  Time-delay measurements of 0957+561 based on radio monitoring now agree with
optical monitoring. Recent models incorporate many recent observations, but the
systematic uncertainties in the models still dominate the uncertainty in the
cosmological results. VLBI observations of the milli-arcsec jet structure have
provided the most important set of modeling constraints. We present new
observations of this structure, made at 18 cm with the Very Long Baseline
Array, the Very Large Array, and the Green Bank 140 ft. telescope. Compared to
the data set of Garrett et al. (1994), ours has a similar theoretical noise
level (20 microJy), but nearly twice as many baselines (66 compared to 36) and
more uniform coverage of the UV plane. Our initial maps confirm the basic jet
structures seen by Garrett et al., with comparable RMS noise (60-80 microJy),
and are in reasonable agreement with the jet structure model of Barkana et al.
(1999). The maps also hint at a new compact component leaving the core, and a
previously undetected diffuse component at the end of the jet. Further
refinements in fringe-fitting, mapping, and self-calibration should yield
significantly lower RMS noise (closer to the theoretical level). We will use a
modified form of the CalTech VLBI Modelfit program (written by Barkana et al.
1999) to determine the magnification matrix between the images. The refined
magnification matrix should provide stronger constraints on the lens model, and
thus reduce the uncertainty in the cosmological results.
