We perform a detailed analysis of the optical gravitational lens ER 0047–2808 imaged with WFPC2 on the Hubble Space Telescope .
Using software specifically designed for the analysis of resolved gravitational lens systems , we focus on how the image alone can constrain the mass distribution in the lens galaxy .
We find the data are of sufficient quality to strongly constrain the lens model with no a priori assumptions about the source .
Using a variety of mass models , we find statistically acceptable results for elliptical isothermal-like models with an Einstein radius of 1.17 ^ { \prime \prime } .
An elliptical power-law model ( \Sigma \propto R ^ { - \beta } ) for the surface mass density favours a slope slightly steeper than isothermal with \beta = 1.08 \pm 0.03 .
Other models including a constant M/L , pure NFW halo and ( surprisingly ) an isothermal sphere with external shear are ruled out by the data .
We find the galaxy light profile can only be fit with a Sérsic plus point source model .
The resulting total M/L _ { B } contained within the images is 4.7 h _ { 65 } \pm 0.3 .
In addition , we find the luminous matter is aligned with the total mass distribution within a few degrees .
This is the first time a resolved optical gravitational lens image has been quantitatively reproduced using a non-parametric source .

The source , reconstructed by the software , is revealed to have two bright regions , with an unresolved component inside the caustic and a resolved component straddling a fold caustic .
The angular size of the entire source is \sim 0.1 ^ { \prime \prime } and its ( unlensed ) Lyman- \alpha flux is 3 \times 10 ^ { -17 } erg s ^ { -1 } cm ^ { -2 } .