Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555.

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For the conversion of angular to physical scales, we assume a flat cosmology with $\Omega_{{\rm m}}=0.3$ and $H_{0} = h_{60}\times 60\,{\rm km\,s^{-1}~Mpc^{-1}}$ , leading to a scale of $3.2\,h_{60}^{-1}~{\rm kpc}$ per second of arc at 3C273's redshift of 0.158.

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... index

We define the spectral index $\alpha$ such that $f_\nu \propto \nu^{\alpha}$ .

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... TELESCOPE

Proposal ID 5980.

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... imaging

Proposal ID 7848.

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... visit

A visit is a set of exposures that should be observed together; the HST equivalent of an observing run. The telescope is aligned to its guide stars only at the beginning of a visit.

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... level

The use of the quasar's diffraction spike by Bahcall et al. 1995 to locate the QSO's center is only expected to work well for sources near the camera center (Krist, priv. comm.). We failed to reproduce their quoted accuracy with our data.

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... images

Data from proposal 5589, PI John Trauger.

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