2 Observations

Observations were made using the Planetary Camera of the second Wide Field and Planetary Camera (WFPC2) (Biretta et al. 2000) on board the HUBBLE SPACE TELESCOPE in three sets of observations on March 23rd and June 5th/6th, 1995, for 35500s through filters F300W (ultra-violet, U) and 10000s through F622W (roughly $R_{\rm C}$ in the Kron-Cousins system). The resulting limiting magnitudes are listed in Table 1. The jet was imaged onto the center of the Planetary Camera (PC) chip which has 800 by 800 pixels with a nominal size of 0 $.\!\!^{\prime\prime}$0455 projected on the sky. The telescope was oriented such that the position angle of the PC chip's y-axis was $\approx$ $154\hbox{$^\circ$ }$. This way, the quasar itself is also mapped on the PC, while each of three neighbouring astrometric reference stars (see Table 3 in Röser & Meisenheimer 1991) was observed on one of the three Wide Field chips. This choice gives distortion-free images of the jet, while allowing the use of the quasar and the reference stars for image alignment. The total exposure time was split into individual exposures of about 2500s or one HST orbit, executed in three separate sets of observations. Each of these exposures was done at a telescope pointing slightly offset from all others (by up to 1 $^{\prime \prime }$) to facilitate the correction for chip artifacts. One short exposure was obtained in each set of observations and filter to measure the positions of the quasar and reference stars, most of which are saturated on the long exposures. Our study of 3C273's jet will be conducted employing a 0 $.\!\!^{\prime\prime}$2 effective beam size. This results in a typical signal to noise ratio (S/N) per resolution element in the red band of around 100 in the brightest regions and 30-40 in inter-knot regions. Because both the jet flux and the WFPC2 and telescope throughput decrease towards the UV, the UV-band S/N is only about 40 in the brightest regions, while the inter-knot regions are barely detected.