Antenna Shadowing
An antenna is shadowed when its line-of-sight to the source is partially or fully blocked by another antenna in front of it. The shadowed antenna will collect less radiation from the source than if it had not been shadowed, reducing the sensitivity on the baselines to that antenna. Shadowing is more likely to occur when sources are at low elevation and when the antennas are in more compact configurations. The OPT will report, per scan, the maximum amount of shadowing, if any, will occur according to the location in the sky of the target and the configuration of the observations. D-configuration is affected by shadowing more than the other configurations because the antennas are at their closest proximity to each other. If you select "Any" configuration, the OPT will calculate the worst-case scenario by assuming the D-configuration to calculate shadowing.
If any calibration scans will be shadowed for any of the valid LST start times, we recommend increasing the observing time of the calibration source to account for the loss of sensitivity. Table 2.1 shows the amount of shadowing in respect to the fraction of an antenna that is blocked with the corresponding loss of sensitivity. All shadowed baselines will be similarly affected. Note that the loss of sensitivity is not linear with the fractional shadowing.
| Shadowing | Fraction of Area Blocked | Baseline Sensitivity Loss |
|---|---|---|
| 1 m | 0.01 | 0.5% |
| 5 m | 0.10 | 5% |
| 12.5 m | 0.39 | 22% |
| 18 m | 0.64 | 40% |
| 25 m | 1 | 100% |
Avoiding antenna shadowing during the D-configuration relies on the azimuth of the antennas, which is dependent on the location of the source and the LST start time. Therefore, to guarantee no more than a single shadowed antenna, restrict the elevations, if possible, to >40° for D-configuration and >25° for C-configuration. Shadowing is worst along the azimuths of the arms (in both directions), so has a six-fold symmetry. See Figure 2.1 below. Note that the maximum elevation (in degrees) for your target is its Declination plus 56°, so it may not be possible to avoid any shadowing for sources at Dec < -16° or Dec < -31° for D and C configuration, respectively.
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Figure 2.1: Plot of shadowed azimuths in D-configuration (black line) and C-configuration (grey line), for three elevations: 25°, 15°, and 8°. (Important note, these plots and the post processing packages (AIPS and CASA) do not calculate shadowing flags for an antenna placed on the Master Pad or the antenna barn.) |
