Chromatic Aberration (Bandwidth Smearing)

The principles upon which synthesis imaging are based are strictly valid only for monochromatic radiation. When radiation from a finite bandwidth is accepted and 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 ( $\Delta \nu /\nu_0$) with the source offset in synthesized beamwidths ( $\theta/\theta_{\rm HPBW}$). Table 11 shows the decrease in peak response and the increase in apparent radial width as a function of this parameter.

Table 11: Reduction in Peak Response Due to Bandwidth Smearing

${\Delta\nu\over\nu_0}{\theta_0\over\theta_{\rm HPBW}}$	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). $\Delta \nu /\nu_0$ is the fractional bandwidth; $\theta_0/\theta_{\rm HPBW}$ is the source offset from the phase tracking center in units of the synthesized beam.

If you wish to obtain maximum sensitivity and resolution over the entire field-of-view of the antennas, then the spectral-line modes of the correlator (also known as multichannel continuum or pseudo-continuum) probably will be required.