3
Performance Degradation

In Table 1, the committee considers the four performance measures listed in the charge, in turn giving the relevant scaling with N, the number of antennas, and D, the dish diameter.

Comparison of the performance of 40-, 50-, and 60-antenna arrays as shown in Table 1 clearly indicates a significant degradation of performance with a descoped array. The image fidelity and single field sensitivity measures are limitations in the science that the array can produce. The imaging speed and mosaiced image sensitivity relate to the time taken to perform an observation. In practice, they, too, will translate into a reduction in performance because of the difficulty of maintaining instrument and tropospheric stability over longer intervals. The severest performance degradation is in the image fidelity.

The committee concludes that speed, image fidelity, mosaicing ability, and point source sensitivity will all be affected if the ALMA array is descoped. The severest degradation is in image fidelity, which will be reduced by factors of 2 and 3 with descopes to 50 and 40 antennas, respectively.



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The Atacama Large Millimeter Array: Implications of a Potential Descope 3 Performance Degradation In Table 1, the committee considers the four performance measures listed in the charge, in turn giving the relevant scaling with N, the number of antennas, and D, the dish diameter. Comparison of the performance of 40-, 50-, and 60-antenna arrays as shown in Table 1 clearly indicates a significant degradation of performance with a descoped array. The image fidelity and single field sensitivity measures are limitations in the science that the array can produce. The imaging speed and mosaiced image sensitivity relate to the time taken to perform an observation. In practice, they, too, will translate into a reduction in performance because of the difficulty of maintaining instrument and tropospheric stability over longer intervals. The severest performance degradation is in the image fidelity. The committee concludes that speed, image fidelity, mosaicing ability, and point source sensitivity will all be affected if the ALMA array is descoped. The severest degradation is in image fidelity, which will be reduced by factors of 2 and 3 with descopes to 50 and 40 antennas, respectively.

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The Atacama Large Millimeter Array: Implications of a Potential Descope TABLE 1 ALMA Configurations Compared with Other Arrays   CARMA PdB SMA Nobeyama ALMA Latitude (degrees) 39 44 19 36 -23 Altitude (ft) 7200 8300 13500 4400 17000 Fr equency Range (in GHz) 35-250 85-250 186-696 85-250 30-900 Maximum Baseline (km) 3 1 0.5 0.35 12 Best Resolution (mas) at Frequency (GHz) 80 at 250 240 at 250 200 at 696 690 at 250 6 at 850 Usable Time Fraction (250 GHz) 0.25 0.25 0.5 0.1 0.9 Usable Time Fraction (350 GHz) - - 0.3 - 0.8 External User Access Fraction 0.3 0.05 0.1 0 0.35 Antenna Number 23 6 8 6 60 50 40 30 Antenna Diameter (in meters) 3.5 - 10 15 6 10 12 50 40 30 Image Fidelity ~N3 0.056 0.001 0.002 0.001 1.00 0.58 0.30 0.13 Imaging Speed ~N 2D4 0.006 0.024 0.001 0.005 1.00 0.69 0.44 0.25 Single Field Sensitivity ~ND2 0.12 0.16 0.03 0.07 1.00 0.83 0.67 0.50 Mosaiced Image Sensitivity ~ND 0.20 0.13 0.07 0.08 1.00 0.83 0.67 0.05

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The Atacama Large Millimeter Array: Implications of a Potential Descope NOTE: Comparison of ALMA performance as a 30-, 40-, 50-, and 60-antenna array and with the performance of other arrays that are operating or will be operating soon, specifically the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California, the Plateau de Beure Millimeter Interferometer in France (PdB), the Submillimeter Array (SMA), and the Nobeyama Millimeter Array (Nobeyama). Performance measures are normalized to a 60-antenna ALMA array. The CARMA array will be a combination of previously constructed arrays, and will therefore have antennas of different diameters, ranging from 3.5 m to 10 m. Image fidelity (quality) is defined as the mean ratio of the median flux to the error as measured by a simulation. It depends on the source strength and the deconvolution technique used. The N3 dependence is based on a series of simulations. Imaging speed is interpreted as the inverse of the time needed to reach a given noise level per pixel. It is most important at high frequencies where the opacity and stability of Earth’s atmosphere severely limit the continuous time available for observation. The single field sensitivity is defined as the minimum flux that can be detected at a fixed significance level and scales inversely as the total collecting area. Mosaicing refers to the mapping of areas larger than the field of view of a single antenna, by using multiple pointings, up to 1000 in extreme cases. It will be important for observing extended molecular line and dust continuum sources, the emission from the disks of external galaxies, and the interaction of protostars with their host molecular clouds. Mosaicing will be particularly important at the highest frequencies, since the field of view of ALMA is inversely proportional to the square of the frequency. ALMA was designed to provide detailed, high-fidelity maps over a large range of linear scales. The value in the table is the speed to produce a map with a given sensitivity.

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