used for the VLT and Gemini, and is very important for ALMA). The total operations budget is expected to be about $40 million per year.
S. Guilloteau, R. Kawabe and M. Yun reported on simulations involving the ACA. S. Guilloteau showed the results of simulations of observations of M51 using ALMA + zero spacings + the ACA comprised of 12 x 7m antennas . The ACA can improve performance by 50-100%, and more if noise and pointing errors are included. R. Kawabe showed the results of Morita’s simulations of an M31 HII region at 230 GHz. The improvement in dynamic range using the ACA was 30-80%. M. Yun showed simulations using 64 x 12m antennas + zero spacings. The total power is very important; the ACA will provide more assurance and a more robust result. Thus, all results indicate the value of the ACA; further work will include noise and pointing errors, and a wider range of images. J. Baars gave cost estimates for the 7-8 m antennas of the ACA.
W. Wild summarized the recent work of the Receiver Group, and the recent receiver PDR. Major current issues include new cost estimates (due for April), the large number of TBD items in the specs, the limited resources, and the very tight schedule to first light, with no time to test propotypes. Questions to the review panel were: (1) are the top level requirements complete and adequate (the ASAC is to assess the 1 in 104 requirement), (2) are the related requirements complete and adequate, (3) have the correct design solutions been selected for study and development (a delta-PDR is proposed in July/Aug), (4) are the interfaces defined adequately, (5) has adequate attention been given to producibility (yes) and maintainability (no). The report of the receiver production meeting in Garching is available on the web now. Only a few bands will be incorporated at first, and concern was expressed about the plan to integrate other bands later at the OSF.
A. Wootten gave a status report on the baseline correlator. The chip design is now complete, and the project is on schedule. A. Baudry summarized the European Future Correlator activities. It will have very high flexibility and efficiency. The final prototype review is due in December 2003. J. Welch commented about the possible impact of Moore’s law on the cost of future correlator systems. T. Hasegawa reviewed the science case for Japan’s enhanced correlator – the advantages of high spectral resolution at the same time as large bandwidth. Y. Chikada gave details of the Japanese correlator plans; the timescale is consistent with that of ALMA, with a PDR now and CDR in December 2001.
A brief summary of the configuration studies was given by A. Wootten; a detailed discussion is to take place at the Grenoble PDR immediately following the ASAC meeting. Simulations have been done for two cases, the spiral zoom and dual ring arrays (single field only). Six images were used: M51, Cyg A, Mars, the Mundy disk, a complex dust region, and an array of dots. The double ring produces near-in sidelobes and increases noise; otherwise the difference is negligible. The main difference is operational, as the spiral would be reconfigured continuously whereas the double ring only at intervals. T. Hasegawa, N. Scoville and G. Blake commented that good instantaneous u-v coverage was essential, both to make complete images at the best times (in terms of atmospheric conditions) and because source structures may change on these angular scales. Full discussion of these issues will take place at Grenoble.
A. Wootten related the CBI experience of operating on the Chajnantor site. There is a microclimate effect, in which sometimes humidity rising at Chajnantor from below falls as snow and results in the loss of about 12 nights per year. This effect may not be present at Pampa La Bola. S. Sakamoto showed contemporaneous statistics for the two sites; Chajnantor has ca 10% better phase stability, and significantly lower opacity in the best conditions. The wind is about the same. R. Brown showed a possible location for the OSF, with a dedicated road to the site.
The implications of the most recent cost reduction proposal were discussed, and a letter responding to the ACC was drafted.
R. Lucas summarized the current status of the Software Science Requirements (SSR). A detailed draft document currently exists, and is being reviewed over the period Feb/Mar. In March work will start on the requirements for off-line analysis. There was a general feeling that, at least at the beginning, observing time should be guaranteed rather than sensitivity. D. Emerson noted that sometimes it may be u-v coverage that is needed, and this would then define the integration time. The pipeline will include the ACA. R. Crutcher commented that the dynamical scheduling is similar to what is done now on supercomputers. Written input from ASAC members on specifics of the SSR document is encouraged.
R. Crutcher introduced this subject; the requirement is 10% polarization accuracy, and a good knowledge is needed of the polarized primary beam, which (for alt-az) rotates on the sky. The coherent signal passband calibration scheme described by D. Emerson would provide one solution. An alternative is to have half the receivers mounted 45 deg from the others (or rotatable), but this would present significant complications.
