LENSVIEW: software for modelling resolved gravitational lens images

We have developed a new software tool, lensview, for modelling resolved gravitational lens images. Based on the lensmem algorithm, the software finds the best-fitting lens mass model and source brightness distribution using a maximum entropy constraint. The method can be used with any point spread function or lens model. We review the algorithm and introduce some significant improvements. We also investigate and discuss issues associated with the statistical uncertainties of models and model parameters and the issues of source plane size and source pixel size. We test the software on simulated optical and radio data to evaluate how well lens models can be recovered and with what accuracy. For optical data, lens model parameters can typically be recovered with better than 1 per cent accuracy, and the degeneracy between mass ellipticity and power law is reduced. For radio data, we find that systematic errors associated with using processed radio maps, rather than the visibilities, are of similar magnitude to the random errors. Hence analysing radio data in image space is still useful and meaningful. The software is applied to the optical arc HST J15433+5352 and the radio ring MG1549+3047 using a simple elliptical isothermal lens model. For HST J15433+5352, the Einstein radius is 0.525 +/- 0.015 arcsec which probably includes a substantial convergence contribution from a neighbouring galaxy. For MG1549+3047, the model has Einstein radius 1.105 +/- 0.005 arcsec and core radius 0.16 +/- 0.03 arcsec. The total mass enclosed in the critical radius is 7.06 x 10(10) M-circle dot for our best model. We finish by discussing issues relating to modelling of resolved lens images for this method and some alternatives.