10. Calibration, Imaging and Science October 2000 Report Major accomplishments October 2000 I. Imaging - Configuration Studies, Site and Water Vapor Radiometry The imaging and calibration group held several phone meetings during October, 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 re-commenced during October. Steve Heddle in the UK continued to progress on the imaging simulation, producing 1km 'C' 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.stevenheddle.co.uk/ALMA/ALMA_IND.HTM). A teleconference will be held when a sufficient body of intermediate array simulations has been assembled. A Review is planned for early 2001. Butler worked on finalizing the memo on the location of the compact configuration, which will be issued late in November. Butler and Radford will visit the Chajnantor Altiplano in December, in part to inspect more closely the sites for the compact configurations. B. Site G. Delgado has presented a report on a 52-year climatological study for the Chajnantor area done by a consultant; Radford circulated this to the site group. Some long term periodicities in weather can be detected in this data but their amplitude isn't extreme. Apparently, the site is now experiencing a weather nadir period. A meeting will take place next month in Charlottesville to review the CBI experience. Radford received site characterization data through October 12 from A. Otarola and updated databases and web pages accordingly. Radford initiated a detailed study of contemporaneous tipping radiometer data at the two sites to determine how the transparency differed between the sites. His preliminary result is that during 1999 June - December tranparency was 15-30% worse over Pampa la Bola than over Chajnantor Butler prepared a proto-ALMA Memo on comparison between the Site Testing Interferometers at the Chajnantor and Pampa la Bola sites. He found that the median phase fluctuations at Pampa la Bola are ~12% worse than at Chajnantor, averaged over all times. During the lowest rms phase fluctuations, Pampa la Bola fluctuations are 43% worse than at Chajnantor. Butler also commenced a calculation of sun position on a given day at the site, to verify which satellites the NRAO and ESO STI's are observing. Butler worked on radiosonde data and analysis (and presentation for the Morocco meeting). Also at that meeting, there will be a Site Characterization Review amongst all of the ALMA folks. This will consist of Lars-Ake Nyman, Angel Otarola, Guillermo Delgado, Martina Weidner, Roy Booth, Seiichi Sakamoto, Simon Radford, and Bryan Butler. Plans for visiting prospective pad locations during upcoming visits to the site were discussed. Radford received repaired Inmarsat M4 (ISDN) terminal and tested voice and data calls. After three days, the terminal failed during a thunderstorm and was returned; the vendor admitted static problems. Deployment of the Inmarsat M4 terminal at Chajnantor will be delayed until 2001 owing to problems inherent in the equipment. C. ACA The ALMA ImCal group discussed the need for nutators on the antennas of the ACA. These should be incorporated in the design for at least one of the antennas. II. Calibration A. Interferometer/Antenna Amplitude Calibration Radford started an analysis of the 12m prototype nutator dynamics and control system. He also reviewed the nutator design and status with J. Baars. Radford revised the nutator mechanical design for easier production. Lab tests of nutator hardware were also performed. Mangum worked on revisions to ALMA Memo 318 (Amplitude Calibration at Millimeter and Submillimeter Wavelengths). Most modifications are just window-dressing (a more general way to express chopper wheel calibration relation, for example). An analysis of the semi-transparent vane calibration will also be included. He continued to look into implementation issues for the ALMA adaptation of the BIMA prime focus amplitude calibration system. Some issues regarding data acquisition and synchronization need to be worked out for this system. Another area of concern is how the hole, and embedded calibration system, will couple to the astronomical receivers. Calculations indicate that there will possibly be quite a lot of reflected power back to the astronomy receivers, which could lead to standing wave problems. As part of his participation in the SSR group, Mangum wrote the use case for the amplitude calibration observing mode. B. Pointing Calibration Mangum made some progress on the software aspects of the optical pointing system. A frame grabber is now commandable and produces gif images at a user-specified rate. Mangum will continue to develop the image producing and analysis capabilities of this software. III. Science A. ALMA studies, including the ASAC Wootten wrote sections of the ASAC report for which he was the responsible party. The report was disseminated to the ALMA Division Heads/Team Leaders. He produced the agenda and moderated the ASAC teleconference which was held 10 October. Payne brought up the question of which widgets might be needed for the ALMA optical train. Imaging and Calibration feels that provision should be made for a chopper wheel and for a solar attenuator. A second wheel, or a polarization widget are unnecessary. A review was carried out of polarization widgets in use today. For ALMA, the most appropriate would be a wire grid over a reflector, as used at OVRO and NRO. However, the group felt that the disadvantages of this system outweighed the advantages. Carter presented a new optics design, which was also discussed. The very far off-axis beams continue to be a source of concern, though no remedies were identified. Butler (with Gurwell and Wootten) presented an ALMA poster at the DPS meeting in Pasadena. The spectral line capability of the instrument will allow for the observation of multiple molecular species in planetary and cometary atmospheres and protoplanetary disks, providing temperature and wind (for the atmospheres) profiles at high spatial and time resolution and clues as to the chemistry in these places. Wootten (with Gallimore) finished a paper illustrating the ability of ALMA to image galaxies in the distant Universe in the submillimeter. ALMA's detailed imaging capabilities will be a major step for astronomy, making it possible to study the origins of galaxies, and the state of gas and dust in forming and interacting distant galaxies. Crystal Brogan has assembled a list of molecular lines (http://www.cv.nrao.edu/~awootten/mmaimcal/Alma_zeeman.pdf) which will be of particular importance for polarization work on ALMA--she noticed that they come at band edges, by a perfidy of nature, and hence might pose a problem for tuning the polarization response of ALMA to the lines of most interest. Cleanup work continued on the volume summarizing ALMA Science from the meeting last October. The ALMA SSR meeting was held in Berkeley; Mangum and Myers attended. Scott and Myers issued a data rate document, and there was lively discussion over many issues relating to the Requirements document and use cases. Mangum represented Imaging and Calibration at the holography CDR in Tucson and at a face-to-face meeting regarding development plans for the Test Interferometer. B. Imaging and Calibration Plans for next month Mangum will represent Imaging and Calibration at the Antenna PDR in Germany. A face-to-face meeting between Brown, Wootten, Butler and Radford and Readhead to discuss operating and observing experience at Chajnantor during the past year will occur in Charlottesville. The ASAC teleconference will be held on 13 November. The Site Characterization Review will occur at the IAU meeting in Morocco. Configuration work will continue, with a focus on finishing the intermediate array set of the simulations. A phone meeting of the configuration group is planned when this has progressed. 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 As mentioned above, the very far off-axis optics design continues to raise vexing issues.