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Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
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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.

Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
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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

Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
×

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.

Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
×

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Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
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Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
×
Page 14
Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
×
Page 15
Suggested Citation:"3 Performance Degradation." National Research Council. 2005. The Atacama Large Millimeter Array: Implications of a Potential Descope. Washington, DC: The National Academies Press. doi: 10.17226/11326.
×
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The 1991 NRC decadal survey for astronomy and astrophysics included a project called the Millimeter Array (MMA). This instrument would be an array of millimeter-wavelength telescopes intended to capture images of star-forming regions and distant star-burst galaxies. With the addition of contributions form Europe, the MMA evolved into the Atacama Large Millimeter Array (ALMA), a proposed array of 64, 12-meter antennas. The project is now part of the NSF Major Research Equipment and Facilities budget request. Increased costs, however, have forced the NSF to reconsider the number of antennas. To help with that review, NSF asked the NRC to assess the scientific consequences of reducing the number of active antennas from 60 to either 50 or 40. This report presents an assessment of the effect of downsizing on technical performance specifications, performance degradation, and the ability to perform transformational science, and of the minimum number of antennas needed.

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