Chromatic Aberration (Bandwidth Smearing)
The principles upon which synthesis imaging are based are strictly valid only for monochromatic radiation. When visibilities from a finite bandwidth are gridded as if monochromatic, aberrations in the image will result. These take the form of radial smearing which worsens with increased distance from the delay-tracking center. The peak response to a point source simultaneously declines in a way that keeps the integrated flux density constant. The net effect is a radial degradation in the resolution and sensitivity of the array.
These effects can be parameterized by the product of the fractional bandwidth (Δν/ν0) with the source offset in synthesized beamwidths (θ0/θHPBW). Table 5 shows the decrease in peak response and the increase in apparent radial width as a function of this parameter. Table 5 should be used to determine how much spectral averaging can be tolerated when imaging a particular field.
| (Δν/ν0)*(θ0/θHPBW) | Peak | Width |
|---|---|---|
| 0.0 | 1.00 | 1.00 |
| 0.50 | 0.95 | 1.05 |
| 0.75 | 0.90 | 1.11 |
| 1.0 | 0.80 | 1.25 |
| 2.0 | 0.50 | 2.00 |
- Note: The reduction in peak response and increase in width of an object due to bandwidth smearing (chromatic aberration). Δν/ν0 is the fractional bandwidth; θ0/θHPBW is the source offset from the phase tracking center in units of the synthesized beam.
