4 Maps of the optical brightness

The calibrated images are presented in Figs. 1 and 2. The morphology of the jet is identical in both images and appears rather similar to the morphology in high-resolution radio maps (Conway et al. 1993; Bahcall et al. 1995). The exception to this is the radio hot spot, being the dominant part in the radio but fairly faint at high frequencies. Our images show structural details of the optical jet which were not discernible on earlier, shallower and undersampled HST WF images of 0 $.\!\!^{\prime\prime}$1 pixel size (Bahcall et al. 1995). Based on our new maps, the term "knots'' seems inappropriate for the brightness enhancements inside the jet, as these regions are resolved into filaments. The higher resolution necessitates a new nomenclature for the jet features (Fig. 1). For consistency with earlier work (Lelièvre et al. 1984; Flatters & Conway 1985; Röser & Meisenheimer 1991), our nomenclature is partly at variance with that introduced by Bahcall et al. (1995). The jet is extremely well collimated - region A has an extent (width at half the maximum intensity) of no more than 0 $.\!\!^{\prime\prime}$8 perpendicular to the average jet position angle of $\sim$ $222\hbox{$^\circ$ }$ (opening angle $5\hbox{$^\circ$ }$). Even region D2/H3 is only 1 $^{\prime \prime }$ wide (opening angle $\approx$ $2.5\hbox{$^\circ$ }$). The optical jet appears to narrow towards the hot spot, in the transition from H3 to H2. Region A is now seen to extend further towards the core than previously known. It may be noteworthy that Lelièvre et al. (1984) reported the detection of an extension of knot A towards the quasar, whose existence at the reported flux level was not, however, confirmed by later work. The criss-cross pattern visible in regions C1 and C2, and less clearly in B1-2 and D1, is reminiscent of a (double?) helical structure (Bahcall et al. 1995), but could also be explained by oblique double shocks (Hardee & Norman 1989). The jet has three "extensions'' (Fig. 1), none of which has been detected at radio wavelengths. The morphology of the outer extension supports the classification as a galaxy based on its colours made by Röser & Meisenheimer (1991). The nature of the other two extensions, however, remains unknown even with these deeper, higher resolution images. The northern inner extension was already resolved into two knots (In1, In2) on a FAINT OBJECT CAMERA image (Thomson et al. 1993). The two knots are extended sources and clearly connected to each other. The southern extension is featureless and an extended source. Comparing the direct images, we can immediately estimate that the jet's colour slowly turns redder outwards from region A. The similarity of the jet images in both filters shows that there are no abrupt colour changes within the jet. For a quantitative assessment of the extensions' and the jet's colour in the following, we derive an optical spectral index map.