extended component bends and diffuses toward the northeast. The absence of short baselines ($uv<30 k\lambda$) results in some negative features (the so-called ‘negative bowl’ in synthesis images) just outside the outer boundary of the envelope.

The integrated flux density over the whole source is 14.36$\pm$1.02 Jy (very close to the single-dish measurement), suggesting that there is not much missing flux on short spacings. The uncertainty we assign includes both the systematic errors and the [*r.m.s.*]{} fluctuations in the image. Since the calibrator of the flux density scale, 3C 286, is resolved on baselines longer than 600 k$\lambda$ [@An04; @Cot97a], a model with a set of CLEAN components was used in flux density calibration instead of a point-source model. We further compared the derived flux density of the phase calibrator DA 193 from our observations with published results [@Sta98; @Con98]. The comparison suggests that the flux density of DA 193 from our MERLIN observation was consistent with that from the VLBI measurements to within 7 per cent. We note that this systematic error includes both the amplitude calibration error of 3C 286 and the error induced by the intrinsic long-term variability of DA 193; the latter is likely to be dominant.