J. Hamaker discussed polarimetry with ALMA, based on the use of matrix algebra. The traditional approach, based on the Morris et al (1964) Stokes equations, assumes a perfect system, and all corrections have to be made first. The matrix approach is more elegant, but challenging. R. Crucher commented that the matrix formalism has been used at BIMA, but the 45 deg rotation is still required. A. Baudry mentioned the need for unpolarized calibrators, which may not be easy. S. Guilloteau raised the case in which one cannot do self-cal due to inadequate S/N. D. Emerson said that tests of the coherent calibration system would probably be made on the 12m in late 2001. J. Welch said that a subgroup is required to study these issues and report back to the ASAC.
C. Wilson reported on the 20 micron water vapour radiometer IRMA. The prototype has the sensitivity at 20 microns to meet the ALMA speccs, and the new one will be 10 times more sensitive. The concerns are the systematics (effects of cirrus etc). The plan is to install two systems on the SMA in late 2001/early 2002 and do side-by-side comparisons with the 183 GHz system. J. Richer gave a progress report on the Cambridge/Onsala 183 GHz system. It might be possible to use the system for the anomalous refraction correction. The work has begun, and the system should be ready in early 2003 for testing.
S. Guilloteau compared two approaches for receiver calibration: (a) a dual-load calibration system located in the subreflector, and (b) a semi-transparent vane calibration system, located in the receiver cabin. His preference is for (b), although development and testing still has to be done. W. Wild commented on some of the possible practical difficulties with the system. J. Welch reported on tests of (a) at BIMA. It does work, and can measure the standing waves also. Further tests are planned. W. Wild commented that there is no rush to decide between the two options, as they are de-coupled from the receiver design.
Possible calibration sources were discussed. Candidates include quasars (but variable and polarized), ultra-compact HII regions (but dust contamination), planets (but large and variable), asteriods (but variable), AGB stars (but variable), K giants (but complex spectra), nuclei of planetary nebulae, and ionized outflows such as MWC 349 (although this is a northern source).
J. Welch described the use of a standard gain horn attached to one antenna, used in an interferometer mode to provide absolute gain calibration to 1%.
D. Emerson and J. Baars summarized the progress being made with the two prototype antennas.
A. Wootten said that new brochures had been prepared in the U.S., and a new display was presented at the recent AAS meeting. The proceedings of the Washington conference are in press. He mentioned the idea of a possible "ALMA Speakers Bureau". K. Menten suggested that there should be an update of the ESO-produced ALMA video including astronomical images. P. Shaver said that there would be PR activities in Europe over the coming months to support the European ALMA proposal.
Brief reports on the ASTE and APEX projects were given by R. Kawabe and K. Menten respectively. Assembly of the ASTE will begin at San Pedro in October 2001, and the first test observations will be made in January/February 2002. The APEX is planned to arrive at Chajnantor in late 2002.
Sections of the ASAC Report will be prepared by the following persons:
Antennas Fukui, Walmsley, Welch
Configurations/ACA Cox, Evans, Kawabe
Calibration Matsuo, Richer, Wilson
Receivers Blake, Nakai, Van Dishoeck
Site Booth, Sakamoto, Bronfman
Software Gurwell, Tatematsu, Benz
Correlator Bachiller, Scoville, Yamamoto
System, Polarization Crutcher, Yamamoto, Menten
The above groups met separately, and then summaries of each report were presented for preliminary discussion.
The timescale for completion is as follows:
1 March (deadline for contributions); 14 March (reports circulated within ASAC); 18 March (final comments); 20 March (final report to ACC)
Two new vice-chairpersons were nominated. G. Blake was nominated by the U.S. ASAC members, and Y. Fukui was nominated by the Japanese ASAC members. G. Blake will become the ASAC chairperson following the next face-to-face meeting in September.
The next face-to-face ASAC meeting will take place in Chile in the first or second week of September. The final dates have not yet been set, but the current plan is as follows: day –1: San Pedro; day 0: Chajnantor site; days 1,2: ASAC meeting in Santiago; day 3: ALMA science day. A day –2 may be added to allow more time in San Pedro for acclimatization and rest.
The next ASAC teleconference will take place on Wednesday March
14 at 10:15 am EST.