3 Data reduction

The data were re-calibrated under IRAF with the STSDAS package provided by STScI, using the "most recent'' calibration reference files as of December 1998, because some of the files had changed since the original observations. The values of background noise measured on the calibrated frames agree well with the values expected from photon statistics, as calculated from the expected read noise, dark current and sky background level. One of the PC chip's charge traps (Whitmore & Wiggs 1995; Voit et al. 1998) lies inside the jet image, in Col. 339. This has no observable effect on the faint UV image, but the effect had to be corrected on the well-exposed red-band images. This was done by replacing the affected portion of each image by the corresponding pixels from an offset image, as a correction according to Whitmore & Wiggs (1995) unduly increased the noise in the corrected part of the image. The images were initially registered using the commanded offsets to the nearest pixels. This alignment is sufficient for the rejection of cosmic rays as these only affect a small number of adjacent pixels. Cosmic rays were rejected using a standard $\kappa$-$\sigma$ algorithm, rejecting all pixel values deviating more than $4 \sigma$ from the local (low-biased) median in a first pass, and neighbouring pixels with more than $2.5 \sigma$ deviation in a second pass. The number of pixels treated this way agrees with the expected cosmic ray hit rate for the images. A model of the sky background and "horizontal smear'' (increased pixel values in rows containing saturated pixels from the quasar's core, Chap. 4 of Biretta et al. 2000) was fitted in the part of the image containing the jet using second-order polynomials along rows. The coefficients of the polynomials were then smoothed in the perpendicular direction. The conversion from photon count rate to physical flux units used the throughput information provided by STScI which is valid after the 1997 SYNPHOT update.

Figure 1: The jet in red light (620nm) after background subtraction. Logarithmic grey-levels run from 0 to 0.04$\mu$Jy/pixel, 0 $.\!\!^{\prime\prime}$08 effective beam size, 0 $.\!\!^{\prime\prime}$045 pixel size. The quasar core lies 10 $^{\prime \prime }$ to the northeast from A. The labelling of the jet features as introduced by Lelièvre et al. (1984) and extended by Röser & Meisenheimer (1991), together with the hot spot nomenclature from Flatters & Conway (1985) is also shown. Note that the labelling used by Bahcall et al. (1995) is slightly different.

Figure 2: The jet in UV light (300nm) after background subtraction. Logarithmic grey-levels run from 0 to 0.014 $\mu$Jy/pixel, 0 $.\!\!^{\prime\prime}$06 effective beam size, 0 $.\!\!^{\prime\prime}$045 pixel size.