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.
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 |