3.8.7 Importance of calibration and editing

It is worth stressing that the final quality in MERLIN images is usually determined by the quality of the calibration, except for very extended sources where the image quality is limited by the $uv$ coverage. The quality of an image is often measured by its dynamic range, defined as the ratio of the peak brightness in the image to the rms fluctuations off-source $P/\sigma_{\rm rms}$. Table 1.1 gives the instrumental noise levels (which are a function of time on source and bandwidth); these are the minimum map noise attainable under perfect conditions.

To first order, the dynamic range due to phase errors $\phi$ radians on a timescale $t_\phi$ for an observation with $N$ antennas for a total time $T$ is:

$$P/\sigma_{\rm rms} \sim N(T/t_\phi)^{1/2}/\phi$$ (3.9)

For a typical observation using external phase calibration only, with a residual phase-error scatter of typically 15$\dg$ on a timescale of 5 minutes in a 12 hr observation, we may expect a dynamic range of order 300 (or less for intrinsically faint sources). Under ideal conditions with close phase-cals it may approach 1000. Self-calibration (see § 3.9.3) can improve the dynamic range to several thousand for bright sources; beyond this the map errors may be limited by the the accuracy in determining the residual baseline-dependent errors (typically 0.1% in amplitude and 0.02$\dg$ in phase). These can in principle be calibrated using the target if they are assumed constant over the run and dynamic ranges of $\sim50000$ have been achieved in this way, e.g. Fig. 3.7.

Figure 3.7: High dynamic range MERLIN map of 3C273 at 1.6 GHz. The peak is 31.676 Jy beam$^{-1}$ and the lowest contour ( $\sim3\sigma_{\rm rms}$) is 1.5 mJy beam$^{-1}$. The MERLIN beam at this $\delta$ is very elongated N-S, hence some sidelobes from the core are seen in this direction.

Careful editing of the data is of paramount importance especially for faint sources where limited or no self-calibration is possible. At the other extreme `reverse phase-referencing' allows the accurate position of a strong target to be found using a weak phase-cal.