Imaging limitations

The technique of Synthesis Imaging is affected by a number of limitations. In this section, we outline how these apply to VLBI generally, and the EVN in particular.

Field of View (FOV) Limitations

For VLBI the unaberrated FOV is always much smaller than the FOV of the primary beam of the individual VLBI antennas. The two main effects responsible for this are bandwidth smearing and time average smearing. Of these, time smearing usually places the most severe limitations on the FOV. Both are discussed below, but there is also a separate EVN Field of View Guide that explains field of view issues with special regard to the capabilities of the EVN MkIV Data Processor at JIVE, in more detail. This can be viewed both in html and postscript formats.

Bandwidth Smearing

Bandwith smearing arises because the telescopes observe over a range of frequencies, the observed, or in the case of VLBI, the recorded bandwidth. Averaging the visibilities over the observed bandwidth is equivalent to averaging over a short radial cut in the uv-plane. If the response of the interferometer varies appreciably over this area or cut through the uv-plane, then structure corresponding to this variation will be reduced in amplitude or "smeared out" altogether.

It turns out that rapidly varying components in the uv-plane correspond to sources which are located far from the phase centre. The effect of bandwidth smearing on the final image is thus to radialy smear sources located far from the phase centre. The observed peak for the smeared source is reduced (compared to the true, unaberrated peak) but the total flux of the source is conserved.

The effects of bandwidth smearing can be minimised (at the expense of computer processing time and disk space) since VLBI data sets are usually multi-channel in nature. For example, a VLBA/MkIV observing mode with 64 MHz bandwidth may typically comprise of 128 individual 0.5 MHz channels. Provided the data are not subsequently averaged, the bandwidth smearing relates to the channel bandwidth i.e. 0.5 MHz.

Here we tabulate the radial distance from the phase-centre for which bandwidth smearing begins to become significant (for e.g. a 1% reduction in brightness and a channel width of 0.5MHz) for some typical baselines: Shanghai-Robledo - the longest possible EVN only baseline, and Onsala-Noto - the longest EVN baseline not including the Chinese stations. Note that the effect of bandwidth smearing is strongest on the longest baselines but is independent of frequency (for a given baseline or array).

Time Smearing

Averaging in time is equivalent to averaging adjacent points in the uv-plane. The effect of time-smearing is therefore strongest on the longest interferometer baselines, since these sweep through the uv-plane more quickly than shorter baselines. Time-smearing is a much more complicated effect than bandwidth smearing (see below) and the exact details of the effect are dependent on the source position and baseline orientation. The total flux density of a smeared component (c.f. the true flux) is not conserved. The effects of time-smearing scale directly with increasing baseline length and observing frequency. We assume an observing frequency of 5 GHz (6cm) here, and tabulate some indicative (conservative) values for the radial distance from the phase-centre for which time smearing begins to become significant on EVN baselines (1% brightness reduction):

Largest Detectable Angular Structure

The angular size of the largest structure detectable (mappable) by the EVN depends on the length of the shortest (projected) baseline (Eb-Wb). A conservative estimate of the largest detectable angular size is therefore about 0.1 arcsec at 18cm.

If the source of interest has radio structure on a larger angular scale-size, then joint EVN+MERLIN observations should be considered. During joint EVN+MERLIN sessions the Cambridge 32-m telescope is available for VLBI, thus providing several short, VLBI baselines to the Jb, Wb, and Eb telescopes. Further, the addition of the shortest internal MERLIN baselines (>= 11 km), allows large-scale structure on the scales of several arcseconds to be accurately recovered.