In order to use a gravitational lens to measure the Hubble constant
accurately, it is necessary to derive a reliable model of the lens surface
potential. If the analysis is restricted to the locations and magnifications of
point images, the derived Hubble constant depends upon the class of mass models
used to fit the data. However, when there is extended emission from an Einstein
ring, it may be possible to derive a potential from the observed surface
brightness in a model-independent manner. This procedure is illustrated with
reference to B1608+656. The multi-band images are de-reddened, de-convolved and
de-contaminated so that the luminous matter and the surface brightness contours
in the Einstein ring are both faithfully mapped. This intensity distribution
can then be used to reconstruct the potential. Progress in implementing this
program is reported.
The observed incidence of multiple-imaged galaxies on the Hubble Deep Fields
is an order of magnitude smaller than naively predicted on the basis of radio
lens surveys, like CLASS, but consistent with the rate computed using surface
photometry of candidate lens galaxies assuming standard mass to light ratios.
In order to resolve this paradox, it is suggested that most galaxy lenses are
located in compact groups.Comment: Latex 10 pages 4 figures. To appear in ``Gravitational Lensing:
Recent Progress and Future Goals'' Editors: Tereasa G. Brainerd and
Christopher S. Kochane
