Bandwidth Smearing

During observations with a bandwidth of 128MHz the Compact Array records 32 channels of 4MHz. Independent analysis of each 4MHz channel can be done to reduce bandwidth smearing (chromatic aberration). Although at 20cm with the 6km array, radial smearing of the images may still be a problem. For a given baseline, the ratio (A) of observed amplitude to true amplitude in a 4MHz channel (effective width 8MHz) is approximately:

$$A = 1 - \mathrm{offset~\times~baseline}/130$$ (8)

where `offset' is the distance from the phase centre (arcmin) and `baseline' is the baseline length (km). For example, if you observe a source at the half-power point of the primary beam at 20cm (radius 16.5') with a 6km baseline, 128MHz bandwidth and process the 32 channels separately, the amplitude will be only 24% of the true amplitude. For the entire array, the effect is diminished because of the contribution of the shorter baselines. In the above case, the 6km array (synthesised beam of 6") will result in 90% of the true amplitude being retained. See Appendix D.1 in Killeen (1993) for the functional form.

The effect of bandwidth smearing can be reduced by dividing a large band into smaller subsets with higher resolution per channel and using the benefits of frequency switching or multi-frequency synthesis. See the Observing Mode page or the Miriad Users Guide for further details.