An unsuitable choice of the IF bandwidth may lead (a) to irrecoverable distortion of the image by chromatic aberration if the bandwidth is too great or (b) to unacceptably low sensitivity if it is too small.
As discussed in detail by Bridle & Schwab (1989), images made with finite bandwidth are degraded away from the delay tracking center in a way that can be described as a radial smearing by a position-dependent distortion function. The width of the distortion function grows with distance from the delay tracking center. The effect can be thought of as a reduction in amplitude, and a simultaneous broadening, of the apparent point source response. Bridle & Schwab derive expressions for the amplitude reduction (where I is the observed amplitude and is the amplitude in the small-bandwidth limit) for three combinations of bandpass shapes and u-v tapering. All three are functions of the simple dimensionless parameter , i.e. of the fractional bandwidth times the angle from the delay tracking center measured in synthesized half-power beamwidths:
The first step in choosing the IF bandwidth for continuum work is to ask over what field radius (arcsec) you want to limit chromatic aberration to a given reduction of amplitude of the point source response. Then find the corresponding maximum value of the normalized parameter from whichever expression for best approximates your instrument. The maximum allowable IF bandwidth (MHz) consistent with these constraints is then given by
where is your observing frequency in MHz and is the half-power beamwidth in arcsec at which you expect to make your images. Unless you are prepared to relax your smearing/attenuation criterion slightly, select the closest allowed bandwidth that is narrower than . If you are prepared to relax it, choose the closest wider bandwidth.
Your choice of may be determined by the need to image an extended structure with minimal distortion, or by the need to include a strong confusing source in the minimally-distorted field of view. The latter need arises because you may wish to subtract or deconvolve a confusing source's sidelobes from the interesting region. The value of will always be greater than, or about equal to, the value of used earlier when selecting the range of baselines. In general, choose the delay and pointing center to minimize the required for your observations. When using a wide field to include a confusing source, consider displacing the delay center away from the target towards the confusing source. This will avoid the use of unnecessarily narrow bandwidths (and thus of unnecessarily low sensitivity). If the field is dominated by a strong point source (more than ten times brighter than other structure), this source should be placed near the delay center and pointing center when high dynamic range is required. This strategy will minimize the total distortion of the image resulting from bandwidth, pointing, averaging time and computational effects (such as u-v truncation) involving the strong source (for more details, see Clark 1981).
For point source detection experiments the above criteria will normally select the widest available bandwidth, unless the search position is exceptionally inaccurate or the field is known to be highly confused. Chromatic aberration affects observing strategy less critically at higher frequencies for several practical reasons--fractional bandwidths ( ) are often smaller; the usable field of view is limited by the primary HPBW of the antennas for the narrower fractional bandwidths; receiving systems are noisier and antennas are less efficient (so sensitivity, not distortion, limits image quality).
When deciding on the value of that is appropriate for an image of an extended source, consider the detectability of the extended emission at the resolution you will be using for your images (see Resolution, Baseline Range and Observing Frequency and Total Integration Time). There is no point ensuring that extended structure is not smeared radially by the bandwidth effect if low signal-to-noise on the same structure introduces uncertainties larger than the bandwidth distortions. As the signal-to-noise on extended emission itself depends on the choice of IF bandwidth, this calculation may need to be iterated until a suitable compromise is reached.